Review on additive manufacturing of tooling for hot stamping

Abstract

Sustainability is a key factor in an automotive OEMs' business strategy. Vehicle electrification in particular has received increased attention, and major manufacturers have already undertaken significant investments in this area. However, in order to fully confront the sustainability challenge in the automotive industry, lightweight design in additional to alternative propulsion technologies is also required. Vehicle weight is closely correlated with fuel consumption and range for internal combustion and electrified vehicles, respectively, and therefore, weight reduction is a primary objective. Over the past decades, advanced steel and aluminium-forming technologies have seen considerable development, resulting in significant weight reduction of vehicle components. Hot stamping is one of the most established processes for advanced steel and aluminium alloys. The process offers low-forming loads and high formability as well as parts with high strength and minimal springback. However, the high temperatures of the formed materials over numerous cycles and the significant cooling required to ensure desirable component properties necessitate advanced tooling designs. Traditionally, casting and machining are used to manufacture tools; although in recent years, additive manufacturing has gained significant interest due to the design freedom offered. In this paper, a comprehensive review is performed for the state-of-the-art hot-forming tooling designs in addition to identifying the future direction of Additive Manufactured (AM) tools. Specifically, material properties of widely used tooling materials are first reviewed and selection criteria are proposed which can be used for the transition to AM tools. Moreover, key variables affecting the success of hot stamping, for example cooling rate of the component, are reviewed with the various approaches analysed by analytical and numerical techniques. Finally, a number of future directions for adopting additive manufacturing in the production of hot stamping tools are proposed, based on a thorough analysis of the literature.

Full text

ORIGINAL ARTICLE
Review on additive manufacturing of tooling for hot stamping
Dimitrios Chantzis
1
&Xiaochuan Liu
1
&Denis J. Politis
2
&Omer El Fakir
1
&Teun Yee Chua
1
&Zhusheng Shi
1
&
Liliang Wang
1
Received: 13 February 2020 /Accepted: 8 June 2020
#The Author(s) 2020
Abstract
Sustainability is a key factor in an automotive OEMs’business strategy. Vehicle electrification in particular has received
increased attention, and major manufacturers have already undertaken significant investments in this area. However, in order
to fully confront the sustainability challenge in the automotive industry, lightweight design in additional to alternative propulsion
technologies is also required. Vehicle weight is closely correlated with fuel consumption and range for internal combustion and
electrified vehicles, respectively, and therefore, weight reduction is a primary objective. Over the past decades, advanced steel
and aluminium-forming technologies have seen considerable development, resulting in significant weight reduction of vehicle
components. Hot stamping is one of the most established processes for advanced steel and aluminium alloys. The process offers
low-forming loads and high formability as well as parts with high strength and minimal springback. However, the high temper-
atures of the formed materials over numerous cycles and the significant cooling required to ensure desirable component prop-
erties necessitate advanced tooling designs. Traditionally, casting and machining are used to manufacture tools; although in recent
years, additive manufacturing has gained significant interest due to the design freedom offered. In this paper, a comprehensive
review is performed for the state-of-the-art hot-forming tooling designs in addition to identifying the future direction of Additive
Manufactured (AM) tools. Specifically, material properties of widely used tooling materials are first reviewed and selection
criteria are proposed which can be used for the transition to AM tools. Moreover, key variables affecting the success of hot
stamping, for example cooling rate of the component, are reviewed with the various approaches analysed by analytical and
numerical techniques. Finally, a number of future directions for adopting additive manufacturing in the production of hot
stamping tools are proposed, based on a thorough analysis of the literature.
Keywords Additive Manufacturing .Design for Additive Manufacturing .Hot stamping .Die design
1 Introduction
Globally, manufacturing is regarded as one of the primary
sources of wealth, and yet, the large amount of energy used
for industrial operations, in combination with the increasing
demand for goods, causes significant environmental repercus-
sions. Until recently, the performance of a production system
has been evaluated by taking into consideration four main
attributes as follows: cost, time, quality and flexibility [1].
However, global megatrends [2] such as climate change are
moving the manufacturing community towards the consider-
ation of sustainability. As a result, sustainability must be con-
sidered in all stages of the manufacturing decision-making
process [3]. The automotive industry is a major sector
adapting to this new framework, which consists of
government-regulated emission standards such as Euro 6 for
Europe or CAFE for the USA, or even more radical political
rulings such as the UK’s decision to ban all petrol and diesel
vehicles by 2040 [4]. The answer of the automotive industry
to this challenge is electrification, and major automotive
OEMs have already announced plans for full electric or
plug-in hybrid vehicles [5].
However, vehicle sustainability should not be addressed by
focussing on electrification alone. Another key element in
efficiency is lightweight design, which can reduce the envi-
ronmental footprint of both internal combustion (IC) and elec-
trified vehicles. Vehicle weight closely correlates with fuel
consumption and range, for IC and electrified vehicles,
*Liliang Wang
liliang.wang@imperial.ac.uk
1
Department of Mechanical Engineering, Imperial College London,
Exhibition Road, London SW7 2AZ, UK
2
Department of Mechanical and Manufacturing Engineering,
University of Cyprus, 1678 Nicosia, Cyprus
https://doi.org/10.1007/s00170-020-05622-1
/ Published online: 30 June 2020
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respectively, and thus its reduction is effective regardless of
vehicle propulsion technology. The objective of lightweight
design is to minimise dead weight of a construction without
impinging on function, safety or useful life [6].
Over the past decades, advanced steel manufacturing tech-
nologies have seen considerable development, resulting in the
significant reduction of vehicle component weight through the
use of advanced steel alloys, such as high-strength steel
(HSS), advanced high-strength steel (AHSS) and ultra-high-
strength steel (UHSS) [7]. In the automotive industry, ultra-
high-strength steels can be considered a relatively ‘low cost’
lightweight material option compared with aluminium alloys,
and OEMs have extensive knowledge regarding their manu-
facturability. Moreover, their capability to absorb impact en-
ergy in a collision as well as their adequate formability and
joining capability make steel alloys a popular choice with
automotive manufacturers. On the other hand, aluminium
has attracted significant attention due to an improved
strength-to-weight ratio compared with steel and unique cor-
rosion behaviour [8]. A characteristic example of the in-
creased adoption of aluminium is in North American OEM
vehicles, which has seen an increase of 27% between the years
2012 and 2015. The increase stems from an equal combina-
tion of higher vehicle production and increased aluminium
content per vehicle (Ducker [9]). According to the literature
[10], aluminium will comprise of more than 75% of pickup
truck body parts, 24% of large sedans, 22% of SUVs and 18%
of minivan body and closure components by 2025.
Hot stamping is considered one of the main forming pro-
cesses for manufacturing advanced steel and aluminium alloy
sheet components [11,12]. The number of hot stamping parts
is increasing in a BiW with most of them used in the crash-
related zones. These zones must resist intrusion and maintain
their integrity. Numerous studies have been published regard-
ing hot stampingof roof rail [13], door impact beam [14]orB-
pillar inner [15,16]. On the other hand, cold forming is mainly
used with mild steel which is mainly used for non-critical
safety parts such as closures [17]. Hot stamping processes
have increased complexity compared with cold stamping, as
successive processes such asquenching are crucial to the con-
trol of cooling rate and subsequent post-form strength [18].
