Study On The Microstructure And Wear Resistance Of Semi-solid Squeeze Mg₂Si/Al Composite

Mg₂Si/Al is a kind of aluminum matrix composite with the characteristics oflight-weight, high strength, inexpensive, low expansion, heat-resisting andwear-resisting. The reinforcement in the composite is easy to be generated and isconvenient for preparation; it has a good compatibility with matrix; its interface isclean; it has high bonding strength and proper hardness and modulus. The biggestweakness of Mg₂Si/Al composite is its low toughness and big brittleness. The adventof semi-solid forming technology makes it possible to improve on the toughening ofMg₂Si/Al composite. The application of such technology can enhance compactnessand mechanical properties of tissue; it can also reduce macrosegregation, refining thealloy grain, decrease the air holes and shrinkage cavities and prevent the internaldefect.This thesis prepare the semi-solid extrusion Mg₂Si/Al composite based onisothermal heat treatment; research the impact of technological parameters such asisothermal temperature, isothermal holding time and extrusion pressure onsemi-solid microstructure; discuss the spheroidization mechanism during constanttemperature process; study the technology for heating processing and wearingcharacter of semi-solid squeeze composite. The structure of the thesis is as follows:（1） The experiment finds that the Mg₂Si/Al composite prepared by three formingmethods including mainly consist of common gravity casting, liquid forging andsemi-solid squeeze mainly consist of aluminium matrix, Mg₂Si reinforced phase,CuAl2phase and eutectic Si, but these compositions have differences in theirform and content in microscopic structure; the experiment shows that in theprocess of die forging, the crystalline Mg₂Si/Al composite gets solidificationunder pressure, the crystalline grain is refining, the appearance of blackreinforced phase Mg₂Si turns to be spheroidized, the size of white α-Al grain getssmall and distributes more evenly and the spheroidized trend also appears, theshape of eutectic structure turns to be acicular or vermicular grains from thickacicular or fibrous; in Mg₂Si/Al composite microscopic structure shapeed bysemi-solid squeeze, both the reinforced phase and α-Al are double spheroidized;the eutectic structure turns to be smaller and distributes as fine network; parts ofthe eutectic structures are melted in the process of isothermal heat treatment and separated out eutectic Al and eutectic Mg₂Si, and other parts of eutectic Al mixtogether and formed fine spheral α-Al phase in eutectic structures.（2） The comparison of Brinell hardness numbers of Mg₂Si/Al composite underdifferent forming conditions turns out that the Brinell hardness number of liquidforging composite is increased by23.5%than common gravity casting composite,and the semi-solid squeeze composite get the highest increase of39%.（3） The research on the technological parameter of semi-solid squeeze Mg₂Si/Alcomposite finds that when the isothermal temperature is565°C and theisothermal time is60min, the reinforced phase and α-Al of microscopic structureof the composite gets the best effect on double spheroidized, and the Brinellhardness number also arrives to the highest; the microscopic structure of thesemi-solid squeeze composite has no obvious change under extrusion specificpressure, but it is good for the densification of the structure and it increases thehardness number of composite.（4） In the process of isothermal heat treatment, the reinforced phase is formed andthe double spheroidized of the matrix happens in microscopic structure, and partsof the eutectic structures with low melting point are melted and separated outeutectic Al and eutectic Mg₂Si. Parts of the eutectic Al mixed together with α-Al,the other parts mixed together in the eutectic structure and formed fine spheralα-Al phase; parts of the eutectic Mg₂Si mix together with primary Mg₂Si phaseand the other parts mix together in the eutectic structure and form fine spheralMg₂Si phase（5） The experiment shows that the hardness of semi-solid squeeze composite hadobvious increase through T6heat treatment; when the aging time arrives to4h,the composite get the maximum hardness number; meanwhile, in themicroscopic structure of the composite, separate out the fine punctate orvermicular grains, which is available to increase the whole performance of thematerial; Through T6heat treatment and compare with the gravity casting, thestrength of extension and ductility of semi-solid squeeze composite materials areincreased by14.2%and138.6%respectively; it means that semi-solid squeezeforming technology can help to enhance the toughness of Mg₂Si/Al composite.（6） The abrasion experiment finds that when the grain granularity is same, thesemi-solid squeeze composite has a stronger wear-resisting property if the load ischanged; when the grain granularity is different, the semi-solid squeezecomposite has a stronger wear-resisting property comparing with common gravity casting composite and liquid forging composite; the research shows thereason that the increase of wear-resisting property of the semi-solid squeezecomposite: during the wear process, the eutectic structure of the semi-solidsqueeze composite is refining, Mg₂Si reinforced phase and α-Al are doublespheroidized and spread more evenly, increase the combining capacity ofreinforced phase and matrix, at the same time, the structure turns to be morecompact under pressure; increase the supporting function of reinforced phase andit is not easy to be broken; prevent the plastic deformation of the material to alarge extent; increase the capacity of resistance to plastic flow of the material,thus wear-resisting property of the semi-solid squeeze composite is higher thancommon gravity casting composite and liquid forging composite.