MACHINING TITANIUM AND ITS ALLOYS

Abstract

Titanium and its alloys are attractive materials due to their unique high strength-weight ratio that is maintained at elevated temperatures and their exceptional corrosion resistance. The major application of titanium has been in the aerospace industry. However, the focus shift of market trends from military to commercial and aerospace to industry has also been reported. On the Other hand, titanium and its alloys are notorious for their poor thermal properties and are classified as difficult-to-machine materials. These properties limit the use of these materials especially in the commercial markets where cost is much more of a factor than in aerospace. Machining is an important manufacturing process because it is almost always involved if precision is required and is the most cost effective process for small volume production. This paper reviews the machining of titanium and its alloys and proposes potential research issues.     

Minimal quantity lubrication-MQL in grinding of Ti–6Al–4V titanium alloy

Titanium and its alloys are attractive materials due to their unique high strength–weight ratio that is maintained at elevated temperatures and their exceptional corrosion resistance. The major application of titanium has been in the aerospace industry. On the other hand, titanium and its alloys are notorious for their poor thermal properties and are classified as difficult-to-machine materials. The problems that arise during grinding of titanium alloys are attributed to the high specific energy and high grinding zone temperature. Significant progress has been made in dry and semidry machining recently, and minimal quantity lubrication (MQL) machining in particular has been accepted as a successful semidry application because of its environmentally friendly characteristics. A number of studies have shown that MQL machining can show satisfactory performance in practical machining operations. However, there has been few investigation of MQL grinding of special alloys like titanium alloys and the cutting fluids to be used in MQL grinding of these alloys. In this study, vegetable and synthetic esters oil are compared on the basis of the surface quality properties that would be suitable for MQL applications. The cutting performance of fluids is also evaluated using conventional wet (fluid) grinding of Ti–6Al–4V. As a result, synthetic ester oil is found to be optimal cutting fluids for MQL grinding of Ti–6Al–4V.     

Comparative investigation on using cryogenic machining in CNC milling of Ti-6Al-4V titanium alloy

Ti-6Al-4V titanium alloy is one of the most important materials in industry, 80% of which is used in aerospace industry. Titanium alloys are also notoriously difficult-to-machine materials owing to their unique material properties imposing a major bottleneck in manufacturing systems. Cryogenic cooling has been acknowledged as an alternative technique in machining to improve the machinability of different materials. Although milling is considered to be the major machining operation for the manufacture of titanium components in aerospace industries, studies in cryogenic machining of titanium alloys are predominantly concentrated on turning operations. To address this gap, this article provides an investigation on the viability of cryogenic cooling in CNC end-milling of aerospace-grade Ti-6Al-4V alloy using liquid nitrogen in comparison with traditional machining environments. A series of machining experiments were conducted and surface roughness, tool life, power consumption, and specific machining energy were investigated for cryogenic milling as opposed to conventional dry and flood cooling. Analysis revealed that cryogenic machining using liquid nitrogen has the potential to significantly improve the machinability of Ti-6Al-4V alloy in CNC end-milling using solid carbide cutting tools and result in a paradigm shift in machining of titanium products. The analysis demonstrated that cryogenic cooling has resulted in almost three times increased tool life and the surface roughness was reduced by 40% in comparison with flood cooling.     

基于热力学的硬质合金刀具加工钛合金的研究

钛合金因其性能的优越性,在航空、航天、石油、化工、汽车等工业领域得到了广泛应用,本文从热力学的角度研究碳化钨基硬质合金刀具和碳化钛基硬质合金刀具加工钛合金时的溶解扩散磨损特点,对两类刀具材料与钛合金的亲和性做了热力学分析,结果表明碳化钨基硬质合金刀具更适合加工钛合金。     

ROTARY ULTRASONIC MACHINING OF TITANIUM ALLOY: EFFECTS OF MACHINING VARIABLES

Titanium and its alloys are finding prime applications in industries due to their unique properties. However, the high cost of machining is one of the limiting factors for their widespread use. Tremendous efforts are being made to improve the existing machining processes, and new processes are being developed to reduce the machining cost in order to increase the titanium market. However, there is no report on the systematic study of the effects of machining variables on output parameters in rotary ultrasonic machining of titanium and its alloys. This paper presents an experimental study on rotary ultrasonic machining of a titanium alloy. The cutting force, material removal rate, and surface roughness (when using rotary ultrasonic machining) of a titanium alloy have been investigated using different machining variables.     