The cooling rate is determined largely by the quantity of ther-
mal energy transferred from the deformed component to the
tool cooling system.
Press tools are typically designed with internal cooling
channels and manufactured by conventional machining pro-
cesses such as drilling, in order to achieve the desirable
cooling rates. A significant challenge to existing cooling chan-
nel production methods is that the channels are unable follow
an equidistant profile to the tool surface, resulting in inconsis-
tent quench rates throughout the component [19]. In addition,
large press tools are manufactured by segmenting a single tool
into multiple pieces, which are individually machined and
subsequently assembled. Tool segmentation is also often re-
quired due to limitations of cooling channel drilling technol-
ogies [20].
To this end, additive manufacturing (AM) offers significant
advantages, as opposed to conventional subtractive technolo-
gies [21]. These include the fabrication of complex geome-
tries, material savings, design flexibility and reduced tooling
costs [22]. The most evident realisation of these advantages
has been in plastic injection moulding, where steel tools for
final production, have been fabricated using additive tech-
niques in several works [23–28]. However, the adoption of
AM techniques for direct hot stamping tooling has seen lim-
ited progress.
The authors acknowledge the significant potential of the
AM on the design and manufacture of hot stamping tooling.
Thus, the aim of this article is to review currently available
literature in the area that AM and hot stamping tooling inter-
sect. Section 2provides an overview of hot stamping and its
process principles. Section 3reviews recently published stud-
ies on Additively Manufactured hot stamping dies. Section 3.1
investigates design approaches of cooling systems for hot
stamping dies and how these can be applied for AM, while
in Section 3.2 simulation models are reviewed and sugges-
tions are proposed for their use in AM. Then, Section 3.3
reviews the thermo-mechanical properties of well-
established hot stamping tooling materials and identifies min-
imum threshold criteria that can be used for the transition to
Additively Manufactured materials. Finally, summary and
concluding remarks of the study are provided as well as the
authors’outlook for AM in hot stamping tooling.
2 Hot stamping of ultra-high-strength steels
and aluminium alloys
2.1 Hot stamping of ultra-high-strength steels
Hot stamping processes for ultra-high-strength steels currently
exist in the following two main variants: direct and indirect
hot stamping processes (Fig. 1). In a direct hot stamping pro-
cess, the blank is heated in a furnace, transferred to the press
and subsequently formed and quenched in the closed tool. An
indirect hot stamping process is characterised by the use of a
nearly complete cold pre-formed part which is subjected only
to a quenching and calibration operation in the press after
austenitisation [30].
A crucial requirement in the hot stamping of steels is the
transformation of the material microstructure from austenite to
martensite to achieve the desirable post-form mechanical
properties. This can be achieved through careful thermal con-
trol at each process stage. During heating, the temperature
must be above the re-crystallisation temperature in order for
the microstructure to achieve austenisation. Moreover, the
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blank temperature must be homogeneous, which is a pre-
condition for the desired fully martensitic transformation
[31]. During the subsequent hot blank transfer operation to
the forming tool, the temperature decreases rapidly due to heat
loss to the surrounding environment. Lengthy transfer times
may result in microstructural transformation to bainite and
ferrite, and therefore, it is crucial to place the blank in the
die at a temperature above 730 °C. The final operation is to
form the blank inside the die and simultaneously quench. The
cold die acts as a heat sink to the hot blank during quenching,
enabling the hot blanks microstructure to be transformed from
austenite to martensite. However, this phase change is realised
only when the cooling/quenching rate is above a criticalvalue,
which is alloy-dependent. If the cooling rate is below this
value, the microstructure of the final part will consist of bainite
or ferrite phases, which will affect the final mechanical prop-
erties. CCT diagrams are used to represent the types of phase
changes that will occur in a material as it is cooled at different
rates. In Fig. 2, the CCT diagram of 22MnB5 is shown.
Studies regarding this material have concluded that the critical
cooling rate to achieve a full martensite transformation is 30
°C/s [33–36].
Since the cooling of the blank occurs within the dies, hot
stamping die design is critical to the cooling performance and
directly affects productivity. It has been found that quenching
time accounts for up to 30% of the total process cycle time [37].
2.2 Hot stamping of high-strength aluminium alloys
Conventional forming of aluminium alloys encounters the fol-
lowing two major issues: (i) the low formability of aluminium
Fig. 2 CCT diagram of 22MnB5 [32]
Fig. 3 CCP diagrams for a6082 aluminium alloys [58] and b7075
aluminium alloys [57].
Fig. 1 Existing hot stamping
technologies for UHSS. aDirect
hot stamping, bindirect hot
stamping [29]
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alloys at room temperature which significantly limits their
application to the forming of complex-shaped components
[38] and (ii) the low stiffness of aluminium alloys at room
temperature which causes high springback ofthe formed com-
ponents [39]. Solution heat treatment, cold die forming and in-
die quenching (HFQ) was developed as a promising hot
stamping technology to address these two issues [40–42].
During an HFQ forming process, an aluminium alloy blank
is first heated to its solution heat treatment temperature (SHT)
followed by a specific period of soaking, in order to fully
dissolve alloying elements and/or precipitates into the material
matrix, resulting in high formability of the aluminium alloy.
After the heat treatment, the hot blank is transferred to a press
machine and then stamped into the desired shape by forming
tools at room temperature. Simultaneously, the hot blank is
quenched at a sufficiently high-cooling rate to achieve a su-
persaturated solid solution state (SSSS). Consequently, pre-
cipitates are uniformly generated and distributed within the
aluminium matrix during the subsequent artificial ageing,
reaching high post-form strength of the formed component.
Due to these pronounced benefits, HFQ was patented globally
[43,44] and commercialised for automotive applications [45].
In recent years, Fast light Alloy Stamping Technology
(FAST) was developed and patented as a novel hot stamping
process, not only increasing the formability of aluminium al-
loys and decreasing springback but also significantly reducing
cycle time [46,47]. During such a forming process, an alu-
minium alloy blank is heated to an elevated temperature at an
ultra-fast heating rate, in order to enhance the ductility of the
material while maintaining its microstructure. Subsequently,
the hot blank is transferred to a press machine, and simulta-
neously deformed and quenched by cold forming tools at a
high cooling rate. Due to minor changes to the microstructure,
the artificial ageing period is dramatically reduced to achieve
the desired post-form strength. The elimination of lengthy
heating, soaking and ageing periods is beneficial to the reduc-
tion of cycle time. Furthermore, the processing parameters,
such as temperature, heating rate, transfer time and cooling
rate, could be customised to satisfy customer requirements
for the post-form mechanical properties of the formed compo-
nents [48–50].
