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Nanoindentation Shows Uniform Local Mechanical Properties Across Melt Pools And Layers Produced By Selective Laser Melting Of AlSi 10Mg Alloy:

January 2016
Advanced Materials Letters 2016(71):13-16

DOI:10.5185/amlett.2016.6171

Authors:

Nesma T. Aboulkhair

University of Nottingham

Ian Maskery

University of Nottingham

Christopher John Tuck

University of Nottingham

Show all 5 authorsHide

Single track and single layer AlSi 10Mg has been produced by selective laser melting (SLM) of alloy powder on an AlSi12 cast substrate. The SLM technique produced a cellular-dendritic ultra-fined grained microstructure. Chemical composition mapping and nanoindentation showed higher hardness in the SLM material compared to its cast counterpart. Importantly, although there was some increase of grain size at the edge of melt pools, nanoindentation showed that the hardness (i.e. yield strength) of the material was uniform across overlapping tracks. This is attributed to the very fine grain size and homogeneous distribution of Si throughout the SLM material.

The principle of SLM.

…

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Research Article Adv. Mater. Lett. 2016, 7(1), 13-16 Advanced Materials Letters

www.amlett.com, www.vbripress.com/aml, DOI: 10.5185/amlett.2016.6171 Published online by the VBRI Press in 2016

Nanoindentation shows uniform local

mechanical properties across melt pools and

layers produced by selective laser melting of

AlSi 10Mg alloy

Nicola M. Everitt*, Nesma T. Aboulkhair, Ian Maskery, Chris J. Tuck, Ian Ashcroft

Department of Mechanical, Materials and Manufacturing Engineering, University of Nottingham,

University Park, Nottingham NG2 7RD, United Kingdom

*Corresponding author. Tel (+44) 115-8466496; Email: nicola.everitt@nottingham.ac.uk

Received: 14 September 2015, Revised: 21 November 2015 and Accepted: 05 December 2015

ABSTRACT

Single track and single layer AlSi 10Mg has been produced by selective laser melting (SLM) of alloy powder on an AlSi12 cast

substrate. The SLM technique produced a cellular-dendritic ultra-fined grained microstructure. Chemical composition mapping

and nanoindentation showed higher hardness in the SLM material compared to its cast counterpart. Importantly, although there

was some increase of grain size at the edge of melt pools, nanoindentation showed that the hardness (i.e. yield strength) of the

material was uniform across overlapping tracks. This is attributed to the very fine grain size and homogeneous distribution of Si

Keywords: Nanoindentation; additive manufacturing; aluminium alloy.

Introduction

Additive manufacturing techniques have recently been

gaining the attention of various industrial sectors to process

a wide range of materials. Fabricating metal parts using an

additive manufacturing approach can be done via a number

of techniques such as direct metal laser sintering (DMLS),

electron beam melting (EBM), and selective laser melting

(SLM). These processes are appealing for the flexibility in

manufacturing that they offer [1] besides enabling weight

reduction through various routes, of which topology

optimization and the use of lattice structures are examples

[2]. During SLM, a part is built in a layer-by-layer fashion

through metallurgical bonding between single scan tracks to

form a layer and layers to form the 3D bulk sample. A

schematic presentation of the process is shown in Fig. 1.

First a layer of powder with a pre-defined thickness is

deposited on a heated build platform that is then scanned

with a laser beam moving in the XY plane. After that, the

piston controlled platform is lowered and the first step is

repeated. These steps are successively repeated until the

full part is built.

There are several studies in the literature investigating

the process ability of various alloys using SLM such as Ti

alloys, stainless steels [3], Ni alloys [4] and Al alloys [5].

Al alloys are extensively used in the automotive and

aerospace industries and being able to successfully process

them using SLM is expected to further increase their

feasibility in the industry. The literature has demonstrated

the possibility of fabricating near fully dense parts from Al

alloys using this technology [6, 7] in addition to reporting

outstanding mechanical performance when compared to the

conventionally processed counterparts [8]. However, the

studies available in the literature are mostly concerned with

micro and macroscopic mechanical properties with little

attention so far to the local mechanical properties, i.e. at the

sub-micron level. From the authors’ perspective, the local

mechanical properties could be rich with information for

selectively laser melted materials, specifically Al alloys as

their processing develops a characteristically fine

microstructure [9].

Fig. 1. The principle of SLM.

This work differs from other in the literature in that it

used nanoindentation to explore the very local mechanical

properties of the SLM material, and can link this nano-scale

investigation to the detailed microstructure and chemical

composition. Knowledge of the homogeneity (or not) of

the mechanical properties of SLM material is necessary if

the full potential of the technique is to be exploited in

design of weight saving structures. The paper first

Everitt et al.

establishes the hardness map across the melt pool produced

below a single track, and then explores the differences

produced by multiple tracks side by side in a single layer.