钛合金切削润滑研究现状与发展趋势*

钛合金在航空航天、生物医疗等领域具有广泛而重要的应用,但钛合金是典型的难加工金属,其切削润滑问题是制约钛合金加工效率与质量的关键所在,目前尚未得到较好的解决。从钛合金切削中的摩擦学问题、切削润滑问题、水基润滑问题三方面介绍钛合金切削润滑研究现状,以有望解决钛合金切削润滑问题的水基润滑为基础,从适合于钛合金切削润滑的微量润滑技术及纳米颗粒增效两方面探讨钛合金切削水基润滑研究的发展趋势,并总结以水基润滑剂为基础的高性能钛合金切削液体系设计是未来研发新型高效环保钛合金切削液的重要途径。     

钛合金大进给铣削数控加工程序优化及应用

钛合金因其优越的比强度、机械性能和抗腐蚀性等优点而广泛应用于航空工业,但由于热导率低、弹性模量小、化学活性高等问题而导致其切削加工性差.介绍了大进给铣削技术这种高速高效的钛合金加工方法,分析了大进给数控加工程序编制带来的效率、品质和成本等方面的问题,提出了相应的加工程序优化方案,并在实际产品上进行了验证.     

Measurement and analysis of surface roughness in turning of aerospace titanium alloy (gr5)

Titanium alloys are extensively used in aerospace, biomedical applications and they are used in corrosive environments. In this study, the effect of cutting parameters on the surface roughness in turning of titanium alloy has been investigated using response surface methodology. The experimental studies were conducted under varying cutting speeds, feed and depths of cut. The chip formation and SEM analysis are discussed to enhance the supportive surface quality achieved in turning. The work material used for the present investigation is commercial aerospace titanium alloy (gr5) and the tool used is RCMT 10T300 – MT TT3500 round insert. The equation developed using response surface methodology is used for predicting the surface roughness in machining of titanium alloy. The results revealed that the feed was the most influential factor which affect the surface roughness.     

Assessment of the machinability of Ti-6Al-4V alloy using the wear map approach

The high strength to density ratio of titanium alloys coupled with excellent corrosion resistance even at elevated temperatures make them ideal for aerospace applications. Moreover, the biocompatibility of titanium also enables its widespread use in the biomedical and food processing industries. However, the difficulty in machining titanium and its alloys along with the high cost of its extraction from ore form presents a major economic constraint. In the context of machining economics, the wear map approach is very useful in identifying the most suitable machining parameters over a feedrate–cutting velocity plane. To date, wear maps have only been prepared for the machining of ferrous alloys. In this article, a review of the machinability of Ti-6Al-4V alloy is presented with emphasis on comparing the wear performance of various tool materials. In addition, a new wear map for Ti-6Al-4V alloy is presented based on unified turning tests using H13A grade carbide inserts. This wear map can be used as a guide in the selection of cutting variables that ensure the least tool wear rates. This article contrasts the occurrence of a safety zone in the case of machining steels to that of an avoidance zone for Ti-6Al-4V alloy.     

On high-speed turning of a third-generation gamma titanium aluminide

Gamma titanium aluminides are heat-resistant intermetallic alloys predestined to be employed in components suffering from high mechanical stresses and thermal loads. These materials are regarded as difficult to cut, so this makes process adaptation essential in order to obtain high-quality and defect-free surfaces suitable for aerospace and automotive parts. In this paper, an innovative approach for longitudinal external high-speed turning of a third-generation Ti-45Al-8Nb-0.2C-0.2B gamma titanium aluminide is presented. The experimental campaign has been executed with different process parameters, tool geometries and lubrication conditions. The results are discussed in terms of surface roughness/integrity, chip morphology, cutting forces and tool wear. Experimental evidence showed that, due to the high cutting speed, the high temperatures reached in the shear zone improve chip formation, so a crack-free surface can be obtained. Furthermore, the use of a cryogenic lubrication system has been identified in order to reduce the huge tool wear, which represents the main drawback when machining gamma titanium aluminides under the chosen process conditions.     

