| With the continuous deepening and improvement of environmental protection and resource conservation awareness,structural lightweight has become the inevitable trend of the development of modern industry.Magnesium and magnesium alloy structure steel material is the lightest is still the backbone of future automotive materials,in order to reduce vehicle weight and ensure the safety performance of multi material structure type body design is more and more favored by the automobile industry and magnesium and steel connections can not only reduce the overall weight of the structure and ensure the strength of the structure which has a good application prospect while the high physical properties difference between the magnesium alloy and steel giant ambassador is difficult to achieve the effective connection that has become a common concern and need to solve the problem of friction stir welding joints at home and abroad.Because of its high quality,efficiency and clean energy advantages has high application value and market potential so various countries in welding field of friction stir welding technology and model of friction stir welding equipment made a lot of research.Based on the analysis of original laboratory without the key of the friction welding equipment and the operating principle,the friction stir welding equipment without keyhole was designed and manufactured.comparing with the original equipment,new equipment in the main body structure,transmission scheme,welding method and control method made improved design,strengthened the stability of the body and reduce the noise and vibration finished "zero transmission" between the spindle motor and the stirring head.By designing the table could rotate change the original method of welding,increase solder joint area,that has been forming beautiful round spot;in the control system,the hardware and software design of the system with PLC and vector the inverter control as the core,the use of manual and automatic two control mode had the advantage of simple operation,easy to use,improving the welding efficiency.The thickness of 3mm magnesium alloy AZ31 B and 1mm DP600 thick galvanized steel were tested overlap spot welding of new equipment,analysed of mechanical properties of spot welded joints by orthogonal experiment,and researched the joint macro,microstructure and fracture analysis of joints with different welding methods and the connection mechanism and fracture mode in the new equipment,the results showed that:(1)The factors affecting the joint quality of friction stir spot welding without hole for magnesium/steel from large to small were as follows: the shoulder penetration,rotating speed,the length of pin.Considering the mechanical properties of the joints and surface shape effect,the optimal process parameters was the rotating speed is 1200r/min,the shoulder penetration is 0.2mm,the length of pin is 1.7mm;(2)compared with the original laboratory equipment,new equipment in the corresponding parameter range has better overall mechanical properties of welded joints,and the maximum shear load of joint reached 10.1KN;(3)the interface of the shoulder affected zone was smooth and generated the "bamboo" middle layer with a width ranging from 6 to 9 m,but has the crack and the length of bamboo varies in a certain range.With the increasing of the rotating speed and the shoulder penetration,middle layer thickness increased;(4)friction stir spot welding joint without keyhole for magnesium/steel has two kinds of fracture mode: shoulder affected zone was cleavage fracture,namely brittle fracture;stirring zone was ductile brittle mixed fracture;the largest position of joint stress concentration found in the edge of spot welding joints between two magnesium and plate in situ tensile process;in addition,the fracture occurred in the joint surface of magnesium and the middle layer surface,the tensile fracture position of magnesium/steel near the magnesium side,and brittle intermediate layer basically remained in the steel side. |
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