Considering these advantages, hot stamping of aluminium
alloys has been studied by numerous researchers, under the
prism of identifying optimum processing windows to realise
customer requirements. In the study by Harrison et al. [51], the
transfer time of a hot blank from a furnace to a press machine
was reduced and precisely controlled byusing a robotic arm to
ensure that a SSSS state for AA7075 could be achieved after
quenching. The stamping speed and blank holding force were
optimised to reduce wrinkling and/or fracture of anti-collision
side beams made fromAA6111, as stated in the study by Zhou
et al. [52]. The application of lubricants is another efficient
method to avoid crack formation in formed components [53,
54]. Additionally, some researchers have employed dedicated
equipment in the hot stamping of aluminium alloys to realise
desirable processing windows. For instance, a servo motor
was applied in the study by Song et al. [55], to enable for a
more uniform distribution of temperature and strain in
AA7075 sheets. Resistance heating equipment was also
employed by Maeno et al. [56]toshortentheheattreatment
of aluminium alloys.
Despite the differences between these processing windows,
all forming processes have the critical requirement of
quenching the aluminium alloys within cold forming tools.
By evaluating the continuous cooling precipitation (CCP) di-
agram of aluminium alloys, it can be found that the cooling
curve intersects the regions where coarse precipices (e.g. β
and β' for 6XXX alloy) are generated. This results in second-
ary particles being precipitated out and consuming alloying
elements which would lead to a decrease in the post-form
mechanical strength. Therefore, critical cooling rates for alu-
minium alloys must be achieved during quenching to avoid
this phenomenon [57,58]. As a heat transfer medium, tools
significantly affect the cooling rate of aluminium alloys, and
therefore the selection of tool materials and designs is of great
importance in order to achieve the critical cooling rate [53,
54].
It is well understood that hot stamping of both high-
strength steels and aluminium alloys is a complex process that
requires precise control of thermomechanical phenomena.
However, while the majority of studies focus on the material
properties of the final formed component, the significance of
the applied tooling must also be emphasised [59].
3 Additive manufacturing of hot stamping
dies
The manufacture of hot stamping dies with internal cooling
channels is a challenging task due to limitations in conven-
tional processes. Currently, there are three methods of
manufacturing internalcooling channelsfor hot stamping dies
[60]asshowninFig.3. The first is the conventional drilling
method. This method, although most cost effective [61], has a
profound disadvantage. Drilling can only produce straight line
bores and cooling channels which are unable to maintain an
equidistance from a complex 3D tool surface geometry.
Consequently, “hot spots”are obtained which are essentially
areas on a final component that have been cooled at different
rates compared with the rest of the component and inherently
exhibit varied mechanical properties. The second method in-
volves the segmentation ofthe die into two parts;the shell and
the core, with the cooling channels divided between them.
Although this approach increases the design flexibility, it is
associated with increased cost and material waste. Moreover,
the sealing between the two parts is a significant challenge
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which contributes to its reduced popularity. The final method
involves producing the die through casting and pre-fixing the
cooling channels inside the casting mould. However, fixing
the cooling channels is not a simple task, and there is the
possibility of the cooling channels melting during the casting
of the die. Moreover, casting a die necessitates additional post-
processing that it is not required by the other two methods.
The manufacturing of hot stamping diesis typically associated
with other research areas such as high performance cutting of
hot work steels [62–64]. However, the review of such studies
is outside the scope of this paper. Fig. 4
There are strong indications that AM can be a disruptive
technology for hot stamping tools. The design flexibility of-
fered can aid designers to realise concepts that are currently
unachievable using conventional manufacturing processes.
Moreover, in the authors’opinion, metal-forming tooling is
the only area that can fully exploit the benefits of AM pro-
cesses in the short term as high production volumes are not
required for such applications. In the automotive industry, the
manufacturing of a die usually takes 21weeks [65], and there-
fore even with current deposition rates, AM technologies can
be considered as a viable alternative. In this section, small-
scale case studies are presented addressing the application of
AM. Although there are several published studies regarding
the application of AM to tooling, this work focuses specifical-
ly on studies addressing hot stamping and does not include
results from other manufacturing sectors such as injection
moulding.
Cortina et al. [66] investigated the design of conformal
cooling ducts, which were additively manufactured on a
CR7V-L hot work steel substrate and then post-processed with
milling. AISI H13 and AISI 316Llaser claddings were used as
the filler material for the manufacture of the cooling channels
(Fig. 5).
The mechanical as well as thermal performance of the
specimen shown in Fig. 5was evaluated, and it was concluded
that laser cladding is a viable alternative manufacturing pro-
cess. In Fig. 6, the thermal performance of the two cooling
channels is shown and it can be observed that they are almost
identical.
The authors subsequently transferred the proposed
manufacturing approach to a complex geometry (Fig. 7)and
their cooling effectiveness was compared with conventional
manufacturing. The results showed that a more homogeneous
temperature distribution could be achieved with AMed
channels.
In addition, the study by Muller et al. [37]has also
highlighted the benefits of additively manufacturing cooling
channels. It was determined that the main benefit of complex
cooling channels following the profile of the parts was the
radically reduced quenching time during hot stamping pro-
cesses, which accounts for 30% of the cycle time. The cooling
system was redesigned through simulations, and the key input
parameters were the compression force, workpiece tempera-
ture, coolant temperature, flow rate and internal surface rough-
ness of the cooling channels. The resulting improvement in
both the temperature gradient and temperature on the die’s
working surface was significant (Fig. 8).
The performances of the cooling systems were evaluated
by simulating the temperature distribution on the final com-
ponent. It is clear that the most critical area is in the die’s
deepest cavity, which conventional cooling channels could
not reach due to manufacturing limitations in following the
geometric profile of the part. The AMed cooling channels
Fig. 4 Different manufacturing
methods for internal cooling
channels (adapted from [60])
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were able to achieve a temperature in this areathat was almost
50% lower than with conventional cooling channels (Fig. 9).
In addition to simulation work, the authors conducted ex-
perimental work that verified that the additively manufactured
tools reduced the quenching time by 50%.
The studies of Cortina et al. [66] and Muller et al. [37]are
the only studies that haveinvestigated the potential of additive
manufacturing for hot stamping processes of metallic parts.
From a review of the literature, the majority of published
papers have focused on the investigation of AM in injection
moulding of plastic parts [23–28]. As the focus of this review
is on AM for metal hot stamping, injection moulding studies
are not taken into consideration due to the variations of ther-
mal and mechanical loads.
3.1 Design of cooling system for hot stamping dies
Quenching is one of the main bottlenecks to the overall hot
stamping cycle time [67]withMulleretal.[37] estimating this
process to account for 30% of the total production process.
Quenching is achieved through a medium which flows
through the cooling system and acts as a heat sink extracting
heat from the blank. It is clear that a cooling system with
tangential cooling channels to the working surface of the die
and to each other would be ideal from a thermodynamic point
of view. However, this would deteriorate the mechanical per-
formance of the die and would compromise its structural in-
tegrity. Moreover, the diameter of a cooling channel plays a
significant role in the thermal performance of the die. A great-
er volume of cooling medium can flow through a large diam-
eter compared with a smaller one. However, there is a limit on
the cooling channel diameter as significant removal of mate-
rial can compromise die integrity.
