Ultrafine grain formation in a Ti-6Al-4V alloy by thermomechanical processing of a martensitic microstructure

In the current study, ultrafine equiaxed grains with a size of 150 to 800 nm were successfully produced in a Ti-6Al-4V alloy through thermomechanical processing of a martensitic starting microstructure. This was achieved through a novel mechanism of grain refinement consisting of several concurrent processes. This involves the development of substructure in the lath interiors at an early stage of deformation, which progressed into small high-angle segments with increasing strain. Consequently, the microstructure was gradually transformed to an equiaxed ultrafine grained structure, mostly surrounded by high-angle grain boundaries, through continuous dynamic recrystallization. Simultaneously, the supersaturated martensite was decomposed during deformation, leading to the progressive formation of beta phase, mainly nucleated on the intervariant lath boundaries.

Variant selection and intervariant crystallographic planes distribution in martensite in a Ti-6Al-4V alloy

The transformation texture was studied in a Ti-6Al-4V alloy for two microstructures produced through different phase transformation mechanisms (i.e. diffusional vs. displacive). Both microstructures revealed qualitatively similar crystallographic texture characteristics, having two main texture components with Euler angles of (90°, 90°, 0°) and (90°, 30°, 0°). However, the overall α texture strength was considerably weaker in the martensitic structure (i.e. displacive mechanism) compared with the α + β microstructure produced through slow cooling (i.e. diffusional mechanism). The intervariant boundary distribution in martensite mostly revealed five misorientations associated with the Burgers orientation relationship. The five-parameter boundary analysis also showed a very strong interface plane orientation texture, with interfaces terminated mostly on the prismatic planes {hki0}, when misorientation was ignored. The highest intervariant boundary populations belonged to the 63.26°/[10 553 ] and 60°/[112 0] misorientations, with length fractions of 0.38 and 0.3, respectively. The former was terminated on (41 3 0), and the latter was a symmetric tilt boundary, terminated on (1 011). The intervariant plane distribution in martensite was determined more by the constraints of the phase transformation than by the relative interface energies.

Microstructure evolution of martensitic Ti-6Al-4V alloy during warm deformation

The microstructure evolution of martensitic Ti-6Al-4V alloy was investigated through uniaxial hot compression at 700°C and a strain rate of 10-3 s-1. A combination of scanning electron microscopy observation in conjunction with high resolution electron back scattered diffraction (EBSD) was used to characterize the microstructure in detail. The development of the microstructure displayed continuous fragmentation of martensitic laths with increasing strain (i.e. continuous dynamic recrystallization), concurrently with decomposition of supersaturated martensite resulting in the formation of equiaxed grains. At a strain of 0.8, an ultrafine equiaxed grained structure with mostly high angle grain boundaries was successfully obtained. The current work proposes a novel approach to produce equiaxed ultrafine grains in a Ti-6Al-4V alloy through thermomechanical processing of a martensitic starting microstructure. © (2014) Trans Tech Publications, Switzerland.

Enhanced mechanical response of an ultrafine grained Ti-6Al-4V alloy produced through warm symmetric and asymmetric rolling

An equiaxed ultrafine-grained (UFG) microstructure was successfully produced in a Ti-6Al-4V alloy with an average grain size of 110-230. nm through symmetric and asymmetric warm rolling of a martensitic starting microstructure. The UFG material displayed a combination of ultrahigh strength and ductility at room temperature. Compared with the conventional symmetric rolling, the asymmetric rolling process led to a more pronounced effect of microstructure refinement and a higher tensile ductility. The optimum mechanical response was obtained though the asymmetric rolling at 70% reduction, offering an ultimate tensile strength of 1365. MPa and a total elongation of ~23%. Apart from the magnitude of grain refinement, the inclination of basal texture component from the normal towards the rolling direction during asymmetric rolling and possible strain induced β to martensite transformation may concurrently contribute to a remarkable tensile strength-ductility balance.

