The Research On Brazing Capability And Bonding Mechanism Of Amorphous Filler Metal

By employing the X-ray diffraction, differential scanning calorimetry(DSC), scanning electron microcopy and electron microproble analysis, the structure evaluation of amorphous Cu65.5Ni16.6P12.9Sn4.9 filler metal and rapidly solidification AlSiCu filler metal, the interaction between filler metal and matrix metal,the forming process and capability of joint have been systematically investigated. The wetting mechanism, element diffusion behavior of amorphous filler metal has also been investigated.At last, the root reason that amorphous filler metal is better than Ordinary crystalline filler metal has been found. Based on above-mentioned investigations, the following results can be obtiained:During the ramp-up period, amorphous filler metal began crystallization and grow up into a fine grain of homogeneous, non-segregation, which is benefit to the element diffusion and the generation of liquid.The Ordinary crystalline filler metal is composed of large crystal and the compound appeared.The thick CU3P intermetallic compounds not only reduce the joint strength, but also for fatigue and impact properties has also adversely affected.The element in amorphous filler metal is stongth diffusion ability, filler metal interactive with matrix metal when it was warming up. The process is behand and incomplety for Ordinary crystalline filler metal in the period. The diffusion coefficient of Sn is caculated, the resulte indcated that the amorphous filler metal have larger diffusion coefficient and little diffusion activation energy which is benefit to the element diffusion.The wetting process of Cu-P is composed of three stages. The first stage is physical wetting, in which a liquid filler metal on a solid surface will spread to some extent on the surface and then comes to rest reaching a first metastable equilibrium contact angle. After the short transient stage a thin continuous solid solution layer is then formed along the solid/liquid interface. Thereafter, since the solid solution layer is better wetted than the original substrate, the drop starts spreading again as the layer expands outward of the tripleline, parallel to the interface.In this second spreading process, the drop spreading rate is controlled, not by the metal viscosity, but rather by the rate at which the reaction product layer can grow at the triple line, parallel to the solid surface. Spreading continues in this manner until the contact angle becomes equal to the equilibrium contact angle of the alloy on the reaction product. Following this, the drop remains stable even though the reaction layer may continue to grow, both in thickness and further out-wards of the triple line, along the substrate free surface. The obviously difference of wettability between amorphous filler metal and crystalline filler metal is the amorphous has a shorter transient stage and a thicker continuous solid solution layer is then formed along the solid/liquid interface. When it was warming up,atoms in amorphous filler metal has diffused into original substrate then a thin continuous solid solution layer is then formed along the solid/liquid interface, however crystalline filler metal need more time to do this,in addition to solid solution layer is thicker than the original filler metal.The amorphous bonding process is characterized as a four-step process, which is the solid diffusion of the filler metal, the wetting and spread of the liquid filler metal, the liquid diffusion of the filler metal and the solidification of the liquid filler metal. The fist and third stage is more important to amorphous filler metal, however the second stage is major process to crystalline filler metal. So the amorphous bonding process is characterized as "diffusion brazing",and the crystalline filler metal bonding process is characterized as "capillary brazing".A remain layer of filler metal was composed of a mount of(Cu,Ni)3P compound and a few Cu base solid solution. Since the remain layer of amorphous filler metal is thinner than that of crystalline filler metal,the joint strngth of brazing with amorphous is better. Due to the clearance between substrate is thinner, after the element diffusion stage, the thickness of remain layer of amorpouse is thinner and thinner. In the solid diffusion stage of the filler metal, a mount of atoms in amorphous filler metal had diffused into matrix,so remain layer of amorpouse is thinner than crystalline filler metal.Comparing with crystalline filler metal, a good joint could been brazed with amorphous filler metal in shorter brazing time. It was attributed to short physical wetting time when brazing with amorphos filler metal.The brazing capabilty of rapidly solidificatio filler metal is almost same as amorphous filler metal, that is a stonger joint stength, a good wettability, a homogeneous micrstrcture.