Therefore, it is clear that the design of a cooling system for
a hot stamping die is a multi-objective optimisation task.
Ideally, conformal cooling design is preferred; however, due
to manufacturing constraints, they may be prohibitively ex-
pensive. Hence, straight circular drilled cooling channels are
the most practical choice in industry thus far.
3.1.1 Design for additive manufacturing of hot stamping die
cooling systems
The almost infinite design flexibility that AM technologies
offer compared with existing die manufacturing technologies
enables the manufacturing of optimised cooling channels for
hot stamping dies. However, due to the additive nature of
these processes, there are several design constraints that the
Fig. 6 Measurements from a
thermal camera (left, coolant is
off; right, coolant is on) [66]
Fig. 5 aInitial CR7V-L substrate
with one drilled channel and
prepared machined surfaces for
cladding. bFinished sample with
one drilled and one additively
manufactured cooling channel
[66]
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designer must consider. For instance, components which are
designed with overhanging features may require sacrificial
supports to build them successfully. These supports increase
build time, consume extra powder material and require addi-
tional post-processing for their removal. Therefore, in the con-
text of AM hot stamping die cooling systems, a design for AM
(DfAM) approach is critical for the production, performance
and cost effectiveness of additive manufacture.
Lim et al. [68] has provided calculations for the average
temperature of the tool surface by using an analytical model
based on the energy balance principle and a specific cycle
time. Based on the average temperature, the diameter of the
cooling channels was determined and a recommended range
of cooling diameters for different blank thicknesses was pro-
vided as shown in Table 1.
It is observed that for blank thicknesses up to 2 mm, which
is the typical blank thickness used in hot stamping, a cooling
channel of 8–10 mm diameter is recommended. Mazur et al.
[69]investigated the AM manufacturability of conformal
cooling channels and concluded that a circular profile section
channel of 2–8 mm would be compromised in cylindrical
accuracy by 6% due to build errors where arc sections ap-
proach the horizontal. To overcome this issue, several studies
proposed a design for cooling channels with easy to build
supportive internal lattice structures [24,70,71]. The concept
was evaluated and the results showed that indeed the proposed
design reduces cooling time. The reason is that the lattice
structures were found to increase heat transfer due to increased
interfacial surface areas and fluid vorticity. Although the re-
sults are positive, the proposed approach has been experimen-
tally validated only in the case of a straight cooling channel
[71](Fig.10). In the case of conformal cooling channels
where the centre line of a cooling channel is complex, the
internal lattices may severely affect the coolant flow and con-
sequently decrease the quenching effect.
Another approach to overcome the issue of circular accu-
racy of the cooling channels is to use self-supported cross
sections such as diamond shaped or teardrop shaped.
However, the designer should always consider the orientation
of the part inside the chamber as this is the main parameter that
characterises whether a structure is self-supported or not. For
instance, a teardrop shaped-cooling channel is self-supported
if the circular section is oriented towards the base plate (Fig.
11) but is not when rotated 180 degrees.
Orientation can impact other aspects such as surface quality
of the part or build time and any effort to develop a holistic
optimisation strategy will give contradictory results. For in-
stance, high volume of support structures will be needed if a
die is horizontally placed. However, in this case the build time
will be reduced since the number of layers will be fewer com-
pared with when the part is placed vertically. Usually, pro-
posed methods for orientation optimisation are focussing on
Fig. 8 aConventional drilled cooling channels (max temperature in the tool 191 °C), boptimized cooling channels (max temperature 81 °C), c
comparison of the temperature gradient in the conventional and optimized cooling system [37]
Fig. 7 aComplex geometry with conformal cooling channels. bTemperature distribution with conformal cooling channels. cTemperature distribution
with conventional channels [66]
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a specific objective. There are part orientation optimisation
methods forminimising the surface roughness of the final part
[72,73], increasing mechanical properties such as strength to
a specific direction [74,75], and reducing support structures
or even minimizing build time [73,76–78]. In the case of hot
stamping dies, the key objective should be the manufacturabil-
ity of cooling channels without support structures. Other as-
pects such as surface roughness are not crucial as an additively
manufactured die will be post-processed. Post-processing is
necessary for AM components as the typical surface rough-
ness values for SLM are 10–15 μm, whereas for EBM are 20–
35 μm[79], which are in excess of the 8 μm required by
industry for hot stamping dies. However, a potential post-
processing of the cooling channels might not be possible due
to difficulty in accessing the required locations. Moreover, an
additional challenge, which derives from the limited build
volumes of current additive manufacturing machines, is that
a hot stamping die for industrial applications cannot beprinted
in one part. Therefore, the designer must develop a segmen-
tation strategy in order to utilise as much of the available
printing volume as possible.
3.2 Simulation approaches for the design of hot
stamping die cooling systems
A short quenching time is dependent, alongside other param-
eters, on the design of the cooling system of the die. There are
three main design parameters that need to be identified for the
design of a die’s cooling system; the distance of the cooling
channel from the working surface of the die (dsp), the distance
between two neighbouring cooling channels (dpp) and the
diameter of the cooling channel (D) [80,81].
Both analytical and computational simulation models have
been used in the literature for the design ofa hot stamping die.
The majority of analytical models use the energy balance
principle as well as heat transfer theory and can provide a
rapid calculation although of reduced accuracy for d
sp
,d
pp
and D. For increased accuracy, many researchers use finite
element analysis (FEA) methods to optimally selectthe design
variables for a hot stamping die cooling system. A summary
of key modelling methodologies and respective studies are
summarised in Table 2.
The main issue of the aforementioned models is that they
have been developed for hot stamping dies manufactured with
conventional processes and materials. Computational and an-
alytical models need to be revised to take into account some
inherent characteristics of AM processes.
AM-produced components encounter a complex cyclic
thermal history consisting of directional heat extraction, re-
peated melting and rapid solidification, which creates aniso-
tropic and heterogeneous microstructures that differ from me-
tallic parts manufactured via conventional methods.
Moreover, AM defects such as pores and lack of fusion layers
Fig. 9 Temperature distribution
in the final part for aconventional
cooling channels (temperature in
the deepest cavity 335 °C) and b
AMed cooling channels
(temperature in the deepest cavity
177 °C) [37]
Fig. 10 Proposed concept of additively manufactured cooling channels
[71]
Table 1 Cooling channel diameter guideline based on blank thickness
Blank thickness, t, (mm) Cooling channel diameter, d, (mm)
t≤28≤d≤10
2≤t≤410≤d≤12
4≤t≤612≤d≤14
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would induce anisotropic and heterogeneous properties such
as thermal conductivity to the final part. Furthermore, the
cooling channels in an additively manufactured die cannot
be circular due to the overhang issue discussed previously.
Thus, analytical models should be revisited and adjusted
accordingly.
Finally, with the design flexibility that AM processes offer,
hot stamping dies with improved thermal performance can be
produced by integrating lattice structures to the main body of
the die. Lattice structures can be used to insulate die sections
in proximity to the cooling channels of the die, reducing ther-
mal mass and thus improving quenching efficiency. Several
researchers have investigated the effect of lattice structures on
the thermal conductivity of hot stamping dies. Cheng et al.
