SSPring-8/SACLA
Pring-8/SACLAResearch
利用研究Report
成果集
2019A1646

Section B
Section

BL14B2

3D Imaging of Liquid Film Migration in Aluminium Alloy after Brazing
using Phase-contrast Tomography
Lars-Åke Näslund a, *, Torkel Stenqvist a, Daiki Shiozawa b, Stephen Hall c, Johan Hektor c, **,Mårten Edwards d, Linda Ahl a, and Robert Moberg da Gränges Sweden AB, b Kobe University, c Lund University, d Uppsala Synchrotronix AB* Current affiliation: Linköping University, ** Current affiliation: Malmö University

Abstract
An application problem when brazing aluminium is that molten braze alloy may penetrate into the bulk.
This may, for example, increase the risk for reduced corrosion resistance and may in the long run lead tocoolant leakages in heat exchangers. The phenomenon is poorly understood, which motivates a closer lookusing synchrotron radiation based 3D imaging. Through phase-contrast tomography at the BL14-B2 beamline
at SPring-8 we have recorded 3D images of aluminium sheet materials with different stages of braze alloy
penetration where the braze alloy penetration process can be visualized step by step. The characteristic signsof braze alloy penetration has been identified in the 3D images and the project has provided new perspectives
on the braze alloy penetration process.
Keywords: Aluminium, Liquid film migration, Controlled atmospheric brazing, Phase-contrast tomography
Background and Purpose
The introduction of braze clad on aluminium (Al) sheets has enabled fast and convenient brazing assemblyof complex heat exchangers (HEX) with hard requirements on heat performance, strength, and corrosion
resistance [1,2]. The braze clad on aluminium sheets is in most cases an aluminium-silicon (Al-Si) alloy witha solidus of 577 °C. Figure 1(a) shows examples of typical braze clad Al sheet structures in a heat exchanger.
A liquid-to-air heat exchanger consists of several parts joined through brazing. The arrows indicate that hot
fluid is collected in one tank, pushed through the thin tubes, and cooled as heat is transferred to the fins, which
in turn are cooled by the surrounding (forced) air. The cooled fluid is collected in the opposite tank and pushed
out in the external circuit that prevents hot parts to become over-heated. Figure 1(b) shows an optical
microscopy cross-section image of a joint between a tube and a fin in a heat exchanger. Molten Al-Si braze
clad alloy has flown to the narrow space between the tube- and fin materials and formed a joint uponsolidification. The microstructure image shows the Al-Si braze clad alloy on the outside and a corrosion
inhibiting Al-Zn alloy on the inside (waterside) of the grayer core Al alloy tube material. The core Al alloy
appears as grayer, compared with the Al-Si alloy and the Al-Zn alloy because of a homogenous distributionof dispersoids.
During brazing at about 600 °C in a controlled nitrogen gas atmosphere the Al-Si braze alloy in the clad
layer liquefies and flows efficiently to the narrow spaces between different heat exchanger parts through thecapillary force forming strong joints. However, under certain conditions, e.g., when the material is exposed
to cold work prior to the brazing, liquid film migration (LFM) may be a significant problem. LFM is a wellknown application problem in the HEX industry and the phenomenon can appear while brazing at about
600 °C. LFM can be described as when the molten braze alloy penetrates into the core material andconsequently the filler metal flow to the joints will be reduced, as shown in Figure 2, which causes poor braze
joints. A non-linear relationship between the extent of LFM and the applied strain shows that the problem
with LFM is high at low material deformation (3-10 %) but reduces with further increases in the applied strain.
The decrease in the extent of LFM was shown to be because of core alloy recrystallization during brazing,
which reduces the LFM significantly [3]. In addition, LFM will also increase the sensitivity for corrosion of
the core aluminium alloy [3]. Figure 3 shows corrosion attacks where LFM has occurred. Hence, underdisadvantageous conditions the molten braze alloy may wet grains and penetrate into the aluminium corematerial causing serious damage to the Al material.
Yoon and Huppman [4] were the first to report LFM during liquid phase sintering of tungsten with nickel.
When placed in contact with pure liquid nickel the tungsten grains dissolved at first and then reprecipitatedas a solid solution saturated with nickel during the migration of the unreacted liquid into the dissolving grains

