Casting Defects in Transition Layer of Cu/Al Composite Castings Prepared Using Pouring Aluminum Method and Their Formation Mechanism

Shuying Chen; Shengnan Ma; Zhilin Chen; Xudong Yue; Guowei Chang

doi:10.1515/htmp-2017-0124

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Casting Defects in Transition Layer of Cu/Al Composite Castings Prepared Using Pouring Aluminum Method and Their Formation Mechanism

Shuying Chen , Shengnan Ma , Zhilin Chen , Xudong Yue and Guowei Chang

Published/Copyright: September 18, 2018

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From the journal High Temperature Materials and Processes Volume 38 Issue 2019

Abstract

In this study, Cu/Al composite castings were prepared using the pouring aluminum method, and the casting defects in the transition layer and their formation mechanism were investigated. Shrinkage cavities, shrinkage porosities, and cracks were easily formed in the transition layer, and the thickness of the transition layer was uneven after recombination of the solid Cu and liquid Al. Shrinkage cavities easily formed within the α + eutectic (α + CuAl₂) phase, and cracks mainly appeared within the Cu₉Al₄ and CuAl₂ phases. The transition layer with uneven thickness and irregular shape readily formed where the metals solidified last during the solidification; the difference in density between solid Cu and liquid Al, as well as the natural convection in the melt, were responsible for these irregular shapes. As the metals shrunk, shrinkage cavities and porosities formed without external melt feeding. Cracks formed in the Cu₉Al₄ and CuAl₂ phases and at the Cu₉Al₄/CuAl₂ and CuAl₂/eutectic (α(Al + CuAl₂) interfaces during the solid shrinkage process after the solidification was complete.

Keywords: copper cladding aluminum; transition layer; solidification process; casting defects; formation mechanism

Introduction

The recombination of liquid Al and solid Cu is one of the most common methods of preparing copper cladding aluminum composites [1, 2, 3, 4]. The copper surface is dissolved, and an Al–Cu alloy layer is formed between the solid Cu and liquid Al after contact; metallurgical bonding is realized after solidification, creating an area called the transition layer. The microstructure and thickness of the transition layer have been shown to determine the bonding strength of the alloy. Therefore, investigation of the formation of the transition layer, particularly its microstructure and mechanical properties, is needed for further enhancement of the strength of copper cladding aluminum composites. The effects of the temperature, time, cooling method, and magnetic field parameters on the transition layer have already been established [5, 6, 7, 8, 9, 10, 11, 12, 13, 14]; however, there has been much less research on the effects of defects in the transition layer. In the present study, copper and aluminum composite castings were prepared using the casting aluminum method, and the castings were analyzed to elucidate the formation mechanism of these defects and the effects of related factors.

Experimental methods

Aluminum and copper with purities of 99.90 % were used in our experiment. The aluminum ingot was placed in a clay graphite crucible, which was then heated in a resistance furnace. The liquid Al was degassed and refined with C₂Cl₆ after reaching the desired temperature (20°C higher than the pouring temperature) and held for 20 min. The crucible was then removed, and the liquid Al was poured into the experimental device (Figure 1) when the temperature of the liquid Al dropped to 780°C or 830°C. The surface of the Cu plate was sanded down to remove the oxide skin before pouring; then, the plate was placed on the experimental device. Argon was pumped into the casting mold to protect the internal surface of the Cu plate from oxidation during the pouring of Al. The temperature of the liquid Al was kept constant by spreading an insulation covering agent on the surface of the liquid. The external surface of the Cu plate was cooled by jet air, spray, or water injection. The time from pouring to forced cooling was 40, 60, or 110 s. As shown in Figure 1(b), thermocouples were fixed on the longitudinal axis of the cavity, and the temperature–time curve was drawn using a DMR2100 paperless recorder. A scanning instrument was used to scan the longitudinal surface of the composite castings, and a SN-300 electron microscope was used to examine the transition layer microstructure and obtain energy spectra.

Figure 1:

Schematic plan of preparing Cu/Al composite castings by the pouring method: (a) vertical pouring and (b) horizontal pouring.

