Effect of ZrO 2 Nanoparticles on the Microstructure of Al-Si-Cu Filler for Low-Temperature Al Brazing Applications
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ALUMINUM alloys are the most attractive choice for lightweight applications in automotive, aerospace, and structural industries due to their various exciting properties such as higher strength to weight ratio, superior corrosion resistance, low cost, etc. Several high strength Al alloys have been designed in the past, e.g., Al-Cu: 2XXX; Al-Mg: 5XXX; Al-Si: 4XXX; and Al-Zn: 7XXX etc. Additional elements are often included to fabricate a wide variety of alloys for industrial applications in aerospace and/or automobiles.[1] After steel and cast iron, automotive industries have shown interest in aluminum alloys to improve the fuel economy and air pollution.[2] The Al alloys are highly electrically and thermally conducting, and prime candidates in the fabrication of heat exchangers. Al-Si filler alloy has been used in industries for brazing various parts in heat exchangers. However, the development of a filler alloy that can be used at low temperatures is always a serious issue in brazing industries.[3] The commonly used Al-12 wt pctSi eutectic alloy has the melting point at 850 K (577 C), which is considered too high to be used in joining of the Al components. This may result in localized ASHUTOSH SHARMA, Research Professor, MYUNG-HWAN ROH, Masters Student, DO-HYUN JUNG, Ph.D. Student, and JAEPIL JUNG, Professor, are with the Department of Materials Science and Engineering, University of Seoul, Seoul 130743, Korea. Contact e-mail: [email protected] Manuscript submitted May 8, 2015. Article published online November 13, 2015 510—VOLUME 47A, JANUARY 2016
melting or degradation at the joint interface deteriorating the joint strength and eventually leading to the failure in some cases.[4,5] Therefore, to avoid these problems, there is a great need of designing a low-temperature Al filler for heat exchanging applications.[6] There are various reports published on the development of filler metal in the recent few years. For example, in one report, an eutectic Al-6.5Si-42Zn and Al-6.5Si-42Zn-0.5Sr filler metals are proposed with a melting point around 793 K (520 C).[7] The addition of zinc depresses the melting point of Al-Si alloy appreciably; however, the Zn vaporizes very quickly and the process is not suitable for vacuum brazing. Niu et al. designed a low melting point alloy of Al-Si-Ge-Zn with an improved brazeability.[8] However, the addition of Ge may not be a good idea as the price of Ge is almost 400 times higher than the Al, making it an extremely expensive process.[9] In another work, a low melting point filler with copper and nickel is developed, e.g., Al-20Cu2Ni-5Si having a melting temperature below 811 K (538 C). The shear stress of the joint is also impressive over 75 MPa.[10] It is also noteworthy that the addition of copper depresses the melting point to a greater extent and increases the mechanical strength. However, the generation of CuAl2 intermetallic compounds (IMCs) is known to cause cracks and pores in various engineering joints.[11,12] These IMCs, if not controlled, may cause complete damage to the joint
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