[88]developed an analytical model which predicts the ther-
mal conductivity of a BCC lattice core sandwich structure.
The model was validated experimentally although applying
analytical methods to predict heat dissipation within a die with
lattice structures is impractical and becomes unfeasible as the
complexity of lattice topology increases. Therefore, the FEA
Table 2 Key modelling studies
on hot stamping cooling systems Authors Modelling methodology Key remarks
[82] Analytical model considering:
i. Thermal conductivity of die
material
ii. Velocity and temperature of
cooling medium
The study concluded that shorter cooling times can be attained
by placing the cooling channels closer to each other and
closer to the die working surface
[81]
[80]
Analytical model considering
stamping force
Most influential factor on cooling effectiveness is d
sp
, followed
by d
pp
and D
[83] Effect of water velocity in the
channel on the die temperature
As the water velocity in a cooling system increased, the
temperature of the die rapidly decreased because of the
turbulent flow state. However, there is a limit to this effect,
where a velocity in excess of 3.5 m/s causes no significant
effect on the die temperature
[84] Analytical model considering only
heat transfer phenomena
Equations for calculation of Dand d
sp
[85] FEA method The dimensions of the flume affect the water velocity and
consequently the flow uniformity. The relationship between
the flow field and flume width is that the wider the flume,
the better the flow uniformity in the cooling channels. On
the other hand, the flume height and flow field relationship
are not straightforward and follow a quadratic curve with the
maximum flow uniformity being achieved at a specific
flume height
[86] FEA method coupled with
evolutionary algorithm
In some cases, coolingchannels withsmaller diameters can be
more effective than larger ones because they can be
positioned closer to the stamping surface without
compromising the integrity of the tooling
[87] FEA method Use of segmented FEA models which reflect the geometric
features of the component. The proposed method reduces
computational time by 92.6%
Fig. 11 Effect of build direction
on printability of a design feature
95Int J Adv Manuf Technol (2020) 109:87–107
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analysis method is utilised in order to increase modelling ac-
curacy. In the numerical analysis of parts with lattice struc-
tures, two different approaches can be distinguished, namely
the micro-scale and the macro-scale approach. In the first ap-
proach, the exact lattice structure’s geometry is modelled.
However, this approach has an impact on the computational
cost of the analysis, making it challenging even for modern
computers, since the geometry is complex. On the other hand,
as the lattice geometry is periodic, each cell can be modelled
as a volume of a homogenous material resulting in a macro-
scale analysis and therefore reducing computational cost. The
new homogenous material must have mechanical properties
that will give the same response as the lattice cell. These
properties are called effective material properties and can be
either isotropic or orthotropic, depending on the lattice cell
topology and dimensional parameters. Finding these proper-
ties requires the use of a technique known as “homogeniza-
tion”which has been applied to other areas successfully [89].
Conclusively, the existing models, both analytical and com-
putational, can act as a basis for the development of cooling
systems. However, the models must be revised to acknowl-
edge the anisotropic nature of AMed materials and be
enriched by taking into account the new design features, such
as cellular structures which can be manufactured by AM.
3.2.1 Topology optimisation
The unique capabilities of AM technologies allow innovative
design approaches that challenge traditional guidelines of sev-
eral major industries including the hot stamping tooling sector.
Among these approaches, topology optimisation (TO) (Fig.
12) offers the greatest design flexibility, as it allows for mate-
rial distribution in terms of physics requirements, offering the
potential to create novel and complex parts with high perfor-
mance and reducing material cost. TO generates a freeform
geometry which is usually optimised against a specific objec-
tive, for example stiffness. However, the objectives of TO can
be more than non-linear optimisation, which gives the opti-
mum combination of two performance criteria such as thermal
and mechanical. The concept of thermomechanical topology
optimisation has been successfully applied in injection mould-
ing and electronics. In the case of injection moulding, results
have shown a reduction of cooling time by 70% without
compromising the structural integrity of the mould [91]. In
the case of hot stamping, there are limited research studies
that investigate the benefits of thermomechanical topology
optimisation. Another use of TO can be the design of
overhang-free components for AM. A simple way to achieve
the overhang-free design is through post-processing of the
optimised geometry. For instance, Leary et al. [92]added
materials to the topology optimisation resulting in the removal
of overhang-free violations, which is effective even though
the optimisation achieved by the TO process is compromised.
A more structured approach was proposed by [93] in which
the structural boundaries of the topology optimised geometry
are measured and the support-required overhang segments are
penalised.
TO can be extremely useful in hot stamping die design as it
can generate a geometry which quenches the blank optimally
and simultaneously generates overhang free geometries.
Another important benefit is that topology-optimised dies re-
quire less material to be printed than fully solid ones. As a
result, printing time can be reduced. However, to the authors’
knowledge there are no available studies applying TO to hot
stamping die design. It would be useful at this point to high-
light that the thermal criterion for hot stamping dies should
derive from the blank quenching rate rather than the temper-
ature profile of the die. The reason is that the required critical
cooling rate differs depending on material and can range from
30 K/s for boron steel to up to 100 K/s for AA7075-T6.
3.3 Material properties for additively manufactured
hot stamping dies
Bulk materials for hot stamping dies require a unique combi-
nation of properties. They must exhibit high tensile strength,
hardness and toughness values as well as good corrosion re-
sistance. Additionally, the die material must exhibit a low
thermal expansion coefficient and high thermal conductivity.
It is common for the die material to be selected depending on
the specific part to be formed. For instance, a die material of
particularly high thermal conductivity is preferable when hot
stamping relatively large part geometry, of relatively thick
gauge, but of simple complexity and limited forming depth.
On the other hand, a die material of high hardness is preferable
when hot stamping complex part geometry with large forming
depth, which generates high friction against the tools.
Naganathan and Penter [94] outlined the most important bulk
Fig. 12 Topology optimised geometries with various self-supporting angles (a26.6 deg, b45 deg, c63.4 deg) Gaynor and Guest, [90]
96 Int J Adv Manuf Technol (2020) 109:87–107
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material properties for a hot stamping die, which are strength,
hardness, impact toughness and thermal conductivity.
Moreover, hot stamping is a process with cyclic thermal pro-
file and since the aforementioned properties are temperature-
dependent, the die must perform sufficiently in the operating
temperature range. For example, UHSS stamping die temper-
atures may reach 300–400 °C [95], whereas for aluminium
stamping, the die can reach up to 180 °C [96]. Temperature-
dependent values of tensile strength, hardness, impact tough-
ness and thermal conductivity of commonly used die materials
are presented in Table 3. The empty cells reflect the lack of
published data for material properties of hot work tool steels at
elevated temperatures.
In the literature, there are no clear material selection criteria
for hot stamping dies, with most studies focussing on the
properties of the final components manufactured by hot
stamping instead. The study by Ye et al. [109] has addressed
many aspects of material selection, although the study is lim-
ited in that thermal conductivity is not considered an “index”
for hot stamping die material selection.