Experimental results and analysis

Effects of technological parameters on compound quality

Scanning images of the profiles of the Cu/Al composite castings poured horizontally using different technological parameters are presented in Figure 2. The compound quality was optimal when spray cooling or water injection cooling were used at a pouring temperature of 780°C and holding time of 40 s, with the transition layer appearing at the junction of Cu and Al. For a higher pouring temperature or longer holding time, the cooling intensity of the outer surface of the Cu plate was weaker and, thus, the transition layer was thicker. Shrinkage cavities formed easily at the junction of Cu and Al under jet air cooling after the pouring, and a longer holding time resulted in more severe shrinkage cavities, as shown in Figure 2(a). Scanning images of the profiles of the Cu/Al composite castings poured vertically are presented in Figure 3. Although the compound was technically formed, the castings were of very low quality under these conditions. The transition layer was very thick with a thin upper part and thick lower part, and holes appeared in the transition layer. In general, horizontal pouring produced Cu/Al composites of much higher quality than vertical pouring (Figures 2 and 3).

Figure 2:

Recombination effects of Cu and Al under horizontal pouring: (a) air jet cooling, (b) spraying cooling, and (c) water injection cooling.

Figure 3:

Recombination effects of Cu and Al at 830°C pouring under vertical pouring.

Solidification microstructure of transition layer

The microstructures of the transition layers of the Cu/Al composite castings are shown in Figure 4. Three types of microstructures were observed in the transition layer: a hypoeutectic microstructure (which was close to the pure Al), eutectic microstructure in the middle of the transition layer, and hypereutectic microstructure (which was close to the pure Cu). An enlarged photograph of the junction region between the pure Cu and transition layer is presented in Figure 4(b). A thin layer was observed between the pure Cu and transition layer; according to the findings of Xie Jianxin [4], this layer is the Cu₉Al₄ phase. Thus, in total, four microstructures were observed in the transition layer: α + eutectic (α + CuAl₂), eutectic (α + CuAl₂), CuAl₂ + eutectic (α + CuAl₂), and Cu₉Al₄.

Figure 4:

Microstructures of transition layer in the Cu/Al composite casting at 780°C pouring: (a) holding for 60 s, air jet cooling; and (b) enlarged photo of A region shown in Figure 4(a).

Casting defects in the transition layer

Shrinkage cavities

The shrinkage cavities in the transition layer are shown in Figure 5. Both top and internal shrinkage cavities were observed in the Cu/Al composite castings poured vertically. The top cavities were located in the area where the Cu plate dissolved, as marked by arrow “A” in Figure 5(a). The internal cavities were dispersed and had different sizes, as marked by the white arrow in Figure 5(a). Conversely, in the castings poured horizontally, the shrinkage cavities were flat and located just below the middle of the transition layer. These cavities were also smaller compared with those in the samples poured vertically, as shown in Figure 5(b).

Figure 5:

Shrinkage cavities photos in the transition layer under jet air cooling: (a) pouring at 830°C and holding for 110 s; and (b) pouring at 780°C and holding for 60 s.

Transition layer shapes

As observed in Figures 3 and 5, the shape of the transition layer for the vertical pouring was thin at the top and thick at the bottom. For the same pouring temperature, a longer holding time led to a larger thickness difference between the top and bottom of the transition layer (Figure 3). The Al–Cu alloy within the transition layer entered the central region of the pure Al ingot, forming a “sandwich” at a pouring temperature of 830°C and holding time of 110 s with jet air cooling, as shown in Figure 5(a). The transition layers for the horizontal pouring took on flat shapes, and their shapes were not related to the technological parameters, as shown in Figures 2 and 5(b).

Shrinkage porosities

The shrinkage porosities within the transition layers mainly appeared in the α + eutectic (α + CuAl₂) and eutectic (α + CuAl₂) microstructures for the horizontal pouring and were specifically located among α dendrites or eutectic cells (Figure 6). Neither shrinkage cavities nor shrinkage porosities were observed within the CuAl₂ + eutectic (α + CuAl₂) microstructure regardless of the technological parameters.

Figure 6:

Shrinkage porosities within the transition layer.

Cracks

Cracks formed easily in the transition layers, primarily within the Cu₉Al₄, CuAl₂, and CuAl₂ + eutectic (α + CuAl₂) microstructures, as observed in Figure 7. The types of cracks can be roughly divided into three categories: (1) Cracks located between two phases and extending along the boundary, such as those marked by arrow “A” in Figure 7(b) between the CuAl₂ phase and CuAl₂ + eutectic (α + CuAl₂) microstructure, or arrow “B” in Figure 7(c) between the Cu₉Al₄ phase and CuAl₂ phase. (2) Cracks within the phases. Many of the cracks were observed within the CuAl₂ phase after the “F” region shown in Figure 7(b) was enlarged, as marked by arrow “C” in Figure 7(c). The shapes of these cracks were flat, and their directions and sizes were different, which was primarily responsible for the comminuted fracture of the CuAl₂ phase. (3) Cracks on the grain boundary, as marked by arrow “D” in Figure 7(a). These cracks appeared within the eutectic (α + CuAl₂) microstructure. They were curved in shape and extended along the interface of the eutectic cells.

