Tensile behavior of Ti-6Al-4V alloy fabricated by selective laser melting: effects of microstructures and as-built surface quality

Selective laser melting(SLM) is a powerful additive manufacturing(AM) technology, of which the most prominent advantage is the ability to produce components with a complex geometry. The service performances of the SLM-processed components depend on the microstructure and surface quality. In this work, the microstructures, mechanical properties, and fracture behaviors of SLM-processed Ti-6 Al-4 V alloy under machined and as-built surfaces after annealing treatments and hot isostatic pressing(HIP) were investigated. The microstructures were analyzed by optical microscope(OM), scanning electron microscope(SEM) and transmission electron microscopy(TEM). The mechanical properties were measured by tensile testing at room temperature. The results indicate that the as-deposited microstructures are characterized by columnar grains and fine brittle martensite and the asdeposited properties present high strength, low ductility and obvious anisotropy. After annealing at 800-900°C for 2-4 h and HIP at 920°C/100 MPa for 2 h, the brittle martensite could be transformed into ductile lamellar(α+β) microstructure and the static tensile properties of SLM-processed Ti-6 Al-4 V alloys in the machined condition could be comparable to that of wrought materials. Even after HIP treatment, the as-built surfaces could decrease the ductility and reduction of area of SLM-processed Ti-6 Al-4 V alloys to 9.2% and 20%, respectively. The crack initiation could occur at the columnar grain boundaries or at the as-built surfaces. The lamellar(α+β) microstructures and columnar grains could hinder or distort the crack propagation path during tensile tests.     

Cast structure and property variability in gamma titanium aluminides

Aspects of cast structure that influence mechanical property variability, and in particular tensile ductility, have been studied in a Ti-48Al-2Cr-2Nb titanium aluminide alloy. Macrostructure, porosity, microstructure and tensile properties have been characterized over a range of casting processing conditions. Local solidification times and subsequent solid-state cooling rates during casting have been characterized via local thermal measurements in combination with solidification modeling. Large variations in cooling rate during casting dramatically influence the initial cast structure as well as the distribution of defects such as porosity. Variations in as-cast structure persist through subsequent thermal and hot isostatic pressing cycles and contribute to the variability of tensile ductility. The current understanding of the relationship of tensile ductility to processing-induced changes in structure will be briefly discussed.     

Research on investment casting of TiAl alloy agitator treated by HIP and HT

Using TiAl alloy to substitute superalloy is a hot topic in aeroengine industry because of its low density, high elevated temperature strength, and anti-oxidization ability. In this research, Ti-47.5AL-2Cr-2Nb-0.2B alloy was used as the test material. By applying a combination process of ceramic shell mold and core making, vacuum arc melting and centrifugal pouring, and heat isostatic pressing (HIP) and heat treatment (HT) etc., the TiAl vortex agitator casting for aeroengine was successfully made. This paper introduced key techniques in making the TiAl vortex agitator with investment casting process, provided some experimental results including mechanical properties and machinability, and explained some concerns that could affect applications of TiAl castings.     

离心力及铸件模数对Ti-6Al-4V合金组织与力学性能的影响

该论文主要研究在石墨型中不同立式离心力场下离心力及铸件模数对Ti-6Al-4V合金组织及性能的影响。实验过程中铸型的旋转速度主要考虑了三种情况：0, 110及 210 rpm。结果表明：晶粒尺寸及片层厚度随铸件模数的减小和离心力的增加而减小,拉伸强度随铸件模数减小和离心力增加而明显增加,但铸件延伸率呈现相反的变化趋势。文中给出了重力系数、铸件模数与Ti-6Al-4V合金组织和力学性能之间的定量关系。作为与重力场下石墨型中Ti-6Al-4V合金铸件对比分析,研究了金属型中Ti-6Al-4V合金阶梯铸件组织的变化情况,研究发现：两种铸型中浇铸的合金铸件晶粒尺寸、片层厚度随冷却速度的变化趋势基本一致,结合两组实验数据,给出了重力场下Ti-6Al-4V合金铸件组织随冷却速度变化的定量关系。     

Tensile properties and fracture characteristics of spray cast alloy IC6 （Ni-7.8Al-14Mo-0.05B）

Tensile testing results of spray cast Ni3Al-based superalloy indicated that the yield strength and tensile strength increase with the increasing of temperature and reached maximum at around 760℃, then decrease with the increasing of temperature. After high temperature isostatic pressing (HIP), yield strength decreased and ductility and tensile strength increased. Stereographic projection showed that no matter at room temperature, medium temperature or high temperature, cracks extend along (111).     

钛合金热等静压模拟本构关键参数确定及工艺优化

对钛合金粉末热等静压进行数值模拟,以预测材料的成形性能;采用本构为MSC.Marc软件中嵌入的Shima模型,通过对该数值模型的分析;制定了本构模型中TC4材料模拟所需材料参数的测定方法。针对航空类典型环形零件的成形,应用数值模型,通过对多种不同工艺制度参数的对比,得到了热等静压过程中的关键参数压力、温度曲线的优化制度。利用热等静压制备了实际的环形样件,数值模拟与试验结果吻合较好,说明数值模拟可对零件的热等静压工艺过程进行预测,同时也为复杂零件的成形预测奠定了基础。     

添加晶粒细化剂硅对Ti-6Al-4V合金组织和性能的影响(英文)

采用真空感应凝壳熔炼工艺在石墨模中制备Ti-6Al-4V和Ti6Al4V0.5Si两种钛合金。将硅作为一种晶粒细化剂加入到Ti-6Al-4V合金中,考察添加硅对铸态和模锻态Ti-6Al-4V合金组织和性能的影响。铸态合金先在900°C下进行热模锻处理,然后分别进行两种不同的热处理。一种是将模锻样品在1050°C下保温30min,然后水淬以获得细小的层片状组织;另一种是将模锻件在1050°C下保温30min,然后再在800°C下保温30min,以获得粗大的层片状组织。Ti6-Al-4V合金中添加0.5%Si后,铸态合金的晶粒尺寸从627μm减小到337μm,其极限抗拉强度增加约25MPa。具有细小、层片状组织的Ti-6Al-4V0.5Si合金的最大极限抗拉强度为1380MPa,在Hank溶液和NaCl溶液中的腐蚀速度分别为1.35×106和5.78×104mm/a。Ti-6Al-4V合金中添加0.5%Si后,在低滑动速度下的磨损率降低50%,在高滑动速度下的磨损率降低约73%。     

Effect of HIP Treatment on Fatigue Notch Sensitivity of Ti-6Al-4V Alloy Fabricated by Electron Beam Melting

The effect of hot isostatic pressing(HIP)treatment on the fatigue notch sensitivity of Ti-6A1-4V alloy fabricated by electron beam melting(EBM)was investigated.The results indicate that the fatigue notch sensitivity of the as-fabricated samples is much lower than that of HIPed samples.The variation of α lamella thickness and the distribution of pore defects have an effect on the fatigue strength in smooth and notched EBM Ti-6A1-4V samples,resulting in the divergence of the as-fabricated and HIPed samples on fatigue notch sensitivity.     

Microstructure evolution and mechanical properties' improvement of NiAl–Cr(Mo)–Hf eutectic alloy during suction casting and subsequent HIP treatment

A NiAl–Cr(Mo)–Hf eutectic alloy was prepared by suction-casting technique and subsequently hot isostatic pressing treatment. Microstructure and mechanical tests were performed and the results revealed that the suction-cast alloy possessed fine NiAl/Cr(Mo) lamellar, large area fraction of eutectic cell and semi-continuously distributed Ni₂AlHf phase at the cell boundaries. After the HIP treatment, the Ni₂AlHf particles became fine and distributed evenly in the alloy. Moreover, some of the Ni₂AlHf particles along the eutectic cell boundaries were transformed into Hf solid solution phase. Compared with the conventionally cast alloy, the room-temperature compressive ductility and strength of the suction-cast alloy attained significant improvement. In addition, the room-temperature ductility and elevated temperature strength of suction-cast alloy were markedly enhanced by HIP treatment.     

The effect of hot isostatic pressing on crack initiation, fatigue, and mechanical properties of two cast aluminum alloys

This article presents the results of an experimental materials testing program on the effect of hot isostatic pressing (HIP) on the crack initiation, fatigue, and mechanical properties of two cast aluminum alloys: AMS 4220 and 4225. These alloys are often used in castings for high temperature applications. Standard tensile and instrumented Charpy impact tests were performed at room and elevated temperatures. The resulting data quantify improvements in ultimate tensile strength, ductility, and Charpy impact toughness from the HIP process while indicating little change in yield strength for both alloys. In addition standard fracture mechanics fatigue tests along with a set of unique fatigue crack initiation tests were performed on the alloys. Hot isostatic pressing was shown to produce a significant increase in cycles to crack initiation for AMS 4225, while no change was evident in traditional da/dN fatigue crack growth. The data permits comparisons of the two alloys both with and without the HIP process.     

Effects of magnesium and copper additions on tensile properties of Al-Si-Cr die casting alloy under as-cast and T5 conditions

Aluminum high pressure die casting(HPDC) technology has evolved in the past decades, enabling stronger and larger one-piece casting with significant part consolidation. It also offers a higher design freedom for more mass-efficient thin-walled body structures. For body structures that require excellent ductility and fracture toughness to be joined with steel sheet via self-piercing riveting(for instance, shock towers and hinge pillars, etc.),a costly T7 heat treatment comprising a solution heat ...     