In Fig. 13, the critical bulk material properties, as havebeen
outlined, are graphically mapped for well-established hot
worked steels. Commercially, these grades of hot work steels
can be found with slight variations in mechanical properties
due to different heat treatment strategies. As can be seen from
Fig. 13, there is no singular material that demonstrates excel-
lent performance for every material property. However, a ma-
terial with minimum 18 J (Charpy V notch test) of impact
Table 3 Tensile strength, hardness and toughness values as a function of temperature of hot work tool steels
Material KPI Steel Grade Operating temperature Reference
25 °C 100 °C 200 °C 300 °C 400 °C 500 °C 600 °C 700 °C
Tensile Strength
(MPa)
H11 1505 - - - 1187 1035 - - [97]
H13 1990 - 1480 - 1360 1050 - 500 [98]
1.2367 1457 - - 1264 1184 985 - - [99]
CR7V-L 1600 - 1400 1350 1200 1150 800 - [100]
HTCS-1301314---1200---[101]
Hardness
(HRC)
H11 56524845423835- [102]
H13 50464543413730- [98]
1.236755- 5452514638- [103]
CR7V-L56-------[100]
HTCS-13050-------[104]
Toughness
(J)
H11 18-------[102]
H13 25283640424341- [97][98]
1.236723-------(P.[97])
CR7V-L56-------[100]
HTCS-13028.5-----47-[105]
Thermal Conductivity
(W/mK)
H11 - 26.8 - 27 27.3 - 29.3 - [106]
H13 22.7 25.5 26.3 26.2 26.7 28.0 29.1 27.3 [107][98]
1.2367 27.0 28.0 30.0 29.0 28.0 27.0 - - [108]
CR7V-L 24.5 26.2 26.9 27.0 27.2 28.2 29.6 31.0 [107]
HTCS-130 50.8 54.6 54.2 51.5 49.2 48.0 47.1 43.5 [107]
Fig. 13 Performance review of established materials for hot stamping (at
25
o
C)
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toughness, 50 HRC hardness, 1300 MPa tensile strength and
22 W/mK could provide adequate performance for hot
stamping operations.
There are two main categories of AM processes: Directed
Energy Deposition (DED) and Powder Bed Fusion (PBF)
[110]. The main difference between these two families of pro-
cesses is that DED typically relies upon the feeding of powder
or wire into the melt path, while in PBF, the powder is already
spread, within a chamber, before an energy source, either laser
or electron beam, selectively fuses regions of powder in order
to form a functional component. Usually, powder or wire
feedstock DED processes exhibit higher deposition rates than
PBF processes since the deposition and fusion are conducted
simultaneously. Moreover, DED processes can be used to
manufacture larger parts than PBF since the fabrication is
not constrained by chamber’s dimensions. However, PBF pro-
cesses can produce complex components with better dimen-
sional accuracy and surface finish [111]. As far as hot
stamping tooling is concerned, the PBF processes are the best
choice, since the existing industrial studies on similar applica-
tions of injection moulds have shown very good performance
[112]. Moreover, according to Roland Berger [113], PBF is
the most mature metal printing technology and still the stron-
gest selling with machine sales rising steadily over the last ten
years. Thus, rest of Section 3.3 reviews solely the material
properties of SLMed materials which are suitable for hot
stamping applications although all the remarks from previous
sections are transferable to any other AM processes.
In Table 4, an overview of SLM powder materials from a
range of suppliers is presented, and it can be seen that for the
same material, large variations exist in their properties. The
data presented were derived from specific build strategies
such as a specific build orientation, layer thickness, laser
speed and laser power, and therefore, the maximum values
for the material properties from each supplier are presented.
The first observation that can be made is that there is a signif-
icant absence of impact toughness and thermal conductivity
data, whereas values for tensile strength and hardness are
widely available. As a result, the evaluation of each material
against the four critical material properties for hot stamping
tooling cannot be conducted. However, there are materials that
can be easily excluded as an alternative choice for hot
stamping tooling applications. For instance, all the aluminium
alloys, although demonstrating acceptable thermal conductiv-
ity values, do not meet either the tensile strength requirement,
which is 1300 MPa, nor the hardness criterion (50 HRC). The
same applies to most stainless steel materials. Therefore, the
only viable alternatives are maraging steels, CL50WS, EOS
Stainless Steel CX and H13. These all meet both the tensile
strength and hardness requirements and are close to the de-
fined 22 W/mK threshold for thermal conductivity. The reason
for not excluding these materials as potential alternatives, de-
spite not meeting the thermal conductivity requirements, is
that there have been limited investigations into whether the
same materials can meet all criteria by using an alternative
build strategy.
Finally, it should be mentioned that there are some studies
that have investigated the performance of an additively
manufactured tool built from powder produced by wrought
material suppliers. Valls et al. [132] used high thermal con-
ductivity steel powder from Rovalma, one of the suppliers of
high thermal conductivity steels (HTCS). Although the mate-
rial properties of this powder were not presented, the authors
managed to produce a final AM tool component with an even
hardness distribution. However, currently, there is a lack of
published data, and future research should focus on investi-
gating HTCS as well. “Conventional”hot stamping tooling
material manufacturers have acknowledged the benefits of
AM in hot stamping and will eventually develop their powder
capability.
3.3.1 Effect of process parameters on selective laser-melted
material properties
Selective Laser Melting (SLM) [133] has over 50 different
process parameters that can impact the quality of the final part,
creating a significant challenge in understanding process
physics and developing an effective build strategy [134]
(Table 5).
Variations in the process parameters can influence not only
the microstructure of the final components but may leadto the
generation of defects [135]. Different values for laser power
and speed parameters have been shown to cause a consider-
able change to the microstructure leading to anisotropic be-
haviour [136][137,138]. Another common defect is porosity,
which can affect almost every aspect of the performance of the
final part [135]. Porosity is generally related to pores inside
the starting powder that are transmitted to the final deposition.
Furthermore, it can be caused by the lack of fusion of the
powder material, due to insufficient energy being delivered
by the laser source to the powder particles. AM processes
are therefore sensitive to process parameter variations.
Several key studies on process development of AMed mate-
rials are presented in Table 6.
However, as the AM process is being assessed for hot
stamping tooling applications, an evaluation of the effect of
AM process parameters on the tensile strength, hardness,
toughness and thermal conductivity is required. Table 7sum-
marises all the key studies on process development of AMed
H13, MS1 and MS300. The reported results in most cases are
in line with the data reported by industrial suppliers (Table 4).
It has been observed that heat treatment after the processing of
components delivers higher tensile strength and hardness but
lower impact toughness values. Moreover, it has been identi-
fied that a minimum laser volumetric energy density of 60
J/mm
3
is suitable for achieving densities higher than 99% on
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both maraging steel and H13. However, there are no available
data regarding thermal conductivity which makestheir assess-
ment against hot stamping applications difficult but promis-
ing. Therefore, research efforts should focus towards this di-
rection. Moreover, another demonstration on the limited adop-
tion of AM in hot stamping is the lack of interfacial heat
transfer coefficient (IHTC) data. IHTC between a hot blank
and cold tools is an important thermophysical parameter in hot
stamping processes that determines the cooling rate and con-
sequently the post-form mechanical strength of formed
components.