Figure 7:

Cracks in the transition layer under pouring at 830°C, holding for 110 s and jet air cooling.

As observed in Figure 7, most of the cracks were flat. The intracrystalline phenomenon was observed, as marked by arrows “A” and “E” in Figure 7, which implied that the cracks in the transition layer were cold cracks.

Discussions

Formation mechanism of shrinkage cavities and porosities

Solidification of transition layer

As demonstrated by the cooling curve in Figure 8, the cooling effect of the Cu plate on the liquid Al did not cause the solidification of the liquid Al. In other words, the liquid Al was in contact with the solid Cu for a long time after pouring. During this period, the Cu atoms on the surface of the solid Cu constantly dissolved into the liquid Al to form an Al–Cu alloy melt (the transition layer) close to the Cu plate. The Cu concentration gradually decreased from the bottom to the top of the transition layer, as shown in Figure 9. In addition, the fast heat dissipation from the surface of the Cu plate caused the temperature of the liquid Al to gradually increase from the bottom to the top of the transition layer, and the temperature gradient was small because of the natural cooling. When the temperature at the top of the transition layer decreased to 660°C, the liquid Al began to solidify. Because the liquidus temperature of the Al–Cu alloy melt was fairly low, the pure Al at the top of the transition layer solidified before the transition layer. The Cu content in the Al–Cu alloy melt next to the Cu plate was the highest; therefore, its liquidus temperature was the lowest. This part remained liquid after the top of the transition layer started to solidify, and the Cu atoms on the surface of the Cu plate continued to dissolve into the melt. These processes are depicted in Figure 10(a) and (d).

Figure 8:

Cooling curves of horizontal pouring under pouring at 830°C, holding for 60 s and water injection cooling.

Figure 9:

EDS analysis of Cu element in the transition layer.

Figure 10:

Schematic plan of solid Cu dissolution, solidification process of the transition layer and the shrinkage cavities formation: (a)–(c) vertical pouring; and (d)–(f) horizontal pouring.

When the temperature continued to decrease or the temperature of the outside surface of the Cu plate began to decrease due to the forced cooling, the transition layer began to solidify, and the CuAl₂ + eutectic microstructure (α + CuAl₂) (close to the pure Cu) and α + eutectic (α + CuAl₂) microstructure (close to the pure Al) grew into the transition layer, as shown in Figure 10(b) and (e). Finally, the eutectic (α + CuAl₂) microstructure formed in the intermediate zone of the transition layer, as observed in Figures 10(c) and (f).

Shrinkage cavity and porosity formation mechanism

Because the pure Al began to solidify first, the Al–Cu alloy melt was closed between the solid Cu and solid Al, as shown in Figures 10(b) and (e). This part of the Al–Cu alloy solidified last because there was no external metal feeding during the liquid stage or solidification shrinkage, which caused the shrinkage cavities to form in the transition layer.

During the vertical pouring, the temperature of the casting upper part was high and the upper melt of the transition layer solidified last; therefore, the shrinkage cavities mainly appeared at the top of the transition layer, as observed in Figures 3 and 10(c) and as marked by arrow “A” in Figure 5(a). Because the melt shape in the transition layer was thin at the upper part and thick at the lower part, the transition layer solidified from the two sides toward the center when forced cooling of the outer surface of the Cu plate was initiated. Solidification of the upper part of the transition layer was rapid. Therefore, the feeding passageway between the upper and lower parts of the transition layer was blocked, which resulted in the formation of the shrinkage cavities in the bottom of the transition layer, as shown in Figure 10(c) and marked by arrow “B” in Figure 5(a).

During the horizontal pouring, it was more difficult for the transition layer to be supplemented by shrinking of the upper part of the melt. The shrinkage cavities were relatively small and randomly distributed in the horizontal direction because of the thin transition layer, as shown in Figures 5(b) and 10(c).

During the solidification, the temperature gradient in front of the solid–liquid interface close to the Cu plate was positive and that close to the pure Al was negative. The solid phase close to the Cu plate grew upward and was easily supplied by the melt during the horizontal pouring; therefore, shrinkage porosities could not easily form. The solid phase close to the pure Al grew downward, and the rest of the melt also flowed downward; thus, some shrinkage porosities were formed because of the difficult feeding among the dendrite crystals, as shown in Figure 6(a). When the eutectic phase transformation occurred in the intermediate zone of the transition layer, because this stage was the final stage of the solidification and there was less remaining melt supply, shrinkage porosities could easily form, as shown in Figure 6(b).