Determining the energy distribution during electric discharge machining of Ti–6Al–4V

The titanium (Ti) alloys are the notoriously “difficult-to-machine” aerospace materials. Compared with the traditional mechanical cutting methods which are costly because of high tooling costs, electrodischarge machining (EDM) is an effective machining method for the Ti alloys. The energy distribution during the EDM process of Ti alloys was rarely reported, though it is a very important factor that can affect the machining performance. In this work, the energy distribution during EDM of Ti–6Al–4V has been investigated by a novel method, at different EDM parameters including interelectrode distance, pulse duration, polarity, and electrode shape. The results of this work show that energy distribution characteristics are greatly affected by the power density applied on the electrodes and more energy is distributed into the anode than into the cathode, which are in good concurrence with the results obtained by other authors. The results of this work will be helpful for further improving the technological performance of this process.     

无涂层硬质合金刀具车削钛合金Ti-6Al-4V实验研究

钛合金是航空航天工业中应用广泛的一种难加工材料。本文研究了无涂层硬质合金刀具干切削钛合金Ti-6Al-4V时切削力、表面粗糙度的变化规律，得到了切削深度、进给量、切削速度对切削力和表面粗糙度的影响规律。对切削力实验结果进行了回归分析得到了切削力的指数公式，并运用校正R^2拟合判定系数、累积概率图和残差图对回归模型进行了检验，检验结果表明：切削力的3个回归方程较好的拟合了实验所测的数据，钛合金切削过程中切削力近似满足指数关系。     

Tool wear and surface quality in milling of a gamma-TiAl intermetallic

Advanced structural materials for high-temperature applications are often required in aerospace and automotive fields. Gamma titanium aluminides, intermetallic alloys that contain less than 60?wt.% of Ti, around 30–35 wt.% of aluminum, and other alloy elements, can be used as an alternative to more traditional materials for thermally and mechanically stressed components in aerospace and automotive engines, since they show an attractive combination of favorable strength-to-weight ratio, refractoriness, oxidation resistance, high elastic modulus, and strength retention at elevated temperatures, together with good creep resistance properties. Unfortunately such properties, along with high hardness and brittleness at room temperature, surface damage, and short and unpredictable tool life, undermine their machinability, so that gamma-TiAl are regarded as difficult to cut materials. A deeper knowledge of their machinability is therefore still required. In this context the paper presents the results of an experimental campaign aimed at investigating the machinability of a gamma titanium aluminide, of aeronautic interest, fabricated via electron beam melting and then thermally treated. Milling experiments have been conducted with varying cutting speed, feed, and lubrication conditions (dry, wet, and minimum quantity lubrication). The results are presented in terms of correlation between cutting parameters and lubrication condition with tool wear, surface hardness and roughness, and chip morphology. Tool life, surface roughness, and chirp morphology showed dependence on the cutting parameters. Lubrication conditions were observed to heavily affect tool wear, and minimum quantity lubrication was shown to be by far the method that allows to extend tool life.     

Effect of microstructure and cutting speed on machining behavior of Ti6Al4V alloy

Machining of aerospace and biomedical grade titanium alloys has always been a challenge because of their low conductivity and elastic modulus. Different machining methods and parameters have been adopted for high precision machining of titanium alloys. Machining of titanium alloys can be improved by microstructure optimization. The present study focuses on the effect of microstructure on machinability of Ti6Al4V alloys at different cutting speeds. Samples were subjected to different annealing conditions resulting in different grain sizes and local micro-strains (misorientation). Cutting forces were significantly reduced after annealing; consequently, sub-surface residual stresses were reduced. Deformation twinning was also observed on samples annealed at a higher temperature due to larger grain size. Initial strain free grains and deformation twinning during machining reduces the cutting force at higher cutting speed.     

Problems and solutions in machining of titanium alloys

Titanium alloys are known as difficult-to-machine materials. The problems of machining titanium are many folds which depend on types of titanium alloys. This paper investigates the underlying mechanisms of basic challenges, such as variation of chip thickness, high heat stress, high pressure loads, springback, and residual stress based on the available literature. These are responsible for higher tool wear and worse machined surface integrity. In addition, many cutting tool materials are inapt for machining titanium alloys as those materials are chemically reactive to titanium alloys under machining conditions. To address these problems, latest techniques such as application of high pressure coolant, cryogenic cooling, tap testing, thermally enhanced machining, hybrid machining, and use of high conductive cutting tool and tool holder have also been discussed and correlated. It seems that all the solutions are not yet well accepted in the industrial domain; further advancement in those fields are required to reduce the machining cost of titanium alloys.     

