| The energy and environment problems caused by the increase of automobile are becoming more and more urgent. Weight reduction of automobile is the most effective way to solve these problems. Dual-phase steel (DP) is such a kind of material used in automobile production to realize automobiles'weight reduction without decrease strength and stiffness. Therefore, the traditional low carbon and low alloy steel used in automobile industry are gradually replaced by high strength steels such as DP. Resistance spot welding process is still the major joining method in automobile industry due to its advantages such as low cost and easy automation. However, the special microstructure of DP will cause asymmetric cooling speed and result in crack and shrinkage voids during resistance spot welding process. The strength of nugget will lower than base material and interfacial fracture failure mode will appear during mechanical test. This failure mode will reduce the cross tension strength by 10% and low cycle fatigue strength by 25%. As a result, studying the mechanism, setting up evaluation model and optimizing welding process parameters to avoid the interfacial fracture mode of DP are becoming more and more important. This dissertation is conducted under this situation.Based on the background mentioned above, 1.4mm thick DP600 is selected as the research material. Firstly, tensile-shear test and fatigue test are conducted to study the influence of interfacial fracture mode DP600 weld on the weld quality to investigate the mechanism of DP's interfacial fracture mode. Secondly, critical weld diameter is determined from the builded module of weld's mechanical test to evaluate the failure modes of DP. Thirdly, transformation rules of martensite during resistance spot welding are investigated. Holding time, post-heat, multi-pulse and forging force are studied to find out their influence on micro-hardeness of spot welds. At last, welding process parameters are optimized using the response surface methodology (RSM) to avoid interfacial fracture failure mode of DP. The purpose of this dissertation is to study the mechanism, improve weld quality and optimize welding parameters for DP welds. The main tasks are listed as follows.(1) Characteristic and mechanism study on DP weld's interfacial fracture modeThe martensite transformed during the quench stage of resistance spot welding process will make the weld joint brittle and cause interfacial fracture failure mode. It will greatly decrease DP weld's quality. This section analyzes the different failure modes of DP welds and defines their fracture degrees. 0 degree represents good weld quality with button pull-out mode while 1~4 degree represents interfacial fracture mode. The fracture magnitude will increase as the degree increases. Tensile-shear and fatigue test are conducted to study the influence of different fracture degrees on weld quality. Results show that interfacial fracture higher than 2 degree will greatly affect the weld quality especially on the fatigue life. The effect will be greater as the increase of fracture degree. Based on these studies, metallographic and SEM experiments are used to investigate the mechanism. It can be concluded that the load capacity of weld, microstructure of martensite are the source of interfacial failure mode of DP weld. The conclusions will provide theoretical support to the following investigation.(2) Study of mechanical evaluation module for DP weld's interfacial fracture modeFor the traditional nugget formula 4 t can not evaluate the high strength steel weld quality effectively, this chapter calculate the critical nugget formula based on the different failure mode of DP weld under tensile-shear test. Mechanical module of weld nugget is builded considering the thickness, indentation and micro-hardeness of weld material. According to the different stress distribution under interfacial fracture mode and button pull-out mode, the maximum force for these two kinds of failure modes are determined based on Tresca criteria and limit stress theory. Then critical weld diameter is calculated. 1.4mm and 1.8mm thick DP600 steel are used to validate the critical weld diameter. Compared to the experiential formula, the critical formula can evaluate interfacial fracture mode of DP weld effectively.(3) Microstructure analysis of DP weld's interfacial fracture modeConsidering that the weld's microstructure is the source of DP's interfacial fracture mode, influence of welding process parameters on microstructure transformation is investigated by theoretical and experimental methods in this chapter. According to the continous cooling transformation (CCT) diagram of Fe-C alloy and cooling stage of spot welding process, martensite transformation is the dominate process of microstructure transformation. Content of martensite and Vickers hardenss are measured for different fracture modes to see the influence of martensite performance on fracture modes. At last, the influence of welding parameters, such as holding time, post-heat conditions, multi-pulse welding, forging force experiments are conducted to do find out their single effect on the microstructure. The results can provide foundation for the parameter optimization in the next stage.(4) Control of DP weld's interfacial fracture mode by optimizing welding process parametersFocus on welding parameters optimization to avoid DP's interfacial fracture failure mode, response surface methodology (RSM) is used to conduct welding parameters design, analysis and optimization. Nugget size ratioηof real nugget diameter d versus critical nugget diameter dCr is used in this section to evaluate interfacial fracture degree of DP welds. Quadradic equation module is concluded from the RSM and uniform experimental design matrix to show that welding current, time and forging force are the most important factors that influence the nugget size ratio while welding force, hold time and post-heat time are the unimportant factors. Considering the maximum of nugget size ratioηand minimum insensitive to welding parametersψ, the optimum welding process parameters are concluded using the SQP method. As a result, control of DP steel weld's interfacial failure mode by optimizing welding parameters is realized.Based on the contents mentioned above, a general and systemic analysis of DP weld's interfacial fracture failure mode can be got. The characteristic, mechanism, evaluation and control of DP weld's interfacial fracture failure mode are studied. It can provide theoretical and experimental support for welding DP steels. Besides, it will help the implementation of DP steel in automobile production.
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