Study On Resistance Spot Welded AZ31B Magnesium Alloy

Magnesium and its alloys have recently attracted great attention in academic research and industrial application owing to some unique properties such as their low density, high specific strength and specific stiffness, good damping capacity, excellent machinability and castability, and recyclable characteristics. Especially within transportation industries, the weight saving effect of replacing steel and aluminium parts is an important factor in reducing fuel consumption and increasing speed. Besides the development of new alloy types, manufacturing techniques such as welding and joining play an important role in exploiting the new fields of applications. The welding and joining of magnesium alloys, principally laser welding (LSW), electron beam welding (EBW), tungsten inert gas welding (TIG), metal inert gas welding (MIG), friction stir welding (FSW) and hybrid laser-TIG welding, have been investigated, and many useful insight and data have been obtained.Resistance spot welding (RSW) is a popular process for assembly lines in the automobile and other industries manufacturing a variety of products made of thin gauge metals, the autobody assembly requiring about 7000-12000 welding spots depending on the size of vehicle. RSW of magnesium alloys has recently shown increasing importance because of magnesium alloys'potential for reducing weight and improving fuel economy in automobiles. As a result, the resistance spot weldability of magnesium alloys is of significant interest for automobile manufacturing process. However, published works on the RSW of magnesium alloys are limited, there is a lack of understanding on the resistance spot weldability of magnesium alloys, and further research and development efforts are required to utilize the full potential of these materials.Effects of welding parameters ( welding time, welding current and electrode force ) on the microstructure and mechanical properties of resistance spot welded magnesium alloy joints have been investigated. Results show that welding time, welding current and electrode force have obvious effects on the microstructure and mechanical properties of joints. An increase in welding time ( 1-16 cycles ) results in an increase in nugget diameter and a coarser microstructure of nugget. The dendrite arm spacing of the nuggets is in 4.76μm-7.12μm. The joint strength of 2930-3050 N can be obtained with welding time longer than 6 cycles under the conditions of this investigation. An increase in welding current ( 15-23 kA ) results in an increase in nugget diameter with attendant increase in joint strength due to increasing the nugget diameter. Higher welding currents ( 25-27 kA ) cause metal expulsion during welding operation owing to excessive heat input and result in reducing joint strength. In the range of 1.5-4.5 kN electrode force, lowering electrode force favors increasing the nugget diameter and improving the joint strength, but too low electrode force ( 1.5 kN ) causes metal expulsion during welding operation.Theα-Mg andβ-Mg₁₇Al₁₂ phases were identified in the XRD pattern. The peak corresponding toα-Mg is much stronger than that ofβ-Mg₁₇Al₁₂, indicating that weld nugget consists ofα-Mg phase and a small amount ofβ-Mg₁₇Al₁₂ phase. The spot welded joints have two failure modes ( interfacial failure and button pullout failure ) under tensile shear loading conditions. For the button pullout failure, the crack initiates at cellular dendritic structure of nuggets and propagates along the cellular dendritic structure, heat affected zone ( HAZ ) and base metal in sequence.Inoculants were used for improving microstructure and refining grain of weld nugget. The inoculant effect of KBF4 on the primaryα-Mg is more effective than that of K₂TiF₄. The reason may be the presence of B in weld nugget. Nugget is nearly occupied by equiaxed dendritic crystals with KBF4 inoculant, and refinement grain is obtained. Visible defects are almost disappeared. Under the condition of equivalent nugget diameter, mechanical properties of joint with KBF4 inoculant are improved to some extent.During the spot welding of magnesium alloy, the physical and mechanical features of magnesium alloys easily raise various welding problems, such as hot cracking, shrinkage cavity, expulsion and electrode degradation in the welded joint.The magnesium alloy weld nugget has a high susceptibility of hot cracking, which is attributed to high coefficients of thermal expansion and volume shrinkage. Hot cracking is very detrimental to mechanical properties of spot welded magnesium alloy joint. Crack features of welded joint and effects of welding parameters on susceptibility of hot cracking in the joint have been investigated. In the spot welded joints, solidification cracking in weld nugget and liquation cracking in heat-affected zone (HAZ) were often observed. The formation of solidification cracking is related to low melting point liquid films between dendrites due to segregation of Al and Mn atoms and tensile stress developed during cooling. The welding parameters (heat input) have an obvious effect on susceptibility of solidification cracking in weld nugget. The solidification cracking appears in the weld nugget when the welding current is higher than 15 kA for 8 cycle welding time and 2.5 kN electrode force, the welding time is longer than 4 cycles for 23 kA welding current and 2.5 kN electrode force, or the electrode force is lower than 4.5 kN for 23 kA welding current and 8 cycle welding time. The susceptibility of solidification cracking increases with