Based on the above analysis and Figures 3, 5, and 6, the concentrated and large shrinkage cavities easily formed in the transition layers of the Cu/Al composite castings during the vertical pouring. For the samples poured horizontally, more dispersed and smaller shrinkage cavities easily formed in the transition layer, and the shrinkage porosities usually appeared within the hypoeutectic microstructure of the transition layer.

Flow in Al-Cu alloy melt flow and its effect on transition layer shapes

When preparing the Cu/Al composite castings using the pouring aluminum method, the Cu atoms dissolved toward the liquid Al to form the Al–Cu alloy close to the Cu plate. The Al–Cu alloy melt automatically sunk in the liquid Al because of the natural convection caused by the density differences. During the vertical pouring, the Al–Cu alloy melt flowed downward along the Cu plate, resulting in the formation of a transition layer with a thin top and thick bottom, as shown in Figure 10(a) and (b). During the horizontal pouring, the Al–Cu alloy was parallel to the Cu plate. If the solid Cu was at the bottom of the casting, the Al–Cu alloy did not flow, and the Cu atoms entered the liquid Al solely via diffusion; the diffusion was difficult because of the effects of gravity, resulting in the formation of a thin transition layer (Figures 2 and 10). If the solid Cu was on top of the casting, the Al–Cu alloy melt flowed rapidly into the liquid Al. Thus, if the flow speed is not carefully controlled, failure of the compound will occur.

According to the experimental results and the above analysis on the flow state of the Al–Cu alloy, the shape of the transition layer was thin at the top and thick at the bottom when the liquid Al was vertically poured, as illustrated in Figure 11(a). The shape became very complex for horizontal pouring, as shown in Figure 11(b).

Figure 11:

Schematic plan of the transition layer shapes in Cu/Al composite casting: (a) vertical pouring and (b) horizontal pouring.

The formation of the Al–Cu alloy and its flow during the compound process between solid Cu and liquid Al is inevitable. The most effective method of obtaining a thin and uniform transition layer is to control the amount of dissolved Cu and to decrease the time of the melt flow.

Cracks caused by thermal stress

The temperature gradually increased from the exterior to interior of the casting after forced cooling of the outer surface of the Cu plate began. The solid shrinkage occurred after the solidification was complete, and the inside of the casting continued to shrink after the surface of the casting stopped shrinking. Thermal stress in the casting developed if the inner shrinkage was blocked. Because the transition layer was close to the surface of the casting, tensile stress likely developed in the transition layer. The solidification microstructures close to the Cu plate were the Cu₉Al₄ and CuAl₂ + eutectic (α + CuAl₂) microstructures, and Cu₉Al₄ and CuAl₂ are both hard, brittle phases. The bonding strengths of the Cu₉Al₄/CuAl₂ and CuAl₂/eutectic (α + CuAl₂) interfaces are low. Under the action of thermal stress, cracks easily formed inside the Cu₉Al₄ and CuAl₂ phases and at the Cu₉Al₄/CuAl₂ and CuAl₂/eutectic (α + CuAl₂) interfaces, as shown in Figure 7(c). The solidification microstructure in the middle areas of the transition layer was the eutectic (α + CuAl₂) microstructure. Shrinkage porosities easily formed between the eutectic cells, and a stress concentration existed around the shrinkage porosities; therefore, it was easy for cracks to extend preferentially between the shrinkage porosities, which resulted in the formation of intergranular cracks between the eutectic cells, as observed in Figure 7(a). The toughness of the α + eutectic (α + CuAl₂) microstructure in the transition layer was relatively high; therefore, cracks were less common in that area.

A thin transition layer could be obtained by decreasing the pouring temperature, shortening the holding time, or strengthening the cooling intensity of the outer surface of the Cu plate. A shift of the tensile concentration formed by the shrinkage into the pure Al was beneficial, effectively preventing the formation of cracks, as illustrated in Figure 4.