Microstructure and texture evolution during symmetric and asymmetric rolling of a martensitic Ti-6Al-4V alloy

In the current study, the effect of deformation mode (i.e., symmetric vs asymmetric rolling) on the extent of grain refinement and texture development in Ti-6Al-4V was examined through warm rolling of a martensitic starting microstructure. During rolling, the initial martensitic lath structure was progressively fragmented, primarily through continuous dynamic recrystallization. This eventually led to an ultrafine-grained (UFG) microstructure composed of equiaxed grains with a mean size of 180 to 230 nm, mostly surrounded by high-angle grain boundaries. Depending on the rolling reduction and deformation mode (symmetric and asymmetric), the rolled specimens displayed different layer morphologies throughout the specimen thickness: a fully UFG surface layer, a partial UFG transition layer, and a partially fragmented lath interior layer. Due to a higher level of effective strain and continuous rotation of the principle axis, asymmetric rolling resulted in a greater extent of grain refinement compared with symmetric rolling at a given thermomechanical condition. A bulk UFG structure was successfully obtained using 70 pct asymmetric rolling. In addition, the rolling texture exhibited various characteristics throughout the thickness due to a different combination of shear and compressive strains. Principally, the basal texture component was displaced from the normal toward rolling direction during asymmetric rolling, differing from the symmetric rolling textures.

Machining of titanium alloy (Ti-6Al-4V)-theory to application

This article correlates laboratory-based understanding in machining of titanium alloys with the industry based outputs and finds possible solutions to improve machining efficiency of titanium alloy Ti-6Al-4V. The machining outputs are explained based on different aspects of chip formation mechanism and practical issues faced by industries during titanium machining. This study also analyzed and linked the methods that effectively improve the machinability of titanium alloys. It is found that the deformation mechanism during machining of titanium alloys is complex and causes basic challenges, such as sawtooth chips, high temperature, high stress on cutting tool, high tool wear and undercut parts. These challenges are correlated and affected by each other. Sawtooth chips cause variation in cutting forces which results in high cyclic stress on cutting tools. On the other hand, low thermal conductivity of titanium alloy causes high temperature. These cause a favorable environment for high tool wear. Thus, improvements in machining titanium alloy depend mainly on overcoming the complexities associated with the inherent properties of this alloy. Vibration analysis kit, high pressure coolant, cryogenic cooling, thermally enhanced machining, hybrid machining and, use of high conductive cutting tool and tool holders improve the machinability of titanium alloy.

Machinability assessment of titanium alloy Ti-6Al-4V for biomedical applications

Titanium alloy (Ti-6Al-4V) has a wide range of application in various fields of engineering. Titanium is mainly used to manufacture aerospace components like landing gear, fuselage, wings, engines etc. and biomedical components like hip joint, knee joint, dental implants etc. Titanium has outstanding material properties such as corrosion resistance, fatigue strength, tensile strength and a very good biocompatibility which makes this material very alluring for biomedical applications. Contrary, the machinability of the material is problematic because of the phase transformations and thus, titanium alloy is a challenge for machining operation. This research is a comparative analysis between the implants manufactured by traditional method of casting and machining. The femoral stem of the hip joint replacement is designed and the component is machined using a five-axis CNC machine.The machined component was subjected to surface roughness testing, tensile testing and bulk hardness testing. The values were compared with the values of titanium implants manufactured by casting. © (2014) Trans Tech Publications, Switzerland.

Low-temperature compaction of Ti-6Al-4V powder using equal channel angular extrusion with back pressure

Equal channel angular extrusion (ECAE), with simultaneous application of back pressure, has been applied to the consolidation of 10 mm diameter billets of pre-alloyed, hydride-dehydride Ti-6Al-4V powder at temperatures ≤400 °C. The upper limit to processing temperature was chosen to minimise the potential for contamination with gaseous constituents potentially harmful to properties of consolidated product. It has been demonstrated that the application of ECAE with imposed hydrostatic pressure permits consolidation to in excess of 96% relative density at temperatures in the range 100-400 °C, and in excess of 98% at 400 °C with applied back pressure ≥175 MPa. ECAE compaction at 20 °C (back pressure = 262 MPa) produced billet with 95.6% relative density, but minimal green strength. At an extrusion temperature of 400 °C, the relative density increased to 98.3%, for similar processing conditions, and the green strength increased to a maximum 750 MPa. The relative density of compacts produced at 400 °C increased from 96.8 to 98.6% with increase in applied back pressure from 20 to 480 MPa, while Vickers hardness increased from 360 to 412 HV. The key to the effective low-temperature compaction achieved is the severe shear deformation experienced during ECAE, combined with the superimposed hydrostatic pressure.