heat input rising due to increasing tensile stress caused by the shrinkage of the nugget, lowering the melting point of liquid films due to solute atom segregation with greater concentration and prolonging time for tensile stress affecting. In HAZ, the grain boundary melting occurred and grain became coarser. The liquation cracking appears in HAZ just adjacent to weld nugget and may be induced by solidification cracking at the edge of weld nugget. The formation of liquation cracking is mainly associated with low melting point liquid films from solute atoms of low solubility diffusing to the melted boundaries and tensile stress developed during cooling. It is favorable to select relatively low heat input (i.e. relatively low weld current, short welding time or high electrode force) for reducing the susceptibility of hot cracking in weld nugget and HAZ (solidification cracking and liquation cracking).The shrinkage cavity is mainly located at the weld nugget centre. Therefore, the strength of spot welds which failed via the pullout failure is not considerably affected by shrinkage cavity. But, shrinkage cavity with hot cracking should be avoided, because it is detrimental to mechanical properties of joint. Besides, a common phenomenon in resistance spot welding is expulsion, i.e., ejection of liquid metal from the nugget during heating. Expulsion happens at either the faying surface or the electrod-workpiece interface. Expulsion at the faying surface helps material flow in weld nugget and induces the high tensile stress and strain in the solid around weld nugget, which should result in the appearance of liquid cracking in HAZ. The reduced joint strength was obtained owing to expulsion easily causing liquid cracking and shrinkage cavity in joints and weld imperfection at the edge of nuggets. It is favorable to select relatively low heat input for improving mechanical properties of spot welded magnesium alloy joint.Detailed investigations of the metallurgical interactions between the copper electrode and magnesium alloy sheet were performed using SEM and EDS. The experimental results indicated that electrode degradation, which eventually leads to weld failure, could form in three basic steps: magnesium pickup and electrode alloying with magnesium, electrode tip face pitting, and cavitation. Magnesium pickup begins even after a few welding spots as tiny drops of molten magnesium alloy are transferred from the sheet surface to the electrode tip face. The molten magnesium alloy adheres to and reacts with the electrode to form local and complex regions of Cu-Mg alloys (CuMg2 and Cu2Mg). The breaking up of the local alloyed regions, either through transfer of molten Cu-Mg mixture or brittle fracture of solidified Cu-Mg intermetallic phase(s), would result in electrode pitting, i.e., material loss from the tip face. Initial pitting occurs on the contact surface and then grows to form large cavities by combining smaller pitted areas. Since pitting and cavitation are results of Mg pickup and alloying, periodic electrode tip face cleaning could extend electrode tip life by limiting the buildup of Mg on the tip face.Since welding quality is related to welding parameters, it is favorable to optimizing welding parameters for saving costing of production and improving welding quality. By means of the quadratic regression combination design process, the regression equations of nugget diameter and tensile shear load of spot welded joint were established. Effects of welding parameters on the nugget diameter and the tensile shear load were investigated. The results show that effect of welding current on nugget diameter is the most evident. And higher welding current will result in bigger nugget diameter. Besides, interaction effect of electrode force and welding current on tensile shear load is the most evident compared with others. The optimum welding parameters corresponding to the maximum of tensile shear load have been obtained by programming using Matlab software, which is 4.7 kN electrode force, 28kA welding current and 4 cycle welding time. Under the condition of the optimum welding parameters, the joint having no visible defects can be obtained, nugget diameter and tensile shear load being 6.8 mm and 3256 N, respectively. The joint consists mainly of weld nugget and heat-affected zone (HAZ). Besides, weld nuggets contain fine cellular-dendritic and equiaxed dendritc structure.An axisymmetrical finite element model was developed for studying the distribution of temperature of resistance spot welding joint to predict weld nugget growth. At the beginning of resistance spot welding, heat generation at faying surface governs nugget formation. The temperature elevation speed at faying surface is so quick that weld nugget has formed in the first cycle. The temperature at faying surface increases slowly after 2 cycles due to dynamic resistance decreasing and heat extracting rising. Besides, the heat generation due to the contact resistance at the electrode-specimen interface was found to be also important during nugget growth. The results of finite element agree well with those of the experiments for nugget diameter. It was approved that the simulation of the welding process is an efficient tool for predetermining welding parameters. Further understanding has been obtained for the resistance spot weldability of magnesium alloys by the above research. A variety of problems have been worked out during spot welded magnesium alloys to some extent. Actual and detailed theories are supplied for the process of manufacturing magnesium alloy products.