Research On Laser Welding Of Brass-stainless Steel

The welding structure of brass and stainless steel has been used more and more widely in the field of refrigeration equipment, mechanical manufacturing and automobile industry. This kind of composite structure not only has excellent thermal conductivity characteristics of brass, but also has superior mechanical properties of the stainless steel. However, there are many differences in the thermal physical properties between the brass and stainless steel, so weldability between brass and stainless steel has attracted increasing attention. Laser welding with the features of high energy density, high heating and cooling speed is conducive to dissimilar metals welding. So, in this paper, laser welding on brass and stainless steel was researched. This research has important theoretical significance and practical application value on welding mechanism for dissimilar welding of high energy density of heat.In this paper firstly researched the macroscopic feature, microstructure, interface characteristics, elements diffusion and mechanical property in the welded joint on the two different lap joint types(brass on top and stainless steel on top). This result showed a fusion zone with “cup” shape during laser lap welding with brass on top, and that a fusion zone with “dish” shape during laser lap welding with stainless steel on top. There was no intermetallic compound formation in the welded joint of brass and stainless steel. Brass and stainless steel existed in the form of mechanical mixture, and non-equilibrium distribution in the welded joint. There was a narrow transition region in the solid-liquid interface. The different lap joint types had the different transition form. In the transition zone was mainly the compound of brass and stainless steel. This research demonstrated that the brass on top was more suitable for the welding of brass sheet to stainless steel sheet.In this paper, the lap joint numerical analysis model was presented on brass to stainless steel laser welding. The temperature field and flow field was researched. The mechanism of chemical composition and distribution of welded joint was analyzed. The mechanism of the weld forming characteristics, and the chemical composition and distribution was researched. The forming characteristics of the weld joint, mixing characteristics of the welded joint and the interface, and the influence factors of non-equilibrium distribution of brass to stainless steel were explained.This paper carried out the continuous and pulsed laser welding of brass sheet to stainless steel sheet in lap joint. The influence of parameters of laser power, welding speed in continuous laser welding and parameters of pulse laser peak power, average power and pulse time in pulsed laser welding on formation, composition and distribution, weld micro-crack and the mechanical properties of the welded joint were researched, in addition, the law was summarized. For continuous laser welding, when the heat input was constant, the welding speed was the main factor to control the welding quality. For pulsed laser welding, peak power and pulse duration have opposite effect on the penetration-to-width ratio of welded joint when the pulsed laser energy was constant.At the same time, this paper also carried out the micro-crack mechanism and influence factors of the brass sheet to stainless steel in laser welding of lap joint. The researches showed that the crack width was less than 0.2μm when brass on top, and was 4.08μm when stainless steel on top the crack width. The appearance probability of the micro-crack was increased with the increase of stainless steel content in the weld. For continuous laser welding, the micro-crack number was increased with the increase of welding heat input. For pulsed laser welding, and the pulsed energy was constant, the micro-crack number was decreased with the increase of laser power density when to the molten pool of the fusion quality of stainless steel was similar to each other. For pulsed laser welding, the micro-crack number and size were increased with the increase of average power when the laser power density was constant. The mechanism of crack appearance was that the molten pool and the transition region had mechanical mixture of the brass and stainless steel. In the weld crystallization process, austenitic stainless steel firstly appears condensing crystallization because of the difference crystallization temperature between the stainless steel and brass. The liquid brass diffused along the austenite grain boundary, and with the crystallization of tensile stress, micro-cracks appeared in the interface.At last, this paper also carried out the tensile-shear force of the lap joint. The researches showed that the welded joint’s tensile-shear force value of continuous laser welding was 200 N larger than pulsed laser welding. Welded joint fracture occurred in the transition zone on the brass side. Cracks number in the transitional zone and in the coarse microstructure in fusion zone were the main factors to influence the tensile-shear force.