| High-pressure die castings of magnesium alloy and aluminum alloy are widely used in the fields of automobile,3C products and electrical appliance manufacturing as lightweight components in engineering.In order to fully exert their respective material characteristic and improve the flexibility of structural design,it is necessary to weld the two in some engineering applications.Due to the historical characteristics of the high-pressure die casting process,there are often a large number of micro pores in die casting components.During fusion welding,these pores will lead to serious pore defects and generate a large number of brittle and hard intermetallic compounds in the weld,which seriously deteriorate performance of the joint.Friction stir welding,as a solid state technology,can avoid serious porosity defects and effectively reduce the generation of a large number of intermetallic compounds than fusion welding,becoming an ideal choice for high quality welding of die castings of magnesium alloy and aluminum alloy.In this subject,the joining mechanism of friction stir welding of dissimilar high-pressure die castings of magnesium alloy and aluminum alloy is studied.AZ91D magnesium alloy and ADC12 aluminum alloy are taken as the research object,and the effects of process parameters such as the relative position of materials,the offset of stirring pin,rotational speed and the like on the macro-morphology and microstructure of the joint are analyzed.The process conditions of friction stir welding process of die casting magnesium/aluminum dissimilar alloy,the evolution law of microstructure and its influence mechanism on the mechanical properties of the joint are emphatically studied.The main conclusions are as follows:?1?Friction stir welding process of high-pressure die castings of AZ91D magnesium alloy and ADC12 aluminum alloy shows that friction stir welded joint with good appearance formation and excellent mechanical properties can be obtained when the ADC12 aluminum alloy was placed on the advancing side and the tool offset to the aluminum alloy by 0.5 mm at the welding speed of 40 mm/min and the rotational speed of 900 rpm.The maximum tensile strength can reach to 102.6 MPa,which is 51%of the base metal.?2?Mg alloy and Al alloy are macroscopically mixed and mechanically interlocked with each other in stir zone,and the structure presents strip-like characteristics.Large pores in the original parent metal are transformed into dispersed micro-pores,and the number and size of micro-pores increase with the increase of rotating speed.The as-cast structure and banded structure coexist in the heat engine affected zone on the forward and backward sides of the joint.The as-cast structure in the heat affected zone is larger than that of the base metal,but the change is not obvious.?3?The as-cast coarse?-Al and?-Mg phases are obviously refined in the stir zone.There are Si,Mg17Al12,Mg2Al3 and Mg2Si phases with granular dispersion distribution in stir zone.With the increase of rotational speed,the particle size shows a certain trend of decreasing first and then increasing.?4?The fracture occurred at the junction of the heat affected zone and the stir zone on the advancing side?aluminum side?.Microstructure mutation and brittle phases were the main factors causing the fracture.The fracture showed typical quasi-cleavage fracture characteristics consisting of cleavage steps,tear edges,micro pores and a few dimples.?5?The effect of Cu on friction stir welding joint was explored.Compared with the welded joint without copper foil,under the same conditions and experimental parameters,copper element is dispersedly distributed in the stir zone.However,magnesium element is accumulated in the advancing side of the stir zone,and distributed in the junction of the heat affected area and the welding stone area in a fiber ribbon shape,which may be related to insufficient plastic flow of materials,thus deteriorating the performance of the joint. |
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