Effects Of P&Ce Modification On Microstructure And Properties Of Al-Si-Zn Filler Metal

Due to the advantages of good fluidity, spreadability, machinability and corrosion resistance, thetraditional Al-Si eutectic alloys become one of the most widely used filler metals in aluminum brazing.However, this series of filler metals have a high melting point, the brazing temperature is above600℃,so dissolution and grain growth are easily happened during the brazing process. In addition, before themodification of Al-Si filler metals, there exist coarse block primary or needle-like eutectic silicon inthe microstructure. They split matrix seriously, worsen the casting and mechanical properties of thefiller metals. This thesis is focused on the above shortcomings of Al-Si filler metals, by adding zinc toreduce their melting point, in order to meet the requirements of low melting point alloy brazing.Moreover, in the basis of Al-Si-Zn filler metal, the effect and mechanism of P and P-Ce complexmodification on melting point, spreadability, microstructure, mechanical properties and fracturemorphology are investigated respectively.Melting point is one of the main indexes in measuring the performance of filler metals. Thesolidus and liquidus temperature of Al-Si-Zn filler metals were524.9℃and548.1℃, which were farbelow the temperature577℃and602℃of Al-Si filler metals. The melting point remained nearly thesame after P or P-Ce complex modification. The experiments showed that, P and Ce element had asignificant effect on spreadability of the filler metals. With the increasing content of P element, thespreading performance was in the trend of rising. The filler metal had the best spreading performancewith0.06wt.%P. In the situation of P-Ce complex modification, with the increasing content of Ceelement, the spreadability increased first, then decreased. The filler metal had a optimum spreadingperformance with0.1wt.%Ce. Continuing to add Ce, the spreadability was deteriorated severely.P, Ce element can effectively refine silicon phase in the Al-Si-Zn filler metals. There existedcoarse block primary and needle-like eutectic silicon in the unmodified filler metals. By adding Pelement, the size of primary and eutectic silicon was decreased. The primary and eutectic silicon hadthe smallest size when the filler metals contained0.06wt.%P, the size were only10.8μm and12.3μm.The size of primary and eutectic silicon would grow when the P content reached to0.12wt.%. Theeffect of P-Ce complex modification was better than P modification. With the increasing content of Ceelement, the size of silicon was changed in this trend that increased first, then decreased, finallyincreased again. The primary and eutectic silicon had the smallest size when the filler metalscontained0.1wt.%Ce, the size were only5.2μm and4.8μm. A large number of needle-like Ce compounds would be formed in the filler metals if too much Ce was added.The results of brazing joints tensile test showed that the fracture of lap joints happened at thebase metal and butt joints happened at brazing seam respectively. The tensile strength of the buttjoints brazed with Al-Si-Zn filler metal increased first, and then decreased with the increasing of Paddition. The effect of P-Ce complex modification was relatively stable. The appropriate Ce contentwas0.05-0.15wt.%when the P content was0.06wt.%. Too much Ce element could lead to thestrength of brazing joints fall sharply. Except the case of too much Ce additives, the rest of modifiedbrazing seam presented ductile fracture characteristics, and all had high fracture strength.