Drilling experiments on a gamma titanium aluminide obtained via electron beam melting

Gamma titanium aluminides are intermetallic structural alloys with many advantages like high temperature and oxidation resistance, low density, high specific strength, rigidity, etc. This makes them promising candidates for critical applications where both mechanical and thermal properties are required. Unfortunately, their machinability is demanding, generating low cutting life and poor surface conditions. A deeper knowledge on the machining parameters is essential for a wider application of these heat-resistant light-weight alloys in aircraft and automotive industry. In this paper, the performance of uncoated carbide drills in drilling a gamma titanium aluminide was analysed. The workpiece material was obtained via electron beam melting (EBM) process, a versatile technology for additive manufacturing of complex metal parts from metal powders. EBM is highly appealing in the field of aeroengine components, and it is particularly interesting in processing gamma titanium aluminides. Cutting performances were measured in terms of tool wear, surface roughness, dimensional and geometric errors. The experimental results show strong dependence of tool wear and part quality on cutting parameters, with poor tool life compared with other work materials.     

[刀具及切削工艺]高速铣削中TaC（NbC）对硬质合金刀具磨损性能的影响

针对航空航天钛合金加工时硬质合金刀具磨损过快的难题,制备了主元素一致、微量合金碳化物TaC（NbC）含量不同的两种WC-Co基硬质合金材料。采用高温维氏硬度计检测两种材料的高温硬度和高温断裂韧性,并制备相同几何参数的立铣刀对钛合金TC4进行铣削加工试验。试验结果表明：在硬质合金中添加微量合金碳化物TaC（NbC）,可以同时提高材料的高温硬度和高温断裂韧性,在相同的切削条件下,添加微量合金碳化物TaC（NbC）的硬质合金立铣刀比未添加微量合金碳化物的立铣刀耐磨性更好,刃口断裂裂纹更少,刀具使用寿命更长,更适合航空航天钛合金材料的高速铣削加工。     

钛合金复杂构件等温锻造实验研究

钛合金复杂构件通常采用机加工成形,导致成本高、材料利用率低,而且常规的锻造工艺很难满足外形复杂、使用性能要求高的钛合金复杂构件成形过程。提出钛合金复杂构件的新工艺,即先等温锻造成复杂构件形状,然后辅以机械加工的方法成形;通过实验研究,结合力学性能测试和超声波探伤,提出非对称变截面钛合金复杂构件等温锻造成形工艺。实验结果表明,采用该等温锻造成形工艺获得的钛合金复杂构件完全满足系统要求,并可替代机械加工产品;按新工艺加工成形的某钛合金复杂构件,不但降低成本,缩短机加工时间,而且材料利用率也提高到60%以上。     

典型钛合金壳体零件加工工艺

随着钛合金的广泛应用,由于其材料强度高,化学活性大,弹性模量较低等原因,其材料的难加工性越来越引起机械加工领域的关注。结合钛合金零件加工领域的技术资料和经验对典型钛合金壳体零件使用加工中心加工的工艺进行了理论探讨,重点介绍了钛合金壳体零件加工中铣削、钻孔、镗孔、攻丝等工艺,在实际运用中可以为钛合金零件加工领域的一线人员提供一定帮助。     

Fibre laser welding of dissimilar alloys of Ti-6Al-4V and Inconel 718 for aerospace applications

Challenges in dissimilar materials welding are the differences of physical and chemical properties between welding materials and the formation of intermetallic brittle phases resulting in the degradation of mechanical properties of welds. However, dissimilar materials welding is increasingly demanded from the industry as it can effectively reduce material costs and improve the design. In aerospace applications, Ti-6Al-4V titanium alloy and Inconel 718 nickel alloy have been widely used because of their superior corrosion resistance and mechanical properties. In this study, a single-mode continuous-wave fibre laser was used in butt welding of Ti-6Al-4V to Inconel 718. Investigations including metallurgical and mechanical examinations were carried out by means of varying processing parameters, such as laser power, welding speed and the laser beam offset position from the interface of the metals. Simple analytical modelling analysis was undertaken to explain the phenomena that occurred in this process. Results showed that the formation of intermetallic brittle phases and welding defects could be effectively restricted at welding conditions produced by the combination of higher laser power, higher welding speed and shifting the laser beam from the interface to the Inconel 718 alloy side. The amount of heat input and position of laser beam to improve the Ti-6Al-4V/Inconel 718 weld quality are suggested.