| With the development of auto industry, application of laser brazing with Zinc-coated steel are more than before. Cu Si3 is selected as filler metal generally during the process of laser brazing. Wetting ability of filler metal, Interface layer metallurgical behavior, forming of weld seam are the key factors which affect the property of joint.In order to study how these key factors affect the process during laser brazing. A fiber laser(IPG YLS-5000) is employed as brazing heat source in the experiment, combining with domestic and foreign studies of galvanized steel laser brazing, the interface mass transfer behavior was systematically studied by OM, Rigaku X-ray Diffractometry(XRD-D/max-RB), Hitachi Scanning Electron Microscopy(SEM, S-3400N), electronic universal material testing machine(AG-25TA) and Vickers hardness tester(HVS-10). This paper includes four parts:(1) The change rule of wetting ability of filler metal induced by laser parameters;(2) The influence of Zinc-layer on the filler metal’s wetting ability on base metal;(3) The growth rhythm of interface layer and the change of compounds in braze seam during the process of filler metal wetting and spreading on base metal;(4) The effects of wire feeding speed on the joint’s micro-structure, mechanical property, micro-hardness are studied in the last chapter.The results show that: all of defocusing distance, laser power and heating time have effects on filler metal wetting and spreading on base metal. With the extension of heating time, the spreading diameter increases as well. Defocusing distance’s influence on the spreading of filler metal is parabolic, present a trend of downward after rising. Laser power and diameter of filler metal present a linear relationship. Among the three parameters, defocusing distance has the greatest effect on diameter of filler metal.Zinc coating can not only improve the corrosion resistance of base metal but also help the filler metal spreading on base metal. A three-phase equilibrium zone were formed after wetting at the front of filler metal, different from the common three-phase point. The research concerns on the growth of interface layer show that: the interface layer between base metal and filler metal is non-uniform interface layer, it changes along with the distribution of laser energy. A special double layer is formed induced by laser agitation and convection of liquid filler metal. The layer is combined by layer A which near filler metal and layer B which near base metal. The thickness and morphology of layer change with laser heating time. The Fe Si Cu intermetallic compounds in the braze seam mainly have 3 sources:(a) more Fe atoms enter into braze seam due to the strengthen dissolved of interface layer, precipitated due to saturation.Combined with Si atoms and form Fe Si Cu intermetallic compounds;(b) Cu-based filler metal also exist in base metal by diffusion effect. At this time Si atoms in Cu-based filler metal react with Fe, then generated Fe Si(Cu) intermetallic compounds;(c) dynamical interface layer promote the formation of wavy-like and whisker-like intermetallic compounds. Some of them were fragmentated by laser beam then generated Fe Si(Cu) intermetallic compounds. Based on this theory, a growing physic model of the compound in laser brazing was proposed.The distribution of Fe Si(Cu) intermetallic compounds are related to the wire feeding speed.Tensile test indicated these Fe Si(Cu) intermetallic compounds are able to reinforce the joint. The overlap joint mechanical property has an obvious improvement even exceed the base metal due to dispersive phases. Overlap joint of laser brazing on zinc-coated steel fails at the braze seam fracture in a combined way: brittle-ductile fracture. Crack initiation at the toe of joint, it is ductile fracture. Failure at top of the joint adjacent to base metal, it is brittle fracture.Results of micro-hardness shows the Fe Si(Cu) intermetallic compounds precipitated could help increase the joint’s micro-hardness distinctively. |
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