Study On High Temperature Mechanical Properties Of Laser Melting Mg-4Al Alloy

The investigated Mg-4Al alloy was made by smelting and casting of pure Mg(99.98 wt. %) and pure Al(99.98 wt. %), ignot casting after cutting was homogenizing annealed for the segregation of composition at 480℃ and a period of 8 hours.The Mg-4Al sheet with a thickness of 4mm was formed through double rollers for rough rolling and six rollers for finish rolling,then grinded to 3mm with sand paper.This project was finished by these tests of laser cladding handle,microhardness, Elongation to Failure(EF) and strain rate change(SRC).Microstructure was observed and analysed with optical microscope(OM),scanning e lectron microscope(SEM) and transmission electron microscope(TEM),and the result is that average grain size of Mg-4Al rolled sheet is 41?m approximately after anneal for stress eliminated. The sheet was dealt with by laser melt at a power value of 1200 W and 1600 W respectively, and got melt layer with different thickness, The maximum value is 0.36 mm and 0.43 mm, respectively. the sheet is re markably comprised of matrix,heat affected zone and melted layer. Compared the hardness of Mg-4Al by laser melted, Vivtorinox hardness decreases gradually from surface to inner cladding layer on the substrate, the acculumation of majority of dislocations in the internal melting zone is the major reason that generated hardness enhancement.There are the dimple and parabolic dimples in the high temperature tensile fracture after high temperature experiments for alloy treated, the size of dimples is not the same,this is a typical ductile fracture. Compared alloy plate treated with the sheet that without disposed by laser, the elongation rate is lower generally but all are more than 100%. Through observation for the melted layer by TEM, there are a small amount of sencond phase which is round, besides plenty of dislocation pile clusters.For Mg-4Al, The test temperature chose is 300、400 and 450℃ respectively, when strain rate is 10-2 s-1, Dislocation climb creep(DCC) is the leading deformation mechanism, and when strain rate is 10-3 s-1, deformation mechanism is a competition mechanism between solute drag creep and dislocation climb creep. Therefore, the mechanism is not changed clearly by laser melted. The creep activation energy were calculated for Mg-4Al whose thickness are different for melted layer. The both values are a little higher than the diffusion energy of solute Al atom in the Mg matrix.