SSPring-8/SACLA
Pring-8/SACLAResearch
利用研究Report
成果集

Section B
Sectionof tungsten. The phenomenon of molten braze material penetration into the core alloy has since been observedin many different alloy systems [5]. Behind the front of the liquid film migration, through the core alloy,
precipitation will form a dislocation free solid solution consistent with the solidus composition at the brazing
temperature [6].

Fig. 1. (a) A liquid-to-air heat exchanger consists of several parts joined throughbrazing. (b) A cross-section image of a joint between a tube and a fin in a heatexchanger.

Fig. 2. Small amounts of cold work (3-5%) sharply reduce braze metal flow.
More cold work brings some improvement [3].

SSPring-8/SACLA
Pring-8/SACLAResearch
利用研究Report
成果集

Section B
Section

LFM where strain is low

No LFM where strain is higher

Fig. 3. Cross-section image of a brazed Al sheet that shows liquid film
migration (LFM) at areas exposed to low material deformation but no LFM
where the strain has been higher. The grain boundaries in the affected areas areprone to corrosion [3].
Recently we presented a three-dimensional X-ray diffraction (3DXRD) study where Al alloy strips,
128 µm thick and 1 mm wide, was investigated [7]. The 3DXRD provided maps of the center-of-mass positionof Al grains in the Al alloy samples. In the 3DXRD image of the sample that was braze simulated at one end
but not on the other we could distinguish the three zones: the non-“brazed” zone, the fully “brazed” zone, andthe transition region between them. Originally for the SPring-8 beamtime at the X-ray absorption beamline
BL14-B2 we planned to perform 3DXRD, although with the access to the X-ray scattering beamline SMS
P21.2 [8] at the synchrotron radiation facility PETRA III at DESY in Hamburg, Germany, the focus for theBL14-B2 beamtime became phase-contrast X-ray computed tomography (PCX-CT) [9-11] to produce 3D
images that show microstructural features in the core Al alloy grains and the distribution of dispersoids in a
braze-simulated sample. Since the sample mainly consist of Al and Si, i.e., atoms with low atomic number Z,
phase-contrast X-ray imaging shows more sensitivity to density variations in the sample compared to
traditional tomography techniques [9-11].

Fig. 4. 1 mm wide and 40 mm long strips along the transition region from thenon-“brazed” zone to the “brazed” zone.
Experimental Summary
The 128 µm thick core material selected for this study was an Al alloy with 1.6 % manganese (Mn), 1.5 %zinc (Zn), 0.8 % silicon (Si), 0.25 % iron (Fe), and < 0.1 % zirconium. On one side of the rolled Al sheet there
was an 18 µm thick layer of an Al braze clad alloy with 10 % silicon. The material was stretched by 6 % tosimulate a sheet forming process prior to brazing. The deformation of 6 % was selected to optimize theoccurrence of LFM, see Figure 2. A 60 x 20 mm2 coupon was cut from the sample and placed in a furnaceSSPring-8/SACLA
Pring-8/SACLAResearch
利用研究Report
成果集