Experimental

Material synthesis

The AlSi 10Mg powder was supplied by LPW technology,

UK. A complete characterisation of the properties of the

powder used in this study can be found in an earlier study

published by the authors [10]. AlSi 10Mg is a speciality

cast alloy widely used in the automotive industry. A cast

slab of AlSi12 was used as a substrate onto which the scan

tracks and layers were processed.

Fig. 2. Schematic of nanoindentation force/displacement data.

The single tracks and layers were produced using a

Realizer SLM-50, Germany. The thickness of the powder

layer laid down before a scan was made was 0.04 mm. The

laser power was 100 W and production was carried out

under an Ar atmosphere with oxygen content within the

processing chamber below 0.1%. The scan speed was

250 mm s-1 and the spacing between the tracks to produce

the layers was 0.05 mm. The hatch spacing 0.05 mm was

used as it was shown to produce sufficient overlap between

the scan tracks to yield a consolidated layer [6]. The single

tracks and layers were cross-sectioned and polished in

preparation for the nanoindentation experiments.

Characterisation

Nanoindentation was carried out on a MicroMaterials

Nanotest platform 3 (Micromaterials Ltd., Wrexham UK).

A diamond Berkovoich indenter (3 sided pyramid) was

used and the system calibrated using a fused silica

reference sample. The test procedure carried out in

accordance with ASTM E2546. In short, the load was

increased at 0.375 mNs-1 up to a maximum load of 7.5mN

and then unloaded at the same rate. A schematic is shown is

Fig. 2.

In order to map the local properties across the melt

pool(s) of the tracks or in the layers, indentations were

spaced every 10 µm within a row and 15 µm between rows.

Fig. 3(a) shows an example of an array spanning across an

individual melt pool and the cast substrate. In the work

reported here, 308 indentations were used for the single

track characterization, whilst 182 indentations were used in

the layer array. A Hitachi TM2020 scanning electron

microscope SEM equipped with an energy dispersive

X-rays EDX detector served to determine the distribution

of the chemical elements within the indented area.

1 1 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2

1 4

1 1 1 1 2 2 2 2 2 2 2 2 3 3 2 2 2 2 2 2

1 1

1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2

2 1

1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 2 2 2

1 2

1 1 1 1 1 2 5 2 2 2 2 2 2 2 2 2 2 2 2 2

1 1

7 1 1 1 2 2 2 2 2 2 2 2 2 3 2 3 3 2 2 2

1 1

1 2 1 1 1 2 3 2 2 2 2 2 3 3 2 2 3 2 2 1

1 1

1 1 1 1 2 3 3 2 2 3 2 2 2 3 2 2 2 2 2 1

1 1

1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 3 1

1 1

3 4 1 1 4 1 1 1 7 2 2 2 2 2 2 2 2 1 1 1

1 1

1 3 1 1 3 1 1 1 1 1 2 2 2 2 2 2 2 2 1 1

2 1

1 1 1 1 1 1 1 1 1 1 3 1 6 2 2 2 1 2 6 1

1 1

4 1 1 5 1 1 1 2 1 1 1 1 1 2 3 3 1 1 1 1

5 8

1 1 5 1 1 1 1 1 1 1 2 2 1 1 2 1 1 1 1 1

3 1

0.7

8.3

Nano-hardness (GPa)

(a)

(b)

Fig. 3. (a) Array of indentations across the melt pool produced in a single

track (b) nanohardness map of that melt pool.

Results and discussion

Single track

The nanohardness of the melt pool of a single track was

found to be 2.21 GPa ± 0.01 (standard error). As

illustrated by the hardness map in Fig. 3(b) (and the small

standard error of the hardness value), the hardness of the

material in the melt pool is uniform with little spatial

variation. This contrasts with the hardness of the as-cast

structure of the AlSi12 alloy substrate with nanohardnesses

varying from less than 1 GPa to more than 8 GPa.

Specifically we have been able to link the high hardness to

the Si flakes (shown in red in the chemical composition

map in Fig. 4) with a nanohardness of 9 ± 1 GPa, whilst the

surrounding matrix in the as cast material microstructure

has a nanohardness of 0.97 ± 1 GPa.

Research Article Adv. Mater. Lett. 2016, 7(1), 13-16 Advanced Materials Letters

(a)

(b)

Fig. 4. (a) Chemical map of the melt pool beneath a single track and the

surrounding substrate material. (Red = Si).

(b) microstructure of the melt pool material.

The melt pool material shows a fine cellular-dendritic

structure of α-Al surrounded by inter-dendritic Si

(Fig. 4(b)). This characteristic microstructure has been

previously reported for selectively laser melted AlSi 10Mg

[11]. The high nanohardness and spatial uniformity of the

hardness can be explained by the very fine grain size and

uniform Si distribution which are all a function of the fast

cooling rate inherent in the manufacturing process.

