Stiliana Raynova

Consolidation of Titanium, and Ti-6Al-4V Alloy Powders by Powder Compact Forging

Consolidation of titanium and titanium alloy powders using thermomechanical powder metallurgy (TPM) processes (powder compact forging, extrusion and rolling) is one way that can lead to cost-effective production of high value-added consolidated titanium and titanium alloy products such as near-net shaped components, tubes and plates. This paper provides an overview of the quality, microstructure (to limited depth), porosity level and mechanical properties of disks produced using open die forging of powder compacts of CP titanium and Ti-6Al-4V alloy powders. The general materials science principles underlying the relationships between processing conditions, microstructure and the mechanical properties of the disks made by using the powder compact forging are discussed.

A Study of Polyvinyl Butyryl Based Binder System in Titanium Based Metal Injection Moulding

Metal injection moulding (MIM) is an innovative injection moulding technique widely used to produce complex shaped components from feedstock composed of metal powders and thermosetting or thermoplastic binders. In MIM, binder selection and formulation are considered as critical processes since binder characteristics dictate the success of MIM. The purpose of this study is to determine the feasibility of polyvinyl butyryl (PVB) based binder system in Ti-6Al-4V(wt.%)/binder feedstock, as well as to understand the effects of key parameters, such as powder loading and mixing conditions on the rheological properties of a feedstock. In this study, PVB, polyethylene glycol (PEG), and stearic acid (SA) were chosen to formulate a multi-component binder system to prepare Ti-6Al-4V based feedstock with the aid of three types of mixers: a compounder, a modified mechanical mixer and a twin screw extruder. Further, morphological analysis was performed using optical microscopy and scanning electron microscopy. Thermal analysis was performed using simultaneous differential thermal analysis and thermogravimetric analysis. Results showed that binder formulation was reasonably successful with the aid of both mechanical mixer and a twin screw extruder under certain mixing conditions, and the critical powder loading was 68 vol.%, resulting in an optimum powder loading of 63 vol.% .

Sintering and open die forging process for P/M titanium alloy with additions of trace amount of erbium

Abstract Er powder with a very large particle size was added to Ti metal and Ti–6Al–4V alloy to achieve a supersolidus liquid sintering effect. However, instead of promoting sintering and homogenisation, very big worm-like voids were created by a substantial Er–Ti liquid phase. Such voids were surrounded by Er segregation and made tensile testing of a Ti–Er alloy impossible. An open die forging process was then employed to diminish these pores and homogenise the Er distribution. Fine Er spots or acicular textures were found in the matrix, but some Er segregation still persisted. The material is weakened by segregation. Good mechanical properties are obtained when the testpieces have reduced levels of Er segregation. Further work will be focused on the homogenisation of Er.

Mechanical Behaviour of Titanium, Ti-6Al-4V (wt %) Alloy and Ti-47Al-2Cr (at %) Alloy Produced Using Powder Compact Forging

Powder compact forging was used to produce bulk consolidated titanium and Ti-6Al-4V (wt %) and Ti-47Al-2Cr (at%) alloy disks from hydrogenated and dehydrogenated (HDH) and gas atomised powders (GA) powders (in the case of titanium and Ti-6Al-4V) and a mechanically milled powder (in the case of Ti-47Al-2Cr alloy). The bulk titanium and Ti-6Al-4V (wt %) alloy have been produced by forging of the powder compacts. The Ti-47Al-2Cr (at %) alloy was produced using canned powder compact forging of a Ti/Al/Cr composite powder. The purpose of the present study is to investigate the deformation and fracture behaviour of the bulk consolidated as-forged materials, by conducting tensile testing at room temperature (RT) and examination of the fractured specimens which had near-α, α + β and  phase structures, respectively. It was found that as-forged bulk titanium disk produced using HDH powder showed a yield point with a yield strength of ~700 MPa and with a considerable amount of ductility. While the as-forged Ti-6Al-4V (wt %) alloy produced using HDH powder, fractured prematurely without any yielding. On the other hand yielding was observed in the as-forged Ti-6Al-4V (wt %) alloy produced using GA powder, showing a yield strength of ~970 MPa and a considerable amount of plastic strain to fracture. The bulk consolidated Ti-47Al-2Cr (at %) alloy also fractured prematurely with fracture strength of ~125 MPa. The mechanical behaviour of the as-forged bulk materials was found to be dependent on several factors such as initial powders used, green density of the powder compact, forging parameters used during forging. It was expected that the entrapped gas in green compacts, absorbed oxygen, porosity and inter-particle bonding play an important role on the quality of the as-forged material, which in turn affected the mechanical behaviour of the bulk material.

Tensile Properties and Fracture Behaviour of Induction Sintered Ti and Ti-6Al-4V (wt %) Powder Compacts

Induction heating of powder compacts could be a very effective method for metal powder consolidation to get the final product or as an intermediate consolidation step to produce feedstock for metal powder forging or extrusion. Our study has proven that only a few minutes of induction heating, of Ti and Ti-6Al-4V (wt %) powder compacts, increases their density dramatically and causes a significant sintering effect, as evidenced by the formation of interparticle diffusion bonding and reflected by the tensile properties of the induction sintered powder compacts. This paper presents and discusses the results of a study on the tensile properties and fracture behaviour of the Ti and Ti-6Al-4V powder compacts sintered under a variety of induction heating conditions. Keywords: Titanium alloys, powder consolidation, