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Due to their large surface-to-volume ratio, nanoparticles exhibit a reduced melting and sintering temperature in comparison to the corresponding bulk material. After melting and sintering of the particles, the material behaves like the bulk material. Therefore, high strength and temperature-resistant joints can be produced at low tempera-tures, which is of great interest for various joining tasks. The paper deals with the joining of two different steels (unalloyed quality steel and stainless steel) using a Ni nanopaste as an alternative for brazing processes. It is shown that high tensile shear strengths can already be achieved at temperatures between 650 °C and 850 °C. In comparison to conventional Ni-based brazing filler metals, the joining temperatures are significantly lower. In addi-tion to the mechanical properties, the resulting microstructure of the joints is investigated and discussed. It is shown that pronounced diffusion zones can be observed between the joining seam and the substrate, which were not expected for the low temperatures.
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Hausner, S.; Weis, S.; Wagner, G.: Joining of steels at low temperatures by Ni nanoparticles.
DVS-Berichte Band 325, 2016, pp. 278-284, ISBN 978-3-945023-64-8
Joining of steels at low temperatures by Ni nanoparticles
Susann Hausner, Sebastian Weis, Guntram Wagner, Chemnitz / D
Due to their large surface-to-volume ratio, nanoparticles exhibit a reduced melting and sintering temperature in
comparison to the corresponding bulk material. After melting and sintering of the particles, the material behaves
like the bulk material. Therefore, high strength and temperature-resistant joints can be produced at low tempera-
tures, which is of great interest for various joining tasks. The paper deals with the joining of two different steels
(unalloyed quality steel and stainless steel) using a Ni nanopaste as an alternative for brazing processes. It is
shown that high tensile shear strengths can already be achieved at temperatures between 650 °C and 850 °C. In
comparison to conventional Ni-based brazing filler metals, the joining temperatures are significantly lower. In addi-
tion to the mechanical properties, the resulting microstructure of the joints is investigated and discussed. It is
shown that pronounced diffusion zones can be observed between the joining seam and the substrate, which were
not expected for the low temperatures.
1 Introduction
Due to the large specific surface area (high surface-
to-volume ratio), nanoparticles exhibit a decreased
sintering and melting temperature with decreasing
particle size, which is known as the Gibbs-Thomson
effect [17] This effect is of great interest for joining
technologies, because after melting and sintering of
the particles, the material behaves like the bulk mate-
rial [78]. Thus, high-strength and temperature-
resistant joints can be produced at low temperatures.
This results in a high potential for various joining pro-
cesses. Previous publications are mostly concerned
with joining of components in power electronics. Es-
pecially the joining of copper with Ag nanoparticles as
a substitute for soldered joints was investigated [9
15]. Nevertheless, there is a much greater potential
for joining with nanoparticles. A possible application is
the joining of high-performance materials respectively
materials with an adapted and optimized microstruc-
ture, where high joining temperatures can result in
undesirable structural damages and therefore to a
loss of the previously optimized properties. For exam-
ple, grain growth of fine-grained steels or recrystalliza-
tion of monocrystalline Ni-based superalloys lead to
strong losses of strength and toughness [1617]. But
also brazing of copper and steels can result in
strength losses by grain growth [1819]. In this con-
text, a reduction of the joining temperature is of great
interest to retain the initial microstructure and there-
fore the mechanical properties. In addition, a low join-
ing temperature would be also of great interest for the
joining of materials with different coefficients of ther-
mal expansion such as carbide-metal joints and ce-
ramic-metal joints, to reduce the often critical thermal-
ly induced residual stresses of the joints [2021]. The
list of possible applications could be extended further.
This demonstrates the great potential of joining with
nanoparticles for highly stressed structural compo-
nents as an alternative for brazing processes.
2 Experimental
2.1 Preparation of the Ni nanopaste
For joining with nanoparticles, mostly so called nano-
pastes are used, in which the nanoparticles are sus-
pended in solvents and through the addition of dis-
persing agents surrounded with an organic shell [78].
This shell leads to repulsive forces between the parti-
cles, so that agglomerations can be avoided [2223].
Due to the limited commercial availability of Ni nano-
suspensions, the Ni nanopaste was produced by our-
selves. The composition of the nanopaste - which was
identified as an optimum regarding to the stability of
the suspension in preliminary works - is shown in
Tab. 1.
Tab. 1: Composition of the produced Ni nanopaste
Metal content [wt%]
Ni
49.0
Content of organic
substances [wt%]
α-Terpineol
24.5
p-Xylene
24.5
1-Tetradecanethiol
(CH3(CH2)13SH)
1.0
1-Octadecanethiol
(CH3(CH2)17SH)
1.0
For the preparation of the nanopaste, the solvent mix-
ture of α-Terpineol and p-Xylene was mixed with two
alkylthiols, which have a stabilizing function (avoiding
agglomeration of the nanoparticles). Subsequently,
spherical Ni nanoparticles with a particle size of
10 nm to 100 nm (average particle size: 20 nm,
ChemPur Feinchemikalien und Forschungsbedarf
GmbH) were added to the mixture. The dispersion of
the nanoparticles was performed using pulsed ultra-
sound (USP 200, Hielscher-Ultraschall-Technologie).
Due to the high metal content, a spreadable paste
could be produced, which allows an application similar
to conventional soldering or brazing pastes.
2.2 Joining with Ni nanopaste
When joining with nanoparticle-containing suspen-
sions, the application of pressure during the joining
process is necessary to achieve a dense microstruc-
ture of the joints and good strengths [7]. Therefore, a
modified hydraulic press was used for the joining ex-
periments, Fig. 1. The heating of the samples was
carried out in air by means of induction. The desired
pressure was applied on the entire joining surface of
the sample with a punch, Fig. 1.
Fig. 1: Schematic figure of the joining system
Two different steels were used as substrates: the
unalloyed quality steel DC01 (EN: 1.0330) and the
stainless CrNi steel X5CrNi18-10 (EN: 1.4301). For
the joining experiments, the influence of the process
parameters joining temperature and joining pressure
on the strength behavior and the resulting microstruc-
ture of the joints was investigated, Tab. 2. Further-
more, different surface treatments respectively the
application of coatings on the stainless steel were
examined to achieve an improved adhesion, Tab. 3.
With the electrolytic pickling (and the subsequent
application of the Ni nanopaste under exclusion of
air), it was intended to investigate whether the
strength can be increased, if an almost oxide-free
substrate surface is present. The electroplated Ni
coating was deposited to examine, whether an im-
proved adhesion and thus higher strengths can be
achieved, if both the substrate surface and the joining
material consist of nickel.
The sample geometry (joining area: 9.0 mm x 7.5 mm)
for tensile shear tests is shown in Fig. 2. The exami-
nation of all tensile specimens at least 5 individual
samples per varied parameter was performed with a
universal tensile testing machine Zwick/Roell Z020 at
room temperature and a strain rate of 10-3∙s-1. Also the
microstructure was analyzed with the sample geome-
try shown in Fig. 2. SEM and EDX analyses were
performed (LEO 1455VP).
Tab. 2: Process parameters and variations for the
joining of the steels DC01 and X5CrNi18-10 with the
Ni nanopaste
Tab. 3: Varied surface treatments of the stainless
steel X5CrNi18-10
Fig. 2: Sample geometry for both steels, overlap:
7.5 mm, sheet thickness: 3 mm
3 Results
3.1 Joining of the steel DC01
Fig. 3 shows the achievable tensile shear strengths of
joints with the unalloyed quality steel DC01 (EN:
1.0330) as a function of joining temperature and join-
ing pressure. The temperature exerts a much greater
influence on the strength behavior in comparison to
the pressure. A significant increase in strength can be
observed with rising joining temperature (from 27 MPa
at a temperature of 650 °C up to 122 MPa at a tem-
perature of 850 °C).
The increase in strength is a result of an increasing
sinter activity caused by the rising temperature, which
can be clearly seen in the images of the microstruc-
ture of the joints, Fig. 4 and Fig. 5. At a temperature
of 650 °C, the inner seam exhibits a porous structure,
1 in Fig. 4. This indicates, that no sufficient sintering
process occurs in the middle of the seam. However,
at a temperature of 850 °C a clear progress in sinter-
ing can be recognized in the inner seam, 1 in Fig. 5.
Therefore, the higher temperature results in an in-
creased diffusibility and thus increased sinterability. At
both temperatures, the edges of the seam exhibit a
much denser structure in comparison to the middle of
the seam, 2 in Fig. 4 and Fig. 5. Probably, an en-
hanced diffusion of Fe from the substrate into the
peripheral areas of the joining seam (Fe content at
850 °C: 46 wt%, 2 in Fig. 5) leads to the significant
densification. Furthermore, at both temperatures, a
dark layer can be seen at the interface between the
joining seam and the substrate, 3 in Fig. 4 and Fig. 5.
According to EDX analyses, the layer most likely is an
iron oxide layer, 3 in Fig. 5. At a temperature of
650 °C, the oxide layer is very thin, Fig. 4. However,
at a temperature of 850 °C the layer is considerably
thicker (Fig. 5), which can be attributed to the strong-
er growth of the oxide layer at higher temperatures
(from temperatures above 700 °C [24]). Despite the
presence of the oxide layers, at both temperatures,
clear diffusion zones can be observed in the sub-
strate, 4 in Fig. 4 and Fig. 5. At a temperature of
650 °C, diffusion depths for Ni into Fe of about 4 µm
and for Fe into Ni > 13 µm can be detected via line
scan, Fig. 6. Accordingly, significant diffusion pro-
cesses between the joining seam and the substrate
take place at a temperature of 650 °C and a holding
time of 10 min. The very thin oxide layer does not or
hardly influences the diffusion between Fe and Ni. At
a joining temperature of 850 °C, comparable diffusion
depths can be determined: Ni diffuses from the joining
seam into the substrate up to 4 µm in depth. Fe can
be detected inside the seam up to a depth of > 17 µm,
Fig. 5 and Fig. 7. However, at a temperature of
850 °C, the diffusion zone exhibits a completely dif-
ferent structure in comparison to 650 °C, see 4 in
Fig. 4 and Fig. 5. While at 650 °C, a homogeneous
zone is formed due to the diffusion of Ni into the sub-
strate (Fig. 4), at 850 °C, the diffusion occurs only
along the grain boundaries (Fig. 5). Therefore, it must
be assumed that the thicker oxide layer at a tempera-
ture of 850 °C (3 in Fig. 5) influences the diffusion in
such a way that the diffusion occurs only along the
grain boundaries and not into the volume.
Fig. 3: Tensile shear strengths of the joints with DC01
as a function of joining temperature and joining pres-
sure (holding time: 10 min)
Fig. 4: Microstructure of a joint with DC01 at a joining
temperature of 650 °C and a joining pressure of
20 MPa (holding time: 10 min)
Fig. 5: Microstructure and EDX analyses of a joint
with DC01 at a joining temperature of 850 °C and a
joining pressure of 20 MPa (holding time: 10 min)
Nevertheless, the results show that at both tempera-
tures significant diffusion processes occur, which
were not expected for the low temperatures and the
holding time of 10 min. For example, in [2527] signif-
icantly lower diffusion coefficients were determined for
the solid phase diffusion of Fe and Ni as bulk materi-
als at similar temperatures. It can be assumed that
nanoeffects are the reasons for the enhanced diffu-
sion: On the one hand, nanoparticles exhibit a signifi-
cantly increased diffusion coefficient [2830], so that
an increased diffusion of Ni into the substrate can be
expected. On the other hand, at the beginning of the
joining process, the seam has a nanoporous structure
(high number of defects) which is compacted during
the joining process (sintering process), but neverthe-
less the resulting sintered structure inevitably exhibits
lattice defects. An increasing number of defects re-
sults in an increasing diffusion coefficient [31] so that
the diffusion of Fe into the seam is facilitated by the
large numbers of defects. It is interesting that in par-
ticular the last-mentioned effect obviously leads to a
significantly increased diffusion: According to the EDX
analyses (Fig. 5), Fe diffused with a much higher
diffusion coefficient into the joining seam than Ni into
the substrate. Also the observed microporosity in the
substrate near the joining seam (Fig. 4 and Fig. 5)
and the displacement of the intersection of the Fe
curve and the Ni curve into the Ni-joining seam in the
line scans (Fig. 6 and Fig. 7), which indicate the
Kirkendall effect, illustrate the significantly higher dif-
fusion of Fe into Ni than vice versa.
Fig. 6: Line scan of a joint with DC01 at a joining tem-
perature of 650 °C and a joining pressure of 20 MPa
(holding time: 10 min)
The joining pressure exerts a significantly lower influ-
ence on the strength behavior in comparison to the
joining temperature, Fig. 3. This is very interesting,
because we also published a paper for the joining of
copper with an Ag nanopaste, in which the pressure
has a much greater influence on the strength behavior
than the temperature [78]. This shows that it is not
possible to transfer the results of one material system
to another, when joining with nanosuspensions. Ac-
cordingly, it is necessary to investigate the influence
of the process parameters for each individual system
of substrate and filler material, which are of interest.
Fig. 7: Line scan of a joint with DC01 at a joining tem-
perature of 850 °C and a joining pressure of 20 MPa
(holding time: 10 min)
3.2 Joining of the stainless steel X5CrNi18-10
Fig. 8 shows the achievable tensile shear strengths of
joints with the austenitic stainless steel (EN: 1.4301)
as a function of joining temperature and joining pres-
sure. When using this steel, both, the joining tempera-
ture as well as the joining pressure exert a decisive
influence on the strength behavior. The highest
strengths (57 MPa) can be achieved at a temperature
of 850 °C and a pressure of 20 MPa, Fig. 8. At lower
temperatures and with a pressureless joining process,
the strengths are significantly lower (750 °C: 10 MPa,
650 °C: 4 MPa, pressureless joining at 850 °C:
16 MPa).
In comparison to the joints with the substrate DC01,
the strengths are significantly decreased, Fig. 3 and
Fig. 8. This may be a result of the passivating oxide
layer of the stainless steel, because the failure mainly
occurs at the interface between the substrate and the
joining seam. The oxide layer of a stainless steel is
formed within milliseconds in atmospheric oxygen, so
that with the used sample preparation in air, no oxide-
free surface can be expected. It must be assumed
that the chemical incompatibility of the non-metallic
oxide layer and the metallic joining seam impedes a
sintering of the particles to the substrate. The diffusion
of Ni into Fe (and vice versa) is presumably limited by
the oxide layer, resulting in lower strength values in
comparison to the DC01.
Fig. 8: Tensile shear strengths of the joints with
X5CrNi18-10 as a function of joining temperature and
joining pressure (holding time: 10 min)
Therefore, the influence of different surface treat-
ments respectively the application of coatings on the
stainless steel (Tab. 3) on the strength behavior was
investigated. Fig. 9 shows, that the strength values
can be increased by surface treatment: While the
strength is 57 MPa when the substrates were ground,
the strengths can be increased to 67 MPa by remov-
ing the oxide layer (electrolytic pickling) and to
82 MPa by the application of an electroplated nickel
layer on the stainless steel substrate. This confirms
the assumption that the passive layer limits the diffu-
sion between the substrate and the joining seam and
thus higher strengths can be achieved with appropri-
ate surface treatments and coatings of the substrate.
Fig. 9: Tensile shear strengths of the joints with
X5CrNi18-10 as a function of the surface treatment
(joining temperature: 850 °C, joining pressure:
20 MPa, holding time: 10 min)
Fig. 10 shows the microstructure of a joint at 850 °C
and 20 MPa (surface treatment: ground). In compari-
son to the microstructure of the DC01 sample, which
was joined with the same process parameters
(Fig. 5), the joining seam is characterized by a much
higher porosity. A multi-zone structure, in particular
with a densification at the edges of the seam cannot
be observed. By EDX analyses, only 5 wt% Fe can be
detected in the joining seam (1 in Fig. 10), while the
DC01 exhibits a significantly higher Fe content (13
46 wt%, Fig. 5). This confirms the assumption in
chapter 3.1 that the strong diffusion of Fe into the
joining seam leads to the densification of the seam
(particularly at the edges of the seam) when using the
DC01 as substrate. In contrast, when using the stain-
less steel, no comparable densification can be
achieved due to the significantly reduced diffusion of
Fe into the seam. This difference in the diffusion coef-
ficients of Fe into Ni between the two substrates may
be due to:
The different structural modifications of the steels
(DC01: ferrite + pearlite, stainless steel: austen-
ite) affect the diffusion behavior. According to
[32], Fe atoms diffuse in austenite (fcc lattice with
high packing density) with a significantly lower
diffusion coefficient than in ferrite (bcc lattice).
This is consistent with the presented results.
However, it should be noted that the joining of
the DC01 occurs in the two-phase region (ferrite
+ austenite). The (small) austenite content is ne-
glected.
It can be assumed that also the formation and
chemical composition of the different oxide layers
influence the diffusivity. While stainless steels
exhibit very dense oxide layers with a low num-
ber of defects, oxide or scale layers of unalloyed
steels like the DC01 are characterized by a high
number of defects [24]. Defects represent diffu-
sion paths, so it can be assumed that the diffusiv-
ity of Fe through the oxide layer is strong pro-
nounced when using the DC01 (many defects).
However, when using the stainless steel, the dif-
fusivity is significantly reduced due to the dense
oxide layer, which acts as a diffusion barrier.
In the stainless steel substrate, a pronounced diffu-
sion zone can be observed, Fig. 10. In contrast to the
diffusion zone of the DC01 (same process parame-
ters, Fig. 5), it is formed as a homogeneous zone.
This suggests that, while the diffusion of Fe into Ni is
limited, the diffusion of Ni into Fe is not or hardly re-
strained: In the diffusion zone of the stainless steel,
65 wt% Ni can be detected by EDX (2 in Fig. 10),
while in the DC01 only 3 wt% Ni were detected (4 in
Fig. 5). Accordingly, when using the stainless steel, Ni
exhibits a significantly higher diffusion coefficient into
Fe than when using the DC01. Once again, this can
be attributed to the different structural modifications of
the steels: Also [25] determined a much higher diffu-
sion coefficient of Ni in austenite than in ferrite.
Fig. 10: Microstructure and EDX analyses of a joint
with X5CrNi18-10 at a joining temperature of 850 °C
and a joining pressure of 20 MPa (holding time:
10 min)
A comparison of the two substrates shows that the
diffusion coefficient of Ni into Fe is significantly higher
than vice versa, when using the stainless steel. In
contrast, when using the DC01, a higher diffusion
coefficient of Fe into Ni than vice versa was detected.
Also a comparison of the line scans of the DC01 and
the stainless steel (Fig. 7 and Fig. 11) shows that the
intersection between the Fe curve and the Ni curve is
shifted towards the joining seam when using the
DC01 (higher diffusion coefficient of Fe into Ni), while
it is shifted towards the substrate when using the
stainless steel (higher diffusion coefficient of Ni into
Fe). Possible reasons for this different behavior have
already been mentioned (structural modifications of
the steels and permeability of the oxide layers for the
diffusion of one element into the other). For a more
detailed explanation of this phenomenon, which is
probably a complex interaction of many factors, fur-
ther studies are required.
4 Conclusion
The results prove that the Ni nanopaste offers a great
potential for the joining of steels at low temperatures.
Although the Ni nanopaste, which was produced by
ourselves, could be optimized (further investigations
regarding the particle size, the particle content and
suitable solvents as well as stabilizing agents would
be of interest), high joint strengths can already be
achieved at temperatures of 650 °C850 °C. The var-
iation of the process parameters joining pressure and
joining temperature shows, that the joining tempera-
ture exerts a significantly stronger influence on the
achievable strengths in comparison to the joining
pressure. A comparison with results for the joining of
copper with an Ag nanopaste [78], where the pres-
sure has a much greater influence on the strength
behavior than the temperature, shows that it is not
possible to transfer the results of one material system
to another when joining with nanosuspensions.
The microstructure investigations show that pro-
nounced diffusion zones can be observed between
the joining seams and the substrates which were not
expected for the low temperatures. It is interesting
that the diffusion behavior differs for both steels:
When using the unalloyed quality steel DC01, in par-
ticular Fe diffuses into the Ni joining seam. In contrast,
when using the stainless steel X5CrNi18-10, Ni exhib-
its a significantly higher diffusion coefficient into the
Fe substrate than vice versa.
The investigations prove the fundamental feasibility of
joining steels with Ni nanoparticles. However, for an
in-depth understanding of the mechanisms, further
investigations are still necessary.
Fig. 11: Line scan of a joint with X5CrNi18-10 at a
joining temperature of 850 °C and a joining pressure
of 20 MPa (holding time: 10 min)
5 Acknowledgement
This work was performed within the Federal Cluster of
Excellence EXC 1075 “MERGE Technologies for Mul-
tifunctional Lightweight Structures” and supported by
the German Research Foundation (DFG). Financial
support is gratefully acknowledged.
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lin, Heidelberg, New York: Springer-Verlag, 2005.
ISBN 978-3-540-21674-2
... Until now, joining with nanoparticles has mostly been investigated as an alternative for electronic packaging, i.e., for components subjected to low mechanical stresses [18][19][20][21][22][23]. However, our own recent studies show that joining with nanoparticles is also suitable for components subjected to high mechanical loads if suitable process parameters are used [24][25][26]. In summary, joining with nanoparticles represents an interesting alternative to perform joining processes at low temperatures, which allows to maintain the specific microstructure and mechanical properties of Q&P steels. ...
... The composition of this paste, which was custom-made, is shown in Table 2. The preparation of the paste is described in Reference [26]. For joining with this nanopaste, a joining temperature of 650 °C, a joining pressure of 20 MPa, and a holding time of 10 min were used. ...
... The composition of this paste, which was custom-made, is shown in Table 2. The preparation of the paste is described in Reference [26]. For joining with this nanopaste, a joining temperature of 650 • C, a joining pressure of 20 MPa, and a holding time of 10 min were used. ...
Article
Full-text available
Quenching and partitioning (Q&P) steels show a good balance between strength and ductility due to a special heat treatment that allows to adjust a microstructure of martensite with a fraction of stabilized retained austenite. The final heat treatment step is performed at low temperatures. Therefore, joining of Q&P steels is a big challenge. On the one hand, a low joining temperature is necessary in order not to influence the adjusted microstructure; on the other hand, high joint strengths are required. In this study, joining of Q&P steels with Ag nanoparticles is investigated. Due to the nano-effect, high-strength and temperature-resistant joints can be produced at low temperatures with nanoparticles, which meets the contradictory requirements for joining of Q&P steels. In addition to the Ag nanoparticles, activating materials (SnAg and Sn) are used at the interface to achieve an improved bonding to the steel substrate. The results show that the activating materials play an important role in the successful formation of joints. Only with the activating materials, can joints be produced. Due to the low joining temperature (max. 237 °C), the microstructure of the Q&P steel is hardly influenced.
... Up to the current state of the art, investigations about nanojoining mainly refer to joining with silver particles in the field of electronic packaging (Ref 4-7), whereby the properties achieved (e.g., reliability) are at least equivalent to those of conventional soldered joints. Recently, nanojoining has also been investigated for higher-stressed components (Ref [8][9][10][11]. For applications in the high temperature range, higher demands are made on the temperature resistance, therefore nickel brazing materials are used ( Ref 12). The challenge of nanojoining is to transfer the promising results of joining with silver nanoparticle to other materials, e.g., nickel, and to characterize them. ...
Article
Thermal joining can lead to high thermal stresses, undesired structural changes, and the associated loss of properties. In the turbine industry, monocrystalline materials are often used to take advantage of their high creep resistance and heat resistance. For process-related reasons, components are mechanically machined, and the contours usually have slightly work-hardened areas due to the mechanical processing. Downstream thermal processes at temperatures above 1100 °C can lead to recrystallization (Rx) at these areas, so that the properties are negatively affected. Usually, the joining temperatures for high-temperature brazing are in the range of 1200 °C, both in new installations and in the case of repairs. It is therefore desirable to reduce the joining temperature without changing the choice of filler material, which can lead to susceptibility to corrosion and oxidation. According to investigations of the last years, nanojoining with nanoparticles offers great potential. The joining temperature can be lowered due to the “surface effect.” A considerable reduction in the size of the particles leads to a significant increase in surface atoms and thus in the specific surface area. The connection of the materials occurs predominantly due to sintering processes. After the joining process, the properties of a bulk material are available again. Mechanical properties comparable to those of brazing have already been achieved with silver nanoparticles (Hausner in WWA 56, 2015). Up to now, publications on the topic of nanojoining have largely referred to silver nanoparticles/silver sintering. Due to the temperature application range, silver filler material cannot be used in gas turbines. Therefore, the first results of nickel nanoparticles for joining of the nickel-based superalloy PWA 1483 using induction heating are described in this paper. During joining, the parameters brazing temperature, holding time and the surface treatment of the base materials were varied. It becomes clear that the microstructure of the joint is dependent on temperature and holding time. Moreover, if the temperature is too low and holding time too short, only insufficiently sintering occurs, which leads to sample failure during the metallographic preparation. On the other hand, samples with a tensile shear strength of up to 165 MPa can be achieved with convenient joining conditions.
... Ni has a melting point over 720 °C higher than the eutectic Cu-Ag temperature and over 350 °C higher than the melting temperature of Cu. Ni NPs have been used by Hausner et al. to join stainless steel using up to 30 MPa of applied pressure and bonding temperature of 850 °C and obtained a joining strength of 120 MPa [23]. Without applied pressure, the joint was noticeably weaker (100 MPa) [4]. ...
Conference Paper
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Ni nanoparticles were successfully used to join Inconel 718 via transient liquid phase (TLP) bonding in a vacuum environment. Ni nanoparticles of 20 nm, 29 nm, and 41 nm in diameter were synthesized by controlling the reducing agent injection rate and joined at up to 1050 °C and heating rate 5-15 °C/min. Based on the Gibbs-Thomson equation and surface melting models, joining using Ni nanoparticles occurs due to competing solid-state sintering and surface melting processes. It was found that faster heating rate; higher maximum bonding temperature, and larger particle size resulted in higher bonding strength. Using a faster heating rate suppresses the amount of solid-state particle-particle sintering that occurs at lower temperatures, where particle-Inconel 718 joining is less active. The suppression of particle-particle sintering as a function of particle diameter is also discussed. The maximum bonding strength achieved is 243 MPa. The fracture surface for Ni nanoparticle-bonded joints demonstrated intergranular fracture (low strength joints) and a combination of cleavage and microvoid coalescence (high strength joints).
... Nanoparticles have been extensively used to augment the thermal or mechanical performance of brazed joints [10,11]. However, pure nanoparticle-based brazing materials were slowly gaining popularity in the last decade [12,13]. A feature that is seldom investigated in the context of soldering or brazing is the diffusion and wetting behaviors as well as the effect of particle shape on the properties of a material. ...
Article
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Ag nanomaterials have been investigated as a filler material for brazing Inconel 718 well below the bulk melting temperature of Ag using both vacuum and laser brazing. However, Ag nanoparticles and Ag nanowires exhibit different bonding strengths. In this study, we focus on different diffusion and wetting behaviors of two kinds of nanopastes. At 550 °C, the areal coverage of the nanopaste decreases as a result of shrinkage in the nanopaste. The shrinkage is followed by rapid wetting and spreading on the Inconel 718 surface. A deeper investigation to the diffusion behavior of Ag nanoparticle and Ag nanowire pastes during laser brazing provides a clear correlation between the diffusion distance and the bonding strength. The diffusion of the base material into the Ag joint is shown to result in a concentration "hump" near the Ag-Inconel 718 interface of different base material elements which is likely caused by a difference in diffusivity of Ag and the Inconel 718 constituent elements. Ag NW joints were found to have a thicker diffusion zone, but also exhibit some Nb/Mo segregation when brazed using 300 W laser power. This may lead in different metallurgic bonding.
... A comparison with results for the joining of copper with an Ag nanopaste [31,32], where the pressure has a much greater influence on the strength behavior than the temperature, shows that it is not possible to transfer the results of one material system to another when joining with nanosuspensions. The investigations shown that high tensile shear strengths can already be achieved at temperatures between 650 °C and 850 °C [50]. In comparison to the joints with the substrate DC01, the strengths joints with the austenitic stainless steel are significantly decreased. ...
Article
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Brazing has a long tradition at the Institute of Material Science and Engineering of the University of Chemnitz, Germany. During the last years, comprehensive and innovative knowledge in brazing and soldering technologies were generated. Originating from high-temperature brazing, topics like metal-ceramic and light metal brazing, ultrasound assisted joining processes through to brazing of metal matrix composites were examined. In addition, new topics like joining by nanoparticles or corrosion behavior of brazed heat exchangers are in the focus of research. Prof. Bernhard Wielage managed the institute for 22 years. Today, Prof. Guntram Wagner introduces new topics like friction stir welding and continues the activities in brazing.
... After joining, nanomaterial properties more closely resemble bulk material properties. The past decade has seen a growing interest in developing nanomaterial pastes (nanopastes, or NPs) for brazing and high temperature applications [6] . ...
Article
Full-text available
Nanobrazing is defined as an innovative brazing technique using nano materials as the filler materials. Incorporating nano brazing into conventional brazing technologies creates a new frontier for repairing turbine blades and vanes.
... Limited studies have been conducted on joining nanomaterials at high temperatures. Hausner et al. (2016) is one of the few nanomaterial brazing studies published at the time of this paper. In this study, Hausner et al. brazed stainless steel using a nickel nanoparticle paste to achieve a maximum bonding strength of 120 MPa using induction brazing. ...
Article
Ag nanopastes composed of Ag nanoparticles or Ag nanowires and Cu-Ag nanopastes with Cu-Ag core-shell nanowires are used as a new brazing material for Inconel® 718. Ag nanoparticles or Ag nanowires are further added to the core-shell paste to adjust to a eutectic composition. Microstructural analysis of the brazed joints was carried out with EDS and XRD. High bonding strength (>100 MPa) was obtained with both Ag and Cu-Ag nanopastes. It was concluded that the Cu-Ag nanopastes form stronger braze joints than the BAg-8 brazing alloy as a result of Hall-Petch strengthening. It has also been concluded that the addition Ag nanoparticles or Ag nanowires to the Cu-Ag core-shell nanowire paste have a significant impact on the bonding strength and fracture of the Cu-Ag joints.
Article
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Nanoparticles exhibit a decrease in sintering and melting temperature with decreasing particle size in comparison to the corresponding bulk material. After melting or sintering of the nanoparticles, the material behaves like the bulk material. Therefore, high-strength and temperature-resistant joints can be produced at low temperatures, which is of big interest for various joining tasks. Joints (substrate: Cu) were prepared with an Ag nanoparticle-containing paste. The influence of the adjustable process parameters joining pressure, joining temperature, holding time, heating rate, thickness of paste application, surface treatment, pre-drying process, and subsequent heat treatment on the strength behavior of the joints was investigated. It is shown that in particular, the joining pressure exerts an essential influence on the achievable strengths. In addition, temperature, holding time, and thickness of paste application have a significant effect on strength behavior. In contrast, the pre-drying process, heating rate, surface pre-treatment, and subsequent heat treatment possess hardly any influence on joint strength.
Thesis
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In der vorliegenden Arbeit werden nanopartikelhaltige Suspensionen auf Ag- und Ni-Basis sowie Ag-Precursoren, die während des Erwärmungsprozesses Nanopartikel bilden, bezüglich ihrer Eignung zum Fügen bei niedrigen Temperaturen untersucht. Dabei wird die, im Vergleich zum entsprechenden Massivmaterial, verringerte Schmelz- und Sintertemperatur von Nanopartikeln ausgenutzt. Da nach dem Schmelz- und Sinterprozess der Partikel die thermischen Eigenschaften des Massivmaterials vorliegen, ergibt sich ein großes Potential für die Herstellung hochfester und temperaturbeständiger Verbindungen bei gleichzeitig niedrigen Fügetemperaturen, was für eine Vielzahl von Fügeaufgaben von großem Interesse ist. In der Arbeit wird zunächst eine kommerzielle Ag-Nanopaste insbesondere bezüglich ihres thermischen Verhaltens charakterisiert. In der Folge werden Fügeverbindungen mit Cu-Substraten hergestellt, die in Abhängigkeit verschiedener Prozessparameter bzgl. der Festigkeiten, der Mikrostruktur sowie der Bruchflächen detailliert charakterisiert werden. Dabei zeigt sich, dass insbesondere der Fügedruck einen signifikanten Einfluss auf die erreichbaren Festigkeiten ausübt. Mit hohen Fügedrücken können bei einer Fügetemperatur von 300 °C höhere Verbindungsfestigkeiten als mit einem konventionellen Hartlot auf AgCu-Basis (Löttemperatur: 780 °C) erreicht werden. Weiterhin werden erste Ergebnisse zum Fügen von Stählen mit einer Ni-Nanopaste vorgestellt, mit der hohe Verbindungsfestigkeiten erzielt werden können. Schließlich wird mit Ag-Precursoren eine weitere Klasse möglicher Fügewerkstoffe vorgestellt, die erst während des Erwärmungs- bzw. Fügeprozesses Nanopartikel bilden, was in einer deutlich vereinfachten Handhabbarkeit resultiert. Die Arbeit liefert zudem Ansätze für weitere Forschungstätigkeiten.
Article
The bondability of copper joints formed using a mixed paste of silver oxide (Ag2O) and copper oxide (CuO) that contained reducing solvents was evaluated in order to achieve bonds that exhibited high migration tolerance and could serve as Pb-free alternatives to the conventional bonds formed using high-melting point solders in electronics packaging. The Ag2O particles reduced into silver nanoparticles at 150°C, whereas the CuO reduced into copper nanoparticles about 300°C. The joints formed using the Ag2O/CuO mixed paste, when heated to the appropriate levels, exhibited bondability superior to that of conventional Pb–5Sn joints. The oxide film formed on the copper substrate was reduced by the combustion of polyethylene glycol 400, and bonding was achieved between the sintered layer and the copper substrate. A longer period resulted in the oxidisation of a few layers of sintered copper layers into Cu2O. The ion-migration tolerance of the Ag2O/CuO mixed paste was approximately four times that of a layer of pure sintered silver.
Article
We have developed a new method for preparing a paste containing a high concentration of Ag nanoparticles for pressureless bonding. A nanoscale layer of polyvinylpyrrolidone coated on the nanoparticles prevents the coalescence of Ag nanoparticles. After heating in air, sintering and bonding occur after the decomposition of polyvinylpyrrolidone. Joint strengths were increased significantly using this new Ag nanoparticle paste as bonding material. Robust joints with shear strength above 20 MPa were formed even without additional bonding pressure.
Article
We propose a novel bonding process using Ag metallo-organic nanoparticles as a new application of nanotechnologies. The average size of the Ag nanoparticles is around 11 nm, and each particle is covered with an organic shell. Therefore, it has the outstanding feature that each nanoparticle exists independently. However, removal of the organic shell is necessary to bring out characteristics of the nanoparticle. Its decomposition temperature measured by thermal analysis is 573 K or less. In addition, it revealed that the thermal characteristic of the organic shell differed completely from Myristyl alcohol, from which the organic shell was derived. At a low bonding temperature of 573 K at a bonding pressure of 1 or 5 MPa, Cu-to-Cu joining using the Ag nanoparticles was achieved. The shear strength of the joints was 25–40 MPa, which was significantly higher than that made using Ag fine particles of 100 nm in size. That is because the reduction of the particle size to a nano-order improved the sintering of Ag particles and the bondability to Cu. Transmission electron microscope observations revealed that metallurgical bonding could be realized at the interface between the Cu and the Ag layer sintered with Ag nanoparticles. This bonding is suggested to originate from the large surface energy contribution caused by the nano-size particles.
Article
The self-diffusion coefficients for Au, Co, and Ni in α-Fe have been determined between approximately 700 and 900°C. The "magnetic effect" is demonstrated by each of the three diffusing elements. The diffusion coefficients in the paramagnetic temperature region are given by the following equations: DAu = 31 exp [-62.4 × 103/RT], DNi = 9.9 exp [-61.9 × 103/RT], DCo = 118 exp [-68.3 × 10-3/RT]. The values of the activation energy for Ni and Co are substantially different from those previously reported by other investigators.
Article
Sintered nanosilver is a lead-free die-attach material that could substitute for solder alloys and conductive epoxies for packaging power semiconductor devices, especially for high-temperature applications. While the maximum use temperature of a solder is limited by its melting point, the sintered silver joint can be used above the processing temperature, thus enabling high-performance power devices based on SiC technology to operate at high temperature. It can be fired at temperatures below 300°C without requiring applied pressure to form a dense interconnection with thermal and electrical conductivities superior to those of common high-temperature solder alloys. Die-shear strengths between 25 and 35 MPa can be obtained which compares favourably to the shear strength of solder. Unlike solder, which tends to form large voids during reflow, the sintered silver has a low elastic modulus and a microstructure containing only randomly distributed micrometer-scale pores that eliminates hot spots in the joint.
Article
A series of experiments investigating the recrystallisation of single crystal superalloy CMSX-4 have been carried out. Indentation atroom temperature has been used to study the effects of annealing time and temperature, and it has been found that a very strong dependence upon temperature is evident. Annealing above the γ′ solvus temperature results in very rapid growth of recrystallised grains whereas annealing below the γ′ solvus greatly suppresses the advancing grain boundaries. Additionally experiments have been carried out using an electrothermal mechanical test (ETMT) machine, to study the effects of degree of plastic strain and the temperature at which the strain is introduced. The strain threshold for recrystallisation under various annealing conditions has been determined and it has been found that recrystallisation occurs more readily if strain is introduced above 950°C. Finally, apparent activation energies for recrystallisation have been determined by measuring the change in resistivity that occurs during recrystallisation.