| In this paper,Al-x%Mg2Si(x=15,20,25,30,40 and 50)and Al-25%Mg2Si-x%Bi-x%A15Ti1B(x=0.2,0.4,0.6 and 0.8)composites were prepared by casting.The microstructures and properties of the composites were modified effectively by hot-extruded.And the vacuum brazing of the hot-extruded Al-x%Mg2Si(x=15,25,and 40)composites were investigated.The main results are as follows:After the Al-x%Mg2Si composites were hot-extruded at 4000C,the outlines of Mg2Sip were rounded,and the flake-like Mg2Sie was broken into separated short rod-like or dot-like shapes.Due to the changes in morphology and reduce of porosity,the flexural strength and elongation values of the hot-extruded composites were 243MPa,217 MPa,168 MPa,144 MPa,99 MPa and 6.1%,4.2%,2.4%,1.5%,0.8%,which were significantly improved,in compared to the as-cast ones which were 197MPa,192 MPa,122 MPa,111 MPa,93 MPa,66 MPa and 1.8%,1.7%,0.9%,0.6%,0.3%,0.2%.The room temperature coefficient of thermal expansion decreased from 14x10-6/K to 10x10-6/K after hot extrusion.Thermal conductivity of the hot-extruded composites were 20.7W/(m-K),18.9W/(m-K),17.7W/(m-K),14W/(m-K)and 8.4W/(m-K),which were significantly improved,in compared to the as-cast ones which were 14W/(m-K),12.5W/(m-K),11W/(m-K),10.5W/(m-K),8W/(m-K)and 5.3W/(m-K).When adding 0.4%Bi,0.6%A15TilB,and 0.4%Bi+0.4%A15TilB modifiers into melts respectively,the average size of Mg2Sip particles DMgaSip in the as-cast Al-25%Mg2Si composites were reduced from 202?m of unmodified composite to 140?m,117?m,and 136?m,respectively.The flexural strength of the composites increased from 122MPa of unmodified composite to 141MPa,145MPa,and 137MPa,respectively.Decreasing extrusion temperature,refinement of the reinforcement of Mg2Si was much significant which in turn contributed to the flexural strength increase of the composites.The largest flexural strength of 183MPa was obtained in our study.The plasticity of the extruded composite increased with increase extrusion temperature.The largest elongation to failure was 1.9%measured with the composite underwent extrusion at 360?.Then,it is found longation to failure will decrease for the composite while with extrusion temperatures above 360?.After modification to the composites hot extrusion,it was found refined Mg2Sip due to modification is more difficult to break and refine further more,so the values of DMg2Sip and flexural strength of the hot-extruded composites do not present significantly improve in compare with those of the unmodified composites.The fracture morphology shows that the better the crushing effect of Mg2Sip,the worse the bonding force of Mg2Si/Al interface.With the increase of the content of Mg2Si,the size of Mg2Sip increase,the combination condition between welding solder and the base composite becomes worse,and the shear strength of the welded joint is reduced.The shear strengths are 71.8MPa(uncut),43.9MPa,and 33.6 MPa,respectively for the composite with 15%,25%and 40%contents of Mg2Si. |
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