Fig. 5. Single layer of SLM AlSi10Mg - Nanohardness map and the

indentation array from which it is produced. (Red dotted lines show the

location of the edge of the tracks).

Layer

Having examined the microstructure of a single track, and

also verified the power of nanoindentation to show up any

spatial variation in nanohardness, we can now investigate

any effect on the microstructure of each single track of the

subsequent heating/cooling (remelting) produced by the

next track. Fig. 5 shows the hardness map across a layer

composed of multiple tracks.

(a)

(b)

Fig. 6. (a) Etched microstructure of the SLM layer (b) chemical map of a

representative area.

The nanohardness of the single layer is remarkably

uniform, with no signs of spatial variation at the edge of the

tracks (melt pool boundaries). The average nanohardness

of the single layer is 2.25 GPa (±0.02 standard error). The

uniformity is perhaps surprising given the coarser

microstructure which can be seen at the melt pool

boundaries where the most secondary heating has taken

place (Fig. 6(a)). However the grain structure is still fine,

and, more importantly, the Si is still homogenously

distributed and has not redistributed to form the flakes seen

in the as cast structure. For the investigated combination of

processing parameters, the average nano-hardness within

the single track was comparable to the single layer,

supporting the observation that re-melting had no

significant effect on altering the local mechanical

properties.

Conclusion

Selective laser melting of AlSi10Mg produced a cellular-

dendritic ultra-fine-grained microstructure due to rapid

solidification. Nanoindentation was able to distinguish the

Everitt et al.

variation in microstructure of the as-cast substrate material.

A higher mean nanohardness was recorded for the SLM

melt pool and also across the surface of a whole layer

which both showed uniform local mechanical properties.

The uniform nano-hardness profile is attributed to the

extremely fine microstructure along with the good

dispersion of the alloying elements.

Acknowledgements

Funding is gratefully acknowledged from BBSRC (Everitt), EPSRC

(Tuck and Ashcroft), and a Dean of Engineering Scholarship, UoN

(Aboulkhair).

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3. Kamath, C.; El-dasher, B.; Gallegos, G.; King, W.; Sisto, A. The

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Mar 2013
ACTA MATER

This study shows that AlSi10Mg parts with an extremely fine microstructure and a controllable texture can be obtained through selective laser melting (SLM). Selective laser melting creates complex functional products by selectively melting powder particles of a powder bed layer after layer using a high-energy laser beam. The high-energy density applied to the material and the additive character of the process result in a unique material structure. To investigate this material structure, cube-shaped SLM parts were made using different scanning strategies and investigated by microscopy, X-ray diffraction and electron backscattered diffraction. The experimental results show that the high thermal gradients occurring during SLM lead to a very fine microstructure with submicron-sized cells. Consequently, the AlSi10Mg SLM products have a high hardness of 127 ± 3 Hv0.5 even without the application of a precipitation hardening treatment. Furthermore, due to the unique solidification conditions and the additive character of the process, a morphological and crystallographic texture is present in the SLM parts. Thanks to the knowledge gathered in this paper on how this texture is formed and how it depends on the process parameters, this texture can be controlled. A strong fibrous 〈1 0 0〉 texture can be altered into a weak cube texture along the building and scanning directions when a rotation of 90° of the scanning vectors within or between the layers is applied.

Flexible manufacturing of metallic products by selective laser melting of powder

Article

Sep 2006
INT J MACH TOOL MANU

In order to produce metallic parts directly from powder material using CAD data, the selective laser melting (SLM) process has been developed. From a series of material tests, nickel-based alloy, Fe alloy and pure titanium powders are found to be feasible for fabrication of metallic models by SLM. Finite element simulation shows stress distribution within the solid single layer formed on the powder bed during forming and some methods for avoiding defects in the products are suggested. The die for metal forming from the nickel-based alloy and the pure titanium models of bone and dental crown are demonstrated. The density of the model made by SLM is higher than 90% of the solid model. The mechanical properties of the formed model can be improved to those of the solid by post-processing.

Additive Manufacturing

Jan 2014

N T Aboulkhair
N M Everitt
I Ashcroft
C J Tuck

Aboulkhair, N. T.; Everitt, N. M.; Ashcroft, I.; Tuck, C. J. Additive Manufacturing 2014, 1. DOI: 10.1016/j.addma.2014.08.001

Jan 2013
1809-1828

L Thijs
K Kempen
J.-P Kruth
J Van Humbeeck

Thijs, L.; Kempen, K.; Kruth, J.-P.; Van Humbeeck, J. Acta Materialia 2013, 61, 1809-19.

Nanoindentation Shows Uniform Local Mechanical Properties Across Melt Pools And Layers Produced By Selective Laser Melting Of AlSi 10Mg Alloy:

Abstract and Figures

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