Section B
Sectionwith a nitrogen gas atmosphere. To simulate a brazing process the furnace was heated according to a heatingramp of 60 °C/min up to 500 °C, 25 °C/min between 500 and 575 °C and 15 °C/min between 575 and 590 °C.
Immediately after reaching 590 °C, the heating process was interrupted. A temperature gradient over the
sample was obtained through cooling blocks attached to both ends of the coupon. Hence, at the furnace
temperature of 590 °C, the braze clad in the middle part of the coupon became liquefied while the braze cladat both ends remained solid. Between the non-“brazed” zone and the “brazed” zone there is a transition region.
After the controlled atmosphere brazing (CAB) simulation the coupon was cut by electrical dischargemachining (wire-cut type) into 1 mm wide and 40 mm long strips along the transition region from thenon-“brazed” zone to the “brazed” zone at one half of the coupon, see Figure 4.
The analyzing method employed in the present study was the propagation-based imaging technique thatuses the difference in refractive index for hard X-rays between particles and base material. The difference inrefractive index will cause interference fringes as a consequence of the photon phase shift obtained when acoherent photon beam is illuminating different parts of the sample. Hence, the changes in the phase of the
X-ray beam that passes through the sample will enhance the contrast of the X-ray absorption image. Imagesof the sample from different angles can then contribute to a 3D reconstruction through filtered back projection
creating a phase-contrast X-ray computed tomography.
The phase-contrast X-ray computed tomography was performed at BL14-B2 beamline at the synchrotron
radiation facility SPring-8 in Japan. The Al sample strip was illuminated by a monochromatic beam of 15 keV
while the sample was rotated 180° with steps of 0.2°. The detector was a Hamamatsu Photonics C4880-41S
combined with the imaging unit M11427-41. The image size was 4000 pixels x 2600 pixels and with aneffective pixel size of 1.37 µm. The distance between the sample and the camera was 200 mm.

Fig. 5. Cross-section slices from the tomography imaging showing (a) the non“brazed” zone, (b) the transition region between the non-“brazed” and the“brazed” zones, and (c) the fully “brazed” zone.

SSPring-8/SACLA
Pring-8/SACLAResearch
利用研究Report
成果集

Section B
SectionResults and Discussion
Three cross-sections of the Al-sample from the 3D image are presented in Figure 5 representing thenon-“brazed” zone with an Al-Si braze clad on the right side of the cross-section, the transition region, andthe fully “brazed” zone. PCX-CT is not able to distinguish different grains and the cross-section of thenon-“brazed” zone in Figure 5(a) appears to be homogeneous with randomly distributed bright spots, which
is precipitates of Al(FeMn)Si. The white bands of particles on the right side of the image are Si-particles that
had segregated out from the Al-Si alloy braze clad. The transition region in Figure 5(b) is similar as thenon-“brazed” zone as no grains are visible. It is, however, clear that the Al-Si alloy braze clad has molten,
which gave the right side of the cross-section image a rough appearance. The 3DXRD showed that the grainsin the braze clad with {001} orientation have grown larger at the elevated temperature [7]. In the fully “brazed”
zone shown in Figure 5(c) the Al(FeMn)Si precipitates are randomly distributed in the left and middle part ofthe cross-section, similar as in Figure 5(a). On the right side, on the other hand, the precipitates are lining up
at about 30 µm from the right end. The line-up formation of precipitates is highlighted with an arrow in
Figure 5(c). Hence, when the Al-Si braze clad is molten in the fully “brazed” zone the PCX-CT shows anaccumulation of relatively large precipitates decorating the borders of newly formed grains, which ischaracteristic features of LFM. For more information about grain formation in this particular sample see Ref.
[7].
Figure 6 presents a cross-section image from a region that is between Figure (b) and (c), where the braze
clad on the top of the Al sheet is molten. The captured image is selected because of the clear appearance of
LFM on the left side where new particle-free Al-Si alloy grains are formed as a characteristic feature(highlighted with arrows). The absence of particles makes the new grains more grayish compared to theoriginal grains in the core. The highlighted grains in Figure 6 show borders that appear white in the PCX-CT
indicating that the new grains are decorated with a band of Si- and Al(FeMn)Si precipitates.