4 Conclusion and Outlook
The aim of this paper was to provide a review and conclusions
on the application of Additive Manufacturing to the design
and production of hot stamping dies. AM, a technology that
enables almost infinite design freedom, can revolutionize the
design and manufacturing of hot stamping dies. Design meth-
odologies such as topology optimisation can deliver dies with
minimum use of material without compromising either their
structural integrity or thermal performance. Moreover, the
manufacture of bespoke cellular geometries such as lattice
structures can selectively substitute solid areas within the die’s
body and enhance its thermal performance. In summary, the
key aspects which a designer must consider during the devel-
opment of an AMed hot stamping die are presented in Fig. 14.
However, there are significant gaps in the research whichmust
be addressed to enable the successful adoption of AM in the
hot stamping tooling industry. These efforts should focus on
three main areas.
The first area is the characterisation of AMed materials
which are suitable for hot stamping die manufacture.
Conventionally manufactured and widely established hot tool
steels demonstrate properties of at least 1300 MPa tensile
strength, 50 HRC hardness, 18 J of impact toughness and 22
W/mK of thermal conductivity. Any AMed materials should
demonstrate at least these properties in order to be considered
areliablealternative.
AM refers to a family of processes such as DED and
PBF processes or even binder jetting. The authors recom-
mend that PBF processes and specifically SLM be a suit-
able process for hot stamping die manufacture, because it
Table 4 Overview of commercially available SLM materials
Powder material Supplier Tensile strength Hardness Impact toughness Thermal conductivity Reference
AlSi10Mg EOS 460 MPa 120HBW - 120 W/mK [114]
AlSi10Mg SLM Solutions 428 MPa 122 HV - - [115]
AlSi10Mg Renishaw 366 MPa 113 HV - 190 W/mK [116]
Cobalt fhrome MP1 EOS 1350 MPa 45 HRC - 13 W/mK [117]
H13 SLM Solutions 1888 MPa - - - [115]
Steel CL50WS Concept Laser 1969 MPa 52HRC - 20 W/mK [118]
Maraging steel Renishaw 1952 MPa 561 HV - 14.2WmK [119]
Maraging steel 1.2709 SLM Solutions 1784 MPa 373 HV[10] - - [115]
Maraging steel MS1 EOS 1930 MPa 56HRC 15 J 20 W/mK [120]
Nickel alloy CL100NB Concept Laser 1047 MPa - - 12 W/mK [121]
Nickel alloy HX EOS 820 MPa - - - [122]
Nickel alloy HX SLM Solutions 800 MPa 248 HV - - [115]
Nickel alloy IN625 EOS 827 MPa 30HRC - - [123]
Nickel alloy IN625 SLM Solutions 961 MPa 285 HV - - [115]
Nickel alloy IN625 Renishaw 1055 MPa 332 HV - 10.7 W/mK [124]
Nickel alloy IN718 EOS 1241 MPa 47HRC - - [125]
Nickel alloy IN718 SLM Solutions 1034 MPa 293 HV - - [115]
Nickel alloy IN718 Renishaw 1379 MPa 456 HV 12 W/mK [126]
Stainless steel 316 L SLM Solutions 633 MPa 209 HV - - [115]
Stainless steel 316 L Renishaw 624 MPa 208 HV - 16.2 W/mK [127]
Stainless steel CL20ES Concept Laser 640 MPa 20HRC - 15 W/mK [128]
Stainless steel CX EOS 1760 MPa 51HRC - - [129]
Titanium alloy Ti6Al4V SLM Solutions 1301 MPa 380 HV - - [115]
Titanium alloy Ti6Al4V Renishaw 1085 MPa 372 HV - 8 W/mK [130]
Titanium alloy CLTI41 Concept Laser 1106 MPa - - 7 W/mK [131]
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Table 5 Summary of key process parameters in SLM/SLS [134]
Laser and scanning parameters Powder properties
(material properties)
Powder bed and recoat parameters Build environment parameters
Parameter Type Parameter Type Parameter Type Parameter Type
Average power Controlled Bulk density Predefined Density Predefined Shield gas Predefined
Laser mode Predefined Thermal conductivity Predefined Thermal conductivity Predefined Oxygen level Predefined
Peak power Predefined Heat capacity Predefined Heat capacity Predefined Shield gas molecular weight Predefined
Pulse width Predefined Latent heat of fusion Predefined Absorptivity Predefined Shield gas viscosity Predefined
Frequency Predefined Melting temperature Predefined Emissivity Predefined Thermal conductivity Predefined
Wavelength Predefined Melt pool viscosity Predefined Deposition system Predefined Heat capacity of Gas Predefined
Polarization Predefined Thermal expansion coefficient Predefined Re-coater type Predefined Pressure Controlled
Beam quality Predefined Surface free energy Predefined Layer thickness Controlled Gas glow velocity Controlled
Intensity profile Predefined Vapor pressure Predefined Powder bed temperature Controlled Convective heat transfer coefficient Predefined
Scan speed Controlled Enthalpy Predefined Ambient temperature Controlled
Spot size Controlled Absorptivity Predefined Surface-free energy Predefined
Hatching Controlled Diffusivity Predefined
Scanning strategy Controlled Solubility Predefined
Particle morphology Predefined
Surface roughness Predefined
Particle size distribution Predefined
Contamination Predefined
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is a fastest growing AM process in the industry, leading to
significant know-how already in place in the supply
chain, combined with continuously developed powder
materials. Moreover, SLM demonstrates greater dimen-
sional accuracy and surface roughness than DED or other
PBF processes as well as the ability to manufacture com-
plex geometries such as lattice structures and conformal
cooling channels. Both industrial suppliers and research
institutes have conducted material characterisation studies
to a variety of SLMed materials, ranging from AlSi10Mg
to Ti6Al4V. Grades of maraging steel or H13 can be con-
sidered a potential alternative to conventional tooling ma-
terials as they meet most of the defined thresholds, al-
though significant research efforts are required to study
the thermal conductivity of these grades. Moreover, AM
is a delicate process with a high-level of anisotropy, and
the quality of the final component can be affected by over
50 process parameters. In addition, conclusions can be
drawn from the currently available literature. Laser
Volumetric Energy Density (VED) between 60 J/mm
3
and 100 J/mm
3
is required to deliver a fully dense micro-
structure. Since VED is a function of laser power, scan-
ning speed, layer thickness and laser overlap, the afore-
mentioned range of VED can be achieved by a combina-
tion of these parameters. For all the investigated studies,
the hatch angle was set at 67 degrees. Moreover, it is
highlighted that heat treatment after printing significantly
increases the hardness of the final component. The
optimal heat treatment strategy for maraging steel is a
heating cycle of 6 hours at 480
o
C. Additionally, in the
case of H13, it was found that parts printed with a powder
bed temperature of 400
o
C demonstrate a homogeneous
microstructure and improved mechanical properties com-
pared with parts produced with or without lower powder
bed temperature. Thus far, published data has demonstrat-
ed the potential for H13 and maraging steel, although
further investigations are required to understand the effect
of each process parameter on the quality of a functional
AMed die. For instance, there are insufficient available
data on temperature-dependent material properties. This
aspect must be further investigated since the operating
temperature of hot stamping significantly affects the per-
formance of an AM die.
