Research On Structure And Properties Of A356 Aluminum Alloy Modified By Friction Stir Processing

Casting aluminum alloy A356 has been widely used in the industrial sector. But its service scope and life is limited due to the coarse α-Al dendrites,eutectic phase,shrinkage porosity and other macroscopic defects. In this paper,Casting A356 alloy was modified by friction stir processing techniques. In order to find the best treatment options, experimenting with different geometry shape stir pins,different passes and different combinations of FSP-heat treatment state have done and microstructure, mechanical properties,corrosion properties of the material have be analyzed. The results show that:Stir zone structure FSPed with threaded tapered pin is most uniform. But obvious eutectic flow line was found in the stir zone which FSPed with tapered pin and square pin. Tensile strength of those materials almost the same,average 38 MPa higher than the base metal and the elongation average increased 16%. The material FSPed with threaded tapered pin have the maximum elongation,which reached to 21%. Threaded tapered pin and tapered pin processed materials have the same hardness,approximately 58 HV. Due to the pulses action,the material FSPed by square pin excess liquidity and it has the minimum hardness,only 51 HV.FSP treated material compared to the casting base metal,eutectic was broken,fracture mode changed from transgranular brittle fracture into porous gather shaped toughness fracture,corrosion tendency decreases and resistance to intergranular corrosion and soak exfoliation corrosion ability increased. Material FSPed by three passes compared to one pass,Al-Si eutectic is more fine and uniform,hardness of the material is slightly decreased while the tensile strength is slightly improved,corrosion tendency further decreased,resistance to intergranular corrosion ability increased but soak exfoliation corrosion ability slightly decreased.In different combinations of FSP-heat treatment state,material modified by FSP+ solution and aging treatment have the most mellow eutectic particles,highest tensile strength and hardness values. Each group of materials’ corrosion potential order is: casting+FSP > solution and aging + FSP > FSP + solution and aging > FSP + aging,while their corrosion current density is opposite and the pitting potential is almost the same. The materials with FSP as the final treatment state have the best intergranular corrosion resistance ability. Materials modified by FSP + solution and aging、solution and aging + FSP have the best soak exfoliation corrosion resistance performance,which is better than casting+FSP treatment,while FSP + aging treatment is worst.