基于增材制造和锻造复合成形的TC4钛合金组织和性能研究

Microstructure and Mechanical Properties of TC4 Alloy Formed by Additive Manufacturing Combined with Forging

采用金相显微镜(OM)、扫描电镜(SEM)观察不同激光功率制备的增材制造/锻造TC4钛合金复合成形试样,并进行了显微硬度测试和拉伸测试。结果表明:微观组织形态均表现为4个区域:基体、过渡区1、过渡区2和增材区。基体为典型的双态组织;过渡区1为部分等轴α溶解中间态组织和β相;过渡区2的组织形态介于网篮组织和过渡区1的组织之间;增材区为典型的网篮组织。拉伸试验表明,过渡区1和过渡区2是复合制造整体试样中强度比较薄弱的地方,对拉伸试样断口观察后发现只有加工激光功率1000 W的水平(I型)拉伸件的韧窝较大且深,其他拉伸试样断裂方式均为半解理半韧性断裂,不同功率复合成形的试样中,激光功率为1000 W的拉伸性能最好,比较水平(I型)和竖直(II型)拉伸试样,II型拉伸件的塑性优于I型拉伸件,抗拉强度、屈服强度与完全锻造拉伸件比较接近,但是延伸率低于完全锻造拉伸件。4个区域中过渡区2的显微硬度最高。

TC4 titanium alloy is a kind of structural material widely used in aerospace,biomedical and other fields,due to the advantages of excellent mechanical properties,high tensile strength,high specific strength,good heat resistance,good corrosion resistance and good biocompatibility. The forging TC4 titanium alloy has good mechanical properties,but it is difficult to form precise parts. Laser solid forming(LSF),a new additive manufacturing technology,has the advantages of fast forming speed,short cycle and near net forming,but it is not suitable for mass production. The combination of forging and additive manufacturing(forging/AM composite forming)has the advantages of both forging and additive manufacturing,and avoids their disadvantages. But there were few researches on forging/AM composite forming. In order to study the feasibility of forging/AM composite forming TC4 titanium alloy components,LSF method was used to manufacture the laser additive materials on the forging substrate of TC4 titanium alloy. The TC4 titanium alloy substrate forging process was 950 ℃ free hammer forging,the deformation was 20%-40% under air cooling condition,and the substrate′s microstructure was typical bimodal structure. The laser powers of additive manufacturing were 1000,1100,1200 and 1300 W,and the reciprocating cycle scanning was used for additive manufacturing processing,other specific parameters were the same:scanning speed was 10 mm·s^(-1),powder feeding speed was 9 g·min^(-1),laser spot diameter was 3 mm. The particle size of TC4 titanium alloy powder used for additive manufacturing was 75-150 μm. The tensile test was conducted on Shimadzu AG-IC100 high temperature stress rupture testing machine at a tensile speed of 1.5 mm·min^(-1). TMVS-1 digital vickers microhardness tester was used to measure the microhardness under the condition of 4.9 N loading and 15 s duration. According to the proportion and direction of additive zone and forging substrate,tensile samples were divided into four types:Type Ⅰ tensile samples with half of the substrate and half of the additive area along the scanning direction,Type Ⅱ tensile samples with half of the substrate and half of the additive area along the stacking direction,complete AM area tensile samples and complete forging substrate tensile samples. Optical microscope(OM)and scanning electron microscope(SEM)were used to observe the microstructure of different area on the forging/AM composite forming TC4 alloy samples prepared at different laser powers as well as tensile fracture. The results showed that the microstructure morphology had four regions:substrate,transition zone 1,transition zone 2 and additive manufacturing area. The microstructure of substrate was a typical bimodel structure;the transition zone 1 was a part of the equiaxed α-dissolved intermediate state with β phase;the transition zone 2 was between the basketweave structure and the transition zone 1;the additive manufacturing area was typical basketweave structure. Under the action of thermal cycle,primary α phase dissolved in β phase in transition zone 1 as the temperature increased,and secondary αphase precipitated out of β-transforming structure as the temperature decreased. In transition zone 2,there was basically no equiaxedα. Hardness of forging area and AM area had little change basically,and the highest hardness was HV 353 in the bonding zone. There was a large cooling rate in the transition zone 2,resulting in the existence of needle-like α’ phase,which increased the hardness of this zone. According to the tensile result,for both Type Ⅰ tensile parts and Type Ⅱ tensile parts,when the laser power was 1000 W for additive manufacturing on the forging substrate,the tensile strength(Type Ⅰ 1091 MPa,Type Ⅱ 1064 MPa)and yield strength(Type I995 MPa,Type Ⅱ 960 MPa)of the samples were the highest,and were slightly better than the tensile strength(989 MPa)and yield strength(881 MPa)of the forging substrate. The elongation of Type Ⅰ and Ⅱ tensile specimens was lower than that of the forging substrate because the fracture location was more likely to occur in the additive manufacturing area or bonding zone. For tensile samples,the plasticity of Type Ⅱ tensile specimen was better than that of TypeⅠtensile specimen. The microscopic reason for this phenomenon was that the microstructure of the AM area in the tensile direction of Type Ⅱ tensile parts was composed of coarse original β-grains and coarser strip α phase,which had better ductility and lower strength compared with the microstructure of β-grains and α phase in the horizontal tensile zone of Type Ⅰ tensile parts. The tensile strength and yield strength were close to that of fully forged tensile sample,but elongation was lower than those of fully forged tensile sample. The tensile test showed that the transition zone 1 and the transition zone 2 were weak places in the overall sample of composite manufacturing. Due to the decrease of equiaxed α phase in transition zone 1 and transition zone 2,the plasticity of transition zone 1 and transition zone 2 was seriously reduced. After observing the fracture of the tensile specimens,it was found that only the Type Ⅰ tensile specimen formed at 1000 W had larger and deeper dimples. The fracture forms of other tensile samples were semi-cleavage semi-ductile fracture. Through the study of mechanical properties and microstructure of forging/AM composite forming TC4 alloy component,it could be concluded that the microstructure,especially the microstructure of the bonding area,determined the mechanical properties of forging/AM composite forming TC4 alloy component. In order to optimize the forging/AM composite forming TC4 alloy component,the microstructure of the bonding zone could be adjusted by heat treatment and other methods,so as to improve the overall mechanical properties.