The second area of study is the development of simula-
tion models for the internal cooling system of hot stamping
dies. It is shown that cooling systems with conformal chan-
nels eliminate hot spots on the dies, providing uniform
properties on the final components, as well as increased
quenching efficiency and consequently decreased process
cycle time. Manufacturing of conformal cooling channels
is limited by conventional manufacturing processes due to
cost implications, although AM technologies can produce
these types of channels without any additional cost. The
existing models, which drive the design of the cooling
systems, address areas related to the diameter of a cooling
channel, its distance from the die working surface and the
Table 6 Key studies on process development of SLM
Authors Scope Material Key remarks
([139,
140],
[141]
Laser average power and scanning
speed
Ti6Al4V,
Mg-9%Al, SS
316 L
Process maps between laser speed and power indicates the available process
window for each powder.
[142] Laser scanning hatch angle SS 304 Changing hatch angle and an increasing interval number (N) also aids in limiting
grain growth in a single direction, minimizing anisotropy in mechanical
properties and residual stress, as well as increasing tensile strength
[143],
[144]
[145]
Effect of powder deposition system
on part quality
Regardless Powder deposition system guarantees uniform layer thickness which leads to a
homogeneous material solidification
Increased blade velocities lead to dynamic powder post-flow and decreased mean
layer thicknesses
Increase of recoating time influences thermal gradient along the building axis. A
higher recoating time allows more time for the part to cool down between layers
[146] Layer thickness impact on material
properties
SS 316 L Layer thickness has a significant effect on microhardness. A thicker layer reduces
hardness and tensile strength
[147] Powder bed temperature effect on
material properties and quality
SS 316 L Increase of powder bed temperature increases part density and improves
dimensional accuracy
([148,
149],
[150]
Gas flow, Velocity and pressure Al-12Si,
SS 316 L
For Al-12Si powder, the type of gas used did not significantly affect the part density
or hardness, with a maximum density of 98%
316 L SS samples fabricated under Ar, N2 and their mixtures with H2 exhibit near
full density with values above 99%
Maximum melt pool temperature should not exceed the vaporization temperature of
the powder and vaporization temperature changes with the gas pressure in the
building chamber
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distance between two consecutive cooling channels.
However, most of the proposed models must be adjusted
under the prism of AM. In the case of analytical models,
thermal conductivity and tensile strength of the die mate-
rial should be adjusted to the “AM equivalent”properties
as their values are heavily dependent on the manufacturing
strategy. Moreover, the cross section of AMed cooling
channels in mostly not circular. As a result, the volume
of the cooling medium flowing through the channel can
be affected leading to different cooling results. On the oth-
er hand, the main challenge in computational models, apart
from the development of material cards for AM materials,
is the discretisation of highly complex geometries that can
be generated by applying a Design for AM (DfAM) ap-
proach. Lattice structures are design features that can be
highly utilised in the design of a hot stamping dies, but
their relatively small dimensions compared with the whole
die can increase the computational cost during a FEA.
Segmented models and homogenisation techniques can
help to direct efficient simulation models.
Finally, the third area of proposed work refers to the
development of Design for Additive Manufacturing
(DfAM) rules for hot stamping tooling applications.
One of the most critical issues in this area is the design
Table 7 Key studies on process development of SLMed materials suitable for hot stamping applications
Authors Scope Material Key Remarks
[151] Microstructure and mechanical properties
characterisation
Maraging
steel
18Ni-300
Maximum hardness 58HRC
Maximum UTS 2216 MPa
Charpy impact 5 J with heat treatment
[152] Microstructure and mechanical properties
characterisation
H13 Ferritic/martensitic steels can be robustly processed by SLM by inducing
retained austenite within the microstructures improving the mechanical
properties of such steels without the necessity of employing a subsequent
heat treatment
[153] Experimental investigation on process
windows of maraging steel 18Ni300
Maraging
steel
18Ni300
Laser power higher than 90 W and laser scanning speed lower than 220 mm/s
can deliver parts with densities greater than 99%. Reported ultimate tensile
strength (UTS) and hardness values are 1085–1192 MPa and 30–35 HRC,
respectively. In this study, the specimens were not heat treated
[154] Effect of preheating on the SLM of H13 H13 Preheating temperatures of 400
o
C during SLM deliver parts with better
mechanical properties than parts produced without or with lower preheating
(200
o
C). SLMed H13 UTS (1965 MPa) is similar to conventionally
produced H13 and the hardness is significantly higher (667HV
0.5
)
[155] Hardness investigation H13 Tempered directly after SLM, H13 specimens exhibit similar hardness level
with conventional heat treated bulk H13 material. However, the peak in
secondary hardness of SLM H13 specimens is shifted to higher
temperatures and this phenomenon should be further investigated
[156] Applicability investigation of MS18Ni300 Maraging
steel
18Ni300
Heat treatment is necessary for SLM of MS18Ni300 in order to achieve
optimum harndess (650 HV) and UTS (2100 MPa)
[157] Experimental investigation on process
windows of MS300 including anisotropy
effect
Maraging
steel 300
Heat treatments were crucial to improve the properties of SLM-produced MS
and eliminating mechanical anisotropy. Density wise, the optimum laser
energy density is found at 67 J/mm
3
. The highest UTS, HRC and charpy
impact energy values were 2014 MPa, 54.6 HRC and 15.5 J, respectively.
[158] Experimental investigation on process
windows of H13
H13 Fully dense parts can be achieved with and without preheating with a
minimum energy density of 60 J/mm
3
. Achieved hardness by SLM is
comparable to a conventionally produced H13 tool steel
[159] Fatigue characterisation of SLMed MS1 Maraging
steel MS1
Fatigue life of SLMed MS1 isidentified at 581 MPa, corresponding to 28% of
the UTS
(Wang
et al.,
2019)
High temperature properties of SLMed H13 H13 Effects of tempering temperature and holding time of the softening resistance
treatment were systematicaly studied. SLMed H13 exhibits higher hardness
than wrought counterparts (650 HV compared to 600 HV). Moreover,
SLMed specimens showed a hardness of 475 HV when tempering for 12
hours at 600 C, which was closed to the wrought parts after 4 hours
treatment (496 HV)
[160] Process parameters effect of MS300 on
residual stresses and porosity
MS300 Strong relationship between process parameters and porosity, residual stresses
and associated distortions. Increasing layer thickness results in a decline in
residual stresses and distortion (less than 0.6 mm) but can increase porosity
by up to 5%
102 Int J Adv Manuf Technol (2020) 109:87–107
Content courtesy of Springer Nature, terms of use apply. Rights reserved.

of self-supported design features to eliminate support
structures, especially in internal cavities of the die such
as cooling channels. DfAM rules can be combined with
topology optimisation algorithms in order to generate
geometries both without supports and be thermo-
mechanically optimised.
Open Access This article is licensed under a Creative Commons
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