| Titanium alloy has been widely used in aviation and aerospace industry due to its high specific strength,excellent high temperature performance and corrosion resistance.The complex integrated titanium alloy structure represented by large titanium alloy frame structure and integral skeleton structure of aerospace vehicles is the key component of national defense equipment.However,traditional manufacturing methods characterize low material utilization,low machining efficiency and high cost.Arc and additive manufacturing technology is one of the important manufacturing means to realize high performance,low cost,light weight of these key components,and it has significant demand in the field of equipment manufacturing.In this paper,TC4 titanium alloy,which is widely used,is used as the research material.This paper carried out researches on thermal filed,formability,microstructural characteristic and mechanical properties of the TC4 thin-wall parts produced by GTAW-based WAAM.The results show that the deposited materials will experience a thermal cycle in which the temperature rises and falls rapidly and the peak temperature decreases gradually during the process of WAAM,and most areas of the samples will undergo four or five phase transitions.The residual stress is basically distributed symmetrically,and the area of the maximum residual stress is located at the root of the thin-wall parts.The unique thermal cycles make the section of the thin-wall parts show three distinct areas.The heat treatment of the following layer to the preceding layer in the ?+? phase field produced a band structure with bundles of ? laths.The hardness in the band region is lower than that of other regions(~20 HV),the hardness in the bottom regions and top regions is slightly higher than that in the middle regions.The tensile strength of the tensile specimens varies between 826 MPa and 948 MPa,the elongation varies between 10 % and 15 %.The horizontal specimens have higher tensile strength and lower elongation than the vertical ones.The results about deposition paths show that the thin-wall part deposited by reciprocation path is relatively well formability,and its side waviness is large at both ends and small at the middle.When the thin-wall part is deposited by reciprocation path,the temperature change rate at both ends of the thin-wall part shows a trend of interlacing layer by layer,which makes the residual stress of the thin-wall part distributed more evenly on the whole than that of the thin-wall part deposited by co-directional path.In terms of microstructure,the ? grains from the thin-wall parts deposited by co-directional path grow epitaxially,which are inclined relative to vertical direction.While those by reciprocating path grow in a relatively disordered direction.In terms of mechanical properties,in the vertical direction,the tensile strength(850MPa)of samples in the reciprocating deposition path is lower than that of samples in the same deposition path(900MPa).The results of oxygen concentration in ambient show that the internal oxygen content meet the requirements of ASTM B381-2013 when the oxygen concentration in ambient is less than 1000 ppm.What's more,the microstructure will be refined to a certain extent,the hardness and tensile strength increase gradually and elongation decrease slightly with the increase of oxygen concentration in ambient.The hardness of samples deposited in the air environment increase to 500 HV,but the tensile strength and elongation of the samples decrease sharply at the same time.Brittle fracture is the main type of fracture at this moment. |
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