Experimental and Numerical Analysis of Injection Molding of Ti-6Al-4V Powders for High-Performance Titanium Parts

Both experimental and numerical analysis of powder injection molding (PIM) of Ti-6Al-4V alloy were performed to prepare a defect-free high-performance Ti-6Al-4V part with low carbon/oxygen contents. The prepared feedstock was characterized with specific experiments to identify its viscosity, pressure–volume–temperature and thermal properties to simulate its injection molding process. A finite-element-based numerical scheme was employed to simulate the thermomechanical process during the injection molding. In addition, the injection molding, debinding, sintering and hot isostatic pressing processes were performed in sequence to prepare the PIMed parts. With optimized processing conditions, the PIMed Ti-6Al-4V part exhibits excellent physical and mechanical properties, showing a final density of 99.8%, tensile strength of 973 MPa and elongation of 16%.     

Effect of hot isostatic pressing processing parameters on microstructure and properties of Ti60 high temperature titanium alloy

Hot isostatic pressing parameters are critical to Ti60 high temperature titanium alloy castings which have wide application perspective in aerospace. In order to obtain optimal processing parameters, the effects of hot isostatic pressing parameters on defects, composition uniformity, microstructure and mechanical properties of Ti60 cast high temperature titanium alloy were investigated in detail. Results show that increasing temperature and pressure of hot isostatic pressing can reduce defects, ...     

Processing and properties of Nb- Ti- base alloys

The processing characteristics, tensile properties, and oxidation response of two Nb-Ti-Al-Cr alloys were investigated. One creep test at 650 °C and 172 MPa was conducted on the base alloy, which contained 40Nb-40Ti-10Al-10Cr. A second alloy was modified with 0.11 at.% C and 0.07 at.% Y. Alloys were arc melted in a chamber backfilled with argon, drop cast into a water-cooled copper mold, and cold rolled to obtain a 0.8-mm sheet. The sheet was annealed at 1100 °C for 0.5 h. Longitudinal tensile specimens and oxidation specimens were obtained for both the base alloy and the modified alloy. Tensile properties were obtained for the base alloy at room temperature, 400,600,700,800,900, and 1000 °C and for the modified alloy at room temperature, 400,600,700, and 800 °C. Oxidation tests on the base alloy and modified alloy, as measured by weight change, were carried out at 600,700,800, and 900 °C. Both the base alloy and the modified alloy were extremely ductile and were cold rolled to the final sheet thickness of 0.8 mm without an intermediate anneal. The modified alloy exhibited some edge cracking during cold rolling. Both alloys recrystallized at the end of a 0.5-h annealing treatment. The alloys exhibited moderate strength and oxi-dation resistance below 600 °C, similar to the results of alloys reported in the literature. The addition of carbon produced almost no change in either the yield strength or ductility as measured by total elonga-tion. A small increase in the ultimate tensile strength and a corresponding decrease in the reduction of area below 600 °C were observed. Carbon addition also served to marginally refine the grain size after annealing. The results of this study and those of similar alloys reported in the literature suggest that 40Nb-40Ti-10Al-10Cr forms a good base alloy suitable for alloying for improvement in its oxidation and high-temperature strength properties.     

微量Zr对铸造TiAl合金高温力学性能的影响

在Ti-47.5Al-2.5V-1.0Cr合金中添加0.2at.％Zr以研究微量Zr对铸造形成的定向层片组织高温力学性能的作用.结果表明,添加0.2at.％Zr显著提高了该合金的持久寿命,但未对其高温拉伸强度产生明显影响.根据两种合金持久试验前后组织对比观察的结果提出,添加微量Zr主要是通过增加定向层片组织的热稳定性而使其持久性能得以改善;另外,微量Zr减少了热等静压过程中等轴晶粒的析出,也有助于延长合金的持久寿命.     

温度和氧含量对Ti-2Al合金拉伸力学行为和断裂方式的耦合影响

研究高氧含量(0.30%,质量分数,下同)、工业水平氧含量(0.16%)和极低氧含量(0.06%) 3种Ti-2Al合金在低温(77 K)、室温和高温(673 K)下的拉伸力学行为。结果表明：低温和室温下,随着氧含量的升高,材料强度提高,塑性降低。然而,高温673 K下,不同氧含量Ti-2Al合金强度和塑性基本相同,材料强度和塑性对氧含量的依赖性显著降低。氧含量较低时,温度对Ti-2Al合金塑性影响很小。微观组织观察表明,随着温度的升高,工业水平及高氧含量Ti-2Al合金拉伸断口由脆性解理转变为延性韧窝。高温环境下,固溶氧原子扩散速率提高使Cottrel气团对位错滑移的阻碍减弱,均匀变形的结果导致高氧含量Ti-2Al合金延性改善。     

