基于丝材电弧增材制造Ti6A14V-xB合金的组织性能及模拟

发布时间：2018-09-10 18:16

【摘要】：钛及钛合金具有高强度、低密度、耐高温、耐腐蚀、无磁性和良好生物相容性等优点被广泛应用于各个领域,然而传统的钛及钛合金成型工艺需要真空熔炼、锻造及大量切削等后续处理,使得材料利用率降低,增加了生产成本,采用增材制造技术可以缩短工艺,提高钛合金材料的利用率,配以合金成分优化,可以提高钛合金的力学性能。基于此,本文研究了钛合金添加微量硼元素结合丝材电弧增材制造技术,并对钛合金丝材电弧增材制造过程中快速凝固组织的演变进行了模拟。首先,采用高真空非自耗熔炼及吸铸方法制备不同硼含量的Ti6Al14V-xB(wt%,x为0,0.05,0.1,0.5)合金,研究了不同微量硼元素添加对Ti6Al4V-xB的铸造显微组织及力学性能的影响;其次,采用Ti6Al4V及Ti6Al4V-0.05B丝材为原料,以电弧为热源,将钛合金丝材熔融并逐层堆积实现快速增材制造,研究了钛合金丝材电弧增材制造的凝固过程、组织形貌和力学性能等;最后,采用元胞自动机-有限元法对Ti6Al4V及Ti6Al4V-0.05B合金丝材电弧增材制造的快速凝固过程进行模拟计算,进一步探究了丝材电弧增材制造不同阶段的固-液转变、初始β晶形核及生长等机理。通过以上研究,得出以下结果:(1)微量B的添加影响了钛合金初始β晶生长,在固-液前沿富集B元素阻碍初始β-Ti长大,有效细化晶粒,当硼含量超过0.1wt%时,则有明显的TiB相析出。Ti6Al4V-xB合金的抗拉强度极限随硼含量的增加单调上升,这是细晶强化和析出强化共同作用的结果;合金的塑性则是先升高后降低,Ti6Al4V-0.05B的塑性提高了 15%,而Ti6Al4V-0.1B与Ti6Al4V-0.5B的塑性则降低超过了 40%,因为析出脆性的TiB相,形成脆性断裂敏感带。(2)在Ti6Al4V丝材电弧增材制造过程中,由于电弧具有高的热量输入,使得每个堆积区-熔合区-堆积区得到了有效的冶金结合,没有明显的堆积分界面和钛马氏体存在,各区域的显微组织均为稳定的α+β片层组织以及接近的显微硬度值。与铸态Ti6Al4V相比,电弧增材制造的钛合金不仅初始β晶粒细小,而且α+β片层间距也较小,其抗拉强度与延伸率相比铸态均有所提高,拉伸断口为细小韧窝状的韧性断裂。(3)在Ti6Al4V-0.05B丝材电弧增材制造过程中,获得了更加细小晶粒的组织,同时有少量不规则针状TiB析出;相比较于铸态Ti6Al4V-0.05B合金,晶粒尺寸有所减小并且更加趋于枝状晶形貌,其抗拉强度提高了 6.2%,延伸率增加了 28.7%。(4)在Ti6Al4V丝材电弧增材制造的凝固组织演变模拟结果中发现,初始阶段β晶取向杂乱且晶粒尺寸细小;随着增材高度的增加,温度梯度变缓,平均固-液转变糊状区域宽度增加,初始β晶平均晶粒尺寸增加,晶体取向趋于热流传递方向(垂直于冷基板方向);其模拟结果与实际增材制造的初始β晶组织形貌相一致;此外,Ti6Al4V-0.05B合金的模拟结果表明,0.05 wt%的硼含量添加提高了形核率与生长速率,从而使得初始β晶粒表现出更多的枝晶生长。
[Abstract]:Titanium and titanium alloys are widely used in various fields because of their high strength, low density, high temperature resistance, corrosion resistance, non-magnetic and good biocompatibility. However, the traditional forming process of titanium and titanium alloys requires follow-up treatment such as vacuum melting, forging and a large number of cutting, which reduces the material utilization rate and increases the production cost. Manufacturing technology can shorten the process, improve the utilization rate of titanium alloy materials, and optimize the alloy composition, which can improve the mechanical properties of titanium alloy. Based on this, this paper studies the manufacturing technology of titanium alloy with trace boron and wire arc augmentation, and the evolution of rapidly solidified microstructure in the process of titanium alloy wire arc augmentation. Firstly, Ti6Al14V-xB (wt%, x 0, 0.05, 0.1, 0.5) alloys with different boron contents were prepared by high vacuum non-consumptive melting and suction casting. The effects of different trace boron addition on the casting microstructure and mechanical properties of Ti6Al4V-xB were studied. Secondly, Ti6Al4V and Ti6Al4V-0.05B wires were used as raw materials and arc as heat source. The solidification process, microstructure and mechanical properties of arc augmented titanium alloy wire were studied. Finally, the rapid solidification process of arc augmented Ti6Al4V and Ti6Al4V-0.05B alloy wire was simulated by cellular automata-finite element method. The mechanism of solid-liquid transition, initial beta nucleation and growth in different stages of wire arc augmentation is studied. The results are as follows: (1) The addition of trace B affects the initial beta crystal growth of titanium alloy. The enrichment of B in the front of solid-liquid hinders the initial beta-Ti growth and refines the grain effectively. When the boron content exceeds 0.1wt%, the grain size is obviously refined. The tensile strength limit of Ti6Al4V-xB alloy increases monotonously with the increase of boron content, which is the result of fine grain strengthening and precipitation strengthening; the plasticity of Ti6Al4V-0.05B alloy increases by 15%, while the plasticity of Ti6Al4V-0.1B and Ti6Al4V-0.5B alloy decreases by more than 40% because of precipitation brittleness. (2) In the arc augmentation process of Ti6Al4V wire, due to the high heat input of the arc, effective metallurgical bonding is achieved between each deposited area, fusion area and deposited area, without obvious deposited interface and titanium martensite, and the microstructure of each area is stable a + beta lamella. Compared with as-cast Ti6Al4V, the titanium alloy made by arc augmentation has not only fine initial beta grain, but also small interlamellar spacing of alpha+beta. Its tensile strength and elongation are higher than that of as-cast titanium alloy, and the tensile fracture surface is fine dimple-like toughness fracture. (3) The titanium alloy made by arc augmentation of Ti6Al4V-0.05B wire has been fabricated. In the process, finer grains were obtained with a small amount of irregular acicular TiB precipitation. Compared with as-cast Ti6Al4V-0.05B alloy, the grain size was reduced and tended to dendrite morphology. The tensile strength increased by 6.2% and the elongation increased by 28.7%. (4) Simulation of solidification microstructure evolution in arc augmented Ti6Al4V wire The results show that the orientation of the initial beta crystal is disordered and the grain size is small; with the increase of the height of the augmented material, the temperature gradient slows down, the width of the average solid-liquid transition paste region increases, the average grain size of the initial beta crystal increases, and the orientation of the crystal tends to the direction of heat transfer (perpendicular to the direction of the cold substrate); the simulation results and the initial production of the actual augmented material. In addition, the simulation results of Ti6Al4V-0.05B alloy show that the addition of 0.05 wt% boron improves the nucleation rate and growth rate, which makes the initial beta grain show more dendritic growth.
【学位授予单位】：西安理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG146.23

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