Production of Ti-6Al-4V billet through compaction of blended elemental powders by equal-channel angular pressing

In this work, compaction by warm equal-channel angular pressing (ECAP) with back pressure was used to produce Ti-6Al-4V billets from both commercially pure (CP) titanium and titanium hydride (TiH 2) powders, which were mixed with pulverised binary Al-V master alloys of two distinct Al/V ratios and with elemental aluminium powder to arrive at the nominal alloy composition. It was demonstrated that the right combination of temperature, high hydrostatic pressure and plastic shear deformation permits consolidation of the powder mixture to maximum green densities of 99.26%. Moreover, after direct compaction of blended elemental powders by equal-channel angular pressing (ECAP) with back pressure, the sintering temperature required for chemical and microstructural homogenisation of the compacts could be reduced by 150-250°C. This was possible due to high green density, increased contact area between powder particles and the formation of fast diffusion paths associated with grain refinement by severe plastic deformation. The sintered Ti-6Al-4V billets exhibited a maximum density of 99.88%, Vickers hardness of 409-445 HV1 and ultimate tensile strength in the range of 1000-1080MPa. In contrast to findings of other authors, the use of TiH 2 powders in conjunction with ECAP processing did not bring any benefits with regard to the production of the Ti-6Al-4V alloy.

A preliminary assessment of machinability of titanium alloy Ti 6Al 4V during thin wall machining using trochoidal milling

Titanium alloys are of great demand in the aerospace and biomedical industries. Most the titanium products are either cast or sintered to required shape and finish machined to get the appropriate surface texture to meet the design requirements. Ti-6Al-4V is often referred as work horse among the titanium alloys due to its heavy use in the aerospace industry. This paper is an attempt to investigate and improve the machining performance of Ti-6Al-4V. Thin wall machining is an advance machining technique especially used in machining turbine blades which can be done both in a conventional way and using a special technique known as trochoidal milling. The experimental design consists of conducting trials using combination of cutting parameters such as cutting speed (vc), 90 and 120 m/min; feed/tooth (fz) of 0.25 and 0.35 mm/min; step over (ae) 0.3 and 0.2; at constant depth of cut (ap) 20mm and using coolant. A preliminary assessment of machinability of Ti-6Al-4V during thin wall machining using trochoidal milling is done. A correlation established using cutting force, surface texture and dimensional accuracy.

One-hit machining of titanium alloy (Ti-6al-4v) wall component with trochoidal milling

This research will definitely give guidelines to industries associated with titanium slot machining.

Machinability assessment of Ti-6Al-4V for aerospace applications

Titanium alloy Ti-6Al-4V is the most commonly used titanium alloy in the aerospace and biomedical industries due to its superior material properties. An experimental investigation has been carried out to evaluate the machinability of high performance aerospace alloys (Ti-6Al-4V) to determine their in service performance characteristics based on different machining strategies. Nearly 80-90% of the titanium used in airframes is Ti-6Al-4V. The experimental design consist of face milling Ti-6Al-4V at 12 different combinations of cutting parameters consisting of Depth Of Cut (DoC)- 1, 2 and 3 mm; speeds- 60 and 100 m/min; coolant on/off and at constant feed rate of 0.04mm/tooth. Post machining analysis consists of cutting force measurement, surface texture analysis and metallographic analysis. The future work consists of in-depth investigation into the phase transformational reactions during machining.

Tool wear and surface integrity analysis of machined heat treated selective laser melted Ti-6Al-4V

In this study, the tool wear and surface integrity during machining of wrought and Selective LaserMelted (SLM) titanium alloy (after heat treatment) are studied. Face turning trails were carried out onboth the materials at different cutting speeds of 60,120 and 180 m/min. Cutting tools and machinedspecimens collected are characterized using scanning electron microscope, surface profiler and opticalmicroscope to study the tool wear, machined surface quality and machining induced microstructuralalterations. It was found that high cutting speeds lead to rapid tool wear during machining of SLMTi-6Al-4V materials. Rapid tool wear observed at high cutting speeds in machining SLM Ti-6Al-4Vresulted in damaging the surface integrity by 1) Deposition of chip/work material on the machinedsurface giving rise to higher surface roughness and 2) Increasing the depth of plastic deformationon the machined sub surface.