The Influence On The Microstructure And Properties For Extrusion On A390Alloy

For the semi-continuous casting A390alloys, the microstructures of as-castalloy, heat treated alloy, and extruded alloy followed by heat treated wereinvestigated by optical microscope and scanning electron microscope(SEM).Their mechanical properties were measured using Instron-5569and5500Relectronic universal testing machine at room temperature and elevatedtemperature. The thermal expansion coefficients were obtained by RJY-1P linedilatometer in the temperature of100-500℃. The wear tests were carried out onthe УТИ TB-1000pintle disc tribometer and the surface morphologies wereobserved by SEM in order to determine the wear mechanism.The results showed that the as-cast semi-continuous casting A390alloy wasconsisted of hard primary silicon with the average size of30-40μm, the needle-like eutectic phases with the maximum length of110μm and the aspect ratiobetween30-80. The fishbone-like Al2Cu distributed on interdendrites. Afterextrusion processing, a small amount of primary Si particles were broken. Theeutectic Si changed into tiny particles with several microns in size and aspectratio around2.The heat treatment was optimized to be solid solution treated at490℃for8h and aged at190℃for8h. The linear expansion coefficient of both semi-continuous cast and extruded A390alloys were measured after heat treatment. At100℃they were about12×10-6℃-1and increased with increased temperature upto26.5×10-6℃-1and23.4×10-6℃-1, respectively at460℃, followed by a slightdecrease.The tensile strength of heat-treated semi-continuous casting A390alloyunder room temperature was243MPa with an elongation to fracture was0.29%,and tensile strength deceased to219MPa with the elongation improved to8.47%when tested at200℃. For the extruded A390followed by heat treated, thetensile strength increased to344MPa with an increased elongation of1.39%, andtensile strength decreased to327MPa with an elongation of14.87%, at200℃.The fractographs of both heat-treated as-cast A390alloy and the extrusionA390alloy followed by heat-treated were all consisted of the brittle fracture ofprimary silicon and the toughness fracture of matrix, showing that their fracturemechanisms were all quasi-cleavage fracture, but the more dimples existed onfractographs of heat-treated semi-continuous cast followed by extrusion A390 alloyThe wear resistance of A390alloy is improved by the extruded treatment.The primary stage is abrasive wear. With the time increasing, it changes to theadhesive wear. Extrusion broke the primary silicon and eutectic silicon anddecreases the size of phases to reduce the probability of silicon particle shedding.Besides, it reduced or eliminated the internal defects such as shrinkage andporosity, resulting in increases in density and the bonding strength of soft andhard phases. Therefore resistant wear properties of A390alloy were increasedafter extrusion..