Conclusions

The most notable conclusions of this study can be summarized as follows. Defects including shrinkage cavities, shrinkage porosities, cracks, and uneven thickness easily formed in the transition layer during the preparation of copper cladding aluminum composites using the pouring aluminum method. Shrinkage porosities easily formed in the α + eutectic (α + CuAl₂) phase, and cracks mainly appeared in the Cu₉Al₄ and CuAl₂ phases. The transition layer solidified last during the spreading of the solid phase from both sides toward the middle of the transition layer. Because there was no external melt supply, the shrinkage cavities formed in the middle of the transition layer. The shrinkage porosities formed preferentially between the dendrite crystals. The shape of the transition layer was thin at the top and thick at the bottom for the vertical pouring, and the shape became very complex for the horizontal pouring. The solidification microstructures close to the Cu plate were Cu₉Al₄ and CuAl₂ + eutectic (α + CuAl₂), and Cu₉Al₄ and CuAl₂ were both hard, brittle phases. The bonding strengths of the Cu₉Al₄/CuAl₂ and CuAl₂/eutectic (α + CuAl₂) interfaces were low. Under the action of thermal stress, cracks easily formed inside the Cu₉Al₄ and CuAl₂ phases and at the Cu₉Al₄/CuAl₂ and CuAl₂/eutectic (α + CuAl₂) interfaces.

Acknowledgements

This study was financially supported by the Provincial Natural Science Fund Guidance Plan (No. 20170540444), the National Natural Science Foundation of China (No. 51674138), and the Growth Plan for Outstanding Young Scholars in Colleges and Universities of Liaoning Province (No. LJQ2014062).

References

[1] H. Liang, Z.Y. Xue, C.J. Wu, Q. Liu and Y. Wu, Acta. Metall. Sin. (Engl. Lett.), 23 (2010) 206–214.Search in Google Scholar

[2] J.T. Luo, S.J. Zhao and C.X. Zhang, J. Cent. South. Univ. Technol., 18 (2011) 1013–1017.10.1007/s11771-011-0796-1Search in Google Scholar

[3] Y.J. Su, X.H. Liu, H.Y. Huang, C.J. Wu, X.F. Liu and J.X. Xie, Metall. Mater. Trans. B., 42 (2011) 104–113.10.1007/s11663-010-9449-2Search in Google Scholar

[4] Y.J. Su, X.H. Liu, H.Y. Huang, X.F. Liu and J.X. Xie. Metall. Mater. Trans. A., 42A (2011) 4088.10.1007/s11661-011-0785-xSearch in Google Scholar

[5] E. Hug and N. Bellidom, Mater. Sci. Eng. A., 528 (2011) 7103.10.1016/j.msea.2011.05.077Search in Google Scholar

[6] M.S. Mohebbi and A. Akbarzadeh, Int. J. Adv. Manuf. Technol., 54 (2011) 1043–1055.10.1007/s00170-010-3016-5Search in Google Scholar

[7] L.Y. Shen, F. Yang, T.F. Xi, C. Lai and H.Q. Ye, Compos. Part. B., 42 (2011) 1468–1473.10.1016/j.compositesb.2011.04.045Search in Google Scholar

[8] W. Xie, T. Yamaguchi and K. Nishio, J. Jpn. Inst. Metals., 75 (2011) 166–172.10.2320/jinstmet.75.166Search in Google Scholar

[9] H. Paul, L.L.S. Ska and M. Prazmowski, Metall. Mater. Trans. A., 44 (2013) 3836–3851.10.1007/s11661-013-1703-1Search in Google Scholar

[10] B. Xu, W.P. Tong, C.Z. Liu, H. Zhang, L. Zuo and J.C. He, J. Mater. Sci. Technol., 27 (2011) 856–860.10.1016/S1005-0302(11)60155-2Search in Google Scholar

[11] K.M.O. Meguro and M. Kajihara, J. Mater. Sci., 47 (2012) 4955–4964.10.1007/s10853-012-6370-xSearch in Google Scholar

[12] I.K. Kim and S.I. Hong, Mater. Des., 47 (2013) 590–598.10.1016/j.matdes.2012.12.070Search in Google Scholar

[13] R. Uscinowicz, Compos. Part. B., 55 (2013) 96–108.10.1016/j.compositesb.2013.06.002Search in Google Scholar

[14] V.Y. Mehr, M.R. Toroghinejad and A. Rezaeian, Mater. Des., 53 (2014) 174–181.10.1016/j.matdes.2013.06.028Search in Google Scholar

Received: 2017-09-11

Accepted: 2018-06-04

Published Online: 2018-09-18

Published in Print: 2019-02-25

This work is licensed under the Creative Commons Attribution 4.0 Public License.

Articles in the same Issue

https://doi.org/10.1515/htmp-2017-0124

Keywords for this article

copper cladding aluminum; transition layer; solidification process; casting defects; formation mechanism

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