| During a production process of an automotive part, when M10 nut and galvanized steel (thickness is 2.5mm) are connected by project-welding process, the unstable welding will appear, which requires reinforcement with CO2 arc welding. This article aims to analyze the reasons of unacceptable welds quality, and gives some suggestions.Optimizing tests on the basis of the present production welding parameters, the results show that adjusting the welding procedure specification can’t meet the welding quality requirements of projection welding nut, when the pressure is 0.4MPa, the welding current is 28KA,and the welding time is 20 cyc, the soldering intensity obtained is not high, it’s hard to nucleate for the salient points; when current is 30KA, the salient points can nucleate but unstably, the nut bumps will be over-burned, and the weld nugget inside arise Stoma; by observing the nucleation of the solder joints section, it is found that all bump nuggets are different, the fundamental reason is that the welder rigidity is not enough and the mold is easy to wear and tear, which will influence the parallelism of the electrodes, making the nut bump forced uneven during the welding process.The welder is short of stiffness, as to this problem, Solidworks simulation software is used to simulate the forces of each nut bump with insufficient welder rigidity, the results show: less stiffness nut bump will be forced uneven, by adjusting the electrodes and cylinder axis distance L can reduce the unevenness force. By simulation analyzing the compact structure of the electrode holder at work site, the results showed the compact type electrode holder has two defects:Firstly, in the welding process, a cylindrical hole in the side of the electrode holder center is easy to cause stress concentration, which leads to easy wear.Secondly, the compact and the contact surface will have a gap, the gap’s width is more than 40μm, which makes the contact surface’s electrical and thermal conductivity lower, affecting the current density distribution and the cooling of the electrode, aggravating the side wear of the center hole in electrode base.In the case of satisfying the electrodes parallel, the welding parameters were optimized. The result showed that the quality of welding could meet the requirements and the scope of the welding specifications was broadened. Through orthogonal experiments, the optimal welding specifications were obtained, in this case:I2=32 KA, P=0.45 MPa,t4=20 cyc. The scope of the welding specifications was defined as below:electric current ranged from 29 to 32 KA, the welding pressure ranged from 0.4 to 0.45 MPa and welding time ranged from 18 to 22 cyc.The analysis on the mechanical properties of the nut project-welding, the ejection forces of the nuts were greater than 35KN and the maximum could reach 46.3KN. The hardness increased gradually from the plate heat-affected zone to the nut heat-affected zone and the maximum hardness of the nut side heat-affected zone was 514 HV. The analysis of the solder joints fracture morphology showed that brittle fracture occurred in the fusion zone. It also showed that the fracture mode occurred in the plate heat-affected zone was a mixture of brittle fracture and ductile fracture.By improving the plate of mold of projection welding, the distance L between the electrode and the axis of the cylinder can be adjusted. The electrode connected with the electrode holder through taper, which make the current density on the electrode holder stable. At this case, the cooling effect on the electrode will be enhanced obviously. And then we verified the parallelism on the mold, the results which showed were consistent with the analysis by simulation. Moreover, by designing the composite insulation positioning pin, the problem on shunt during the welding process on nut was solved absolutely. |
PDF Full Text Request