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Influence of Grain Size on Microstructure and Mechanical Properties of Ti Alloy in TIG
Abstract
Fine grain TC4 alloy with various grain size(2μm,8μm,19μm)were welded by conventional TIG respectively,the microstructure and mechanical properties of welded joint were tested.The results show that the microstructure of weld center(WC)and heat-affected zone(HAZ)are α martensites.With the same parameters,and the reduction of grain size of base metal(BM),the microstructure of WC and HAZ change from lamellar α structures to basket α structures.With the reduction of grain size of BM,the grain growth tendency of HAZ become more apparent,the width of FHAZ is reduced,and the width of CHAZ is increased obviously,the crystal grain gradient of WC-HAZ-BM is increased,the grain transition of these three zones got worse,and the tensile strength and elongation were increased slightly.Tensile fractures of welded joint present character of quai-cleavage crack.Cleavage features of fractographs of weld joint are more obvious with increasing grain size of the base metal.
TC4 wires of Φ4.0 mm, Φ3.6 mm, Φ3.0 mm, and Φ2.8 mm were welded via TIG welding, then were drawn to Φ2.6 mm, the drawing deformation was 13.8%, 19%, 24.9% and 57.8%, respectively. The mechanical properties between the solder and base metal were analysed. The results showed that the drawing deformation of 13.8% and 19% lead to brittle fracture; while there was no brittle fracture in the sample with the drawing deformation of 24.9%, but the elongation was only 5.5%; drawing deformation of 57.8% showed good strength and ductility, the tensile strength was 1060 MPa, and the elongation was 14%, microhardness was 311 HV0.3. Drawing after welding could improve the property of solder metal, with the increase of drawing deformation, and the impact of welding was gradual elimination. Therefore, to ensure the properties of welded metal after hot drawing, there should be enough drawing deformation.
Recent research and development on high-strain-rate superplasticity has led to the concept of an accommodation helper mechanism
and a new opportunity for superplastic forging. In this article, works about processing for ultrafine-grained materials and
deformation mechanisms and high-strain-rate superplasticity applications are reviewed.