Microstructure,Mechanical Properties,and Flatness of SEBM Ti-6Al-4V Sheet in As-Built and Hot Isostatically Pressed Conditions

Sheet (0.41–4.80 mm thick) or thin plate structures commonly exist in additively manufactured Ti-6Al-4V components for load-bearing applications. A batch of 64 Ti-6Al-4V sheet samples with dimensions of 210/180 mm × 42 mm × 3 mm have been additively manufactured by selective electron beam melting (SEBM). A comprehensive assessment was then made of their density, surface flatness, microstructure, and mechanical properties in both as-built and hot isostatically pressed conditions, including the influence of the hot isostatic pressing (HIP) temperature. In particular, standard long tensile (156 mm long, 2 mm thick) and fatigue (206 mm long, 2 mm thick) test sheet samples were used for assessment. As-built SEBM Ti-6Al-4V sheet samples with machined surfaces fully satisfied the minimum tensile property requirements for mill-annealed TIMETAL Ti-6Al-4V sheet products, whereas HIP-processed samples (2 mm thick) with machined surfaces achieved a high cycle fatigue (HCF) strength of 625 MPa (R = 0.06, 10⁷ cycles), similar to mill-annealed Ti-6Al-4V (500–700 MPa). The unflatness was limited to 0.2 mm in both the as-built and HIP-processed conditions. A range of other revealing observations was discussed for the additive manufacturing of the Ti-6Al-4V sheet structures.     

Effects of hot isostatic pressing temperature on casting shrinkage densification and microstructure of Ti6Al4V alloy

The Ti6Al4V alloy castings were produced by the investment casting process, and the hot isostatic pressing(HIP) was used to remove shrinkage from castings. The processing pressure and holding time for HIP were 150 MPa and 20 min, respectively. Four different HIP temperatures were tested, including 750 ℃, 850 ℃, 920 ℃ and 950 ℃. To evaluate the effects of temperature on densification and microstructure of Ti6Al4V alloy treated by HIP, non-destructive testing and metallographic observation was performed. The experimental results show that the shrinkage was completely closed at 920 ℃ and 950 ℃. The densification of Ti6Al4V alloy increased as the HIP temperature increased below 920 ℃. The lamel ae were more uniform, the thickness of lamel ae was obviously broadened and the structure was coarsen. Besides, the Norton creep equation was used to simulate the effect of different temperatures on the densification of Ti6Al4V alloy during HIP. The simulation results were in good agreement with the experimental results. It was also found that 920 ℃ is a suitable temperature for HIP for Ti6Al4V alloy.     

Interface reaction during titanium alloys investment casting by residue gas in ceramic mold

The chemical reaction between mold material and titanium melt during investment casting was studied intensively. However, the influence of residue gas in ceramic mold on interface reaction remains unclear. In this investigation, the effect of residue gas in Y_2O_3-silica sol shell mold on interface reaction during Ti-6Al-4V alloy investment casting was investigated. Two groups of shell molds were prepared by adding different kinds of pore formers, i.e., spherical starch particles or nylon fibers,respectively. Ti-6Al-4V alloy was cast under vacuum by gravity casting through cold crucible induction melting(CCIM) method. Porosity of different shell molds was measured based on Archimedean method. Scanning electron microscopy(SEM) and energy-dispersive X-ray spectroscopy(EDS) were employed to characterize the micromorphology and composition of the reaction area,respectively. White light interferometer(WLI) was used to obtain the surface topography of the shell mold. The results show that the direct chemical reaction is very weak for all specimens. The release of residue gas in closed pores is the key factor influencing surface defects. However, open pores make nearly no difference on the interface reaction.     

Ti-6Al-4V合金的轧制与组织性能

采用光学显微镜、透射电镜和拉伸试验等手段,研究了多道次两向轧制和单向轧制对不同原始状态(热轧态、水淬态和空冷态)Ti-6Al-4V合金显微组织和力学性能的影响。结果表明,热轧态Ti-6Al-4V合金的组织为片状α相+β相+少量等轴α相,水淬态Ti-6Al-4V合金形成了针状马氏体组织,空冷态Ti-6Al-4V合金形成了网状组织。Ti-6Al-4V合金适宜的两向轧制温度为700 ℃,此时合金中可见颗粒状β相弥散分布在α基体上。两向轧制Ti-6Al-4V合金的抗拉强度和屈服强度从高至低顺序为:水淬态>热轧态>空冷态,且轧向强度要高于横向;相较于单向轧制,两向轧制明显降低了Ti-6Al-4V合金板材拉伸性能的各向异性,且水淬态Ti-6Al-4V合金的轧向和横向强度差异最小,700 ℃轧制Ti-6Al-4V合金的主要细化机制为位错细化。