Fig. 6. A cross-section slice from the tomography imaging showing the molten
braze clad on the top of the Al sample. Examples of new grains from LFM arehighlighted with arrows. The balls under the Al sample are glass beads glued
on the backside of the sample making it easier to identify the positions in thesample.
A closer look at one of the new grains formed after the LFM, the leftmost highlighted grain in Figure 6,
is displayed in Figure 7(a). This new Al-Si alloy grain is free from particles, because the LFM has pushed allparticles in front of the LFM frontline. Figure 7(b) shows an illustration of the new grains, colored gray, on
top of the large grain in the core material (orange) [7]. The grain in the core material has randomly distributed
Al(FeMn)Si-particles while the new Al-Si alloy grains are free from particles as they are pushed by the LFM
frontline to the grain boundary.
The white band of Si- and Al(FeMn)Si-particles in the grain boundary of the new grains is sensitive towardcorrosion attacks and may lead to inter-granular corrosion that severely weakens the material.

SSPring-8/SACLA
Pring-8/SACLAResearch
利用研究Report
成果集

Section B
SectionFig. 7. (a) A new Al-Si alloy grain after LFM without particles. The new grain
is decorated with Si- and Al(FeMn)Si-particles. (b) Illustration showing the
new Al-Si alloy grain (gray) decorated with Si-particles (green) andAl(FeMn)Si-particles (red).
Figures 5-7 show that LFM in braze clad Al sheets can be identified in phase-contrast X-ray computed
tomography after brazing. While the “brazed” zone showed severe occurrence of liquid film migration, the
PCX-CT of the transition region between the non-“brazed” zone and the “brazed” zone shows that there areindividual grains that could be distinguished by the particle accumulation around a particle-free surface grain,
which is characteristic features of the LFM phenomenon. Hence, the result shows promises of new valuable
information that can be extracted in combination with other 3D-imageing techniques, such as 3D-X-ray
diffraction (3DXRD) and/or diffraction contrast tomography (DCT), as well as 2D-imaging techniques, suchas light optical microscopy (LOM), scanning electron microscopy (SEM), and electron backscatter diffraction(EBSD).
Challenges
The performed study shows that it is possible to identify the transition region between the solid braze cladand the braze clad that was molten using phase-contrast X-ray computed tomography. Hence, it should bepossible to find isolated grains formed after LFM in the tomography image. In addition, a closer investigation
where the tomography image is merged with a 3D diffraction analysis might provide information regarding
the close environment of the new grain formed after LFM. It should then be possible to reveal the grainorientation of the neighboring grains and if that has influences on the LFM phenomenon. Merging the twodifferent images is, however, a delicate challenge.
Acknowledgements
We acknowledge SPring-8 for the provision of experimental facilities and the beam line scientist Kentaro
Kajiwara at the Japan Synchrotron Radiation Research Institute. The research was partly funded by Sweden’sInnovation Agency (Vinnova) through grants 2018-03268 and 2019-05299. The authors thank themetallurgical department at Gränges Finspång AB for microscopy work.
References[1] M. Nylén et al., Materials Science Forum, 217–222, 1703 (1996).
[2] J. Lacaze et al., Mater. Sci. Eng. A, 413-414, 317 (2005).
[3] R. Woods, Liquid film migration during aluminium brazing, SAE International, 1997, 639.
[4] D. N. Yoon and W. J. Huppmann, Acta Metallurgica, 27, 693 (1979).
[5] D. Y. Yoon, Int. Mater. Rev., 40, 149 (1995).
[6] M. J. Benoit et al., Metallurg. Mater. Transact. A, 48, 4645 (2017).
[7] T. Stenqvist et al., Adv. Eng. Mater., 23, 2100126 (2021).
[8] Z. Hegedüs et al., IOP Conf. Ser.: Mater. Sci. Eng., 580, 012032 (2019).
[9] D. McMorrow and J. Als-Nielsen, Elements of Modern X-Ray Physics, John Wiley & Sons, Inc., 2011,318.
[10] S. C. Mayo et al., Opt. Express, 11, 2289 (2003).

SSPring-8/SACLA
Pring-8/SACLAResearch
利用研究Report
成果集[11] S. C. Mayo, A. W. Stevenson and S. W. Wilkins, Materials, 5, 937 (2012).

(Received: September 21, 2022; Accepted: February 2, 2023; Published: April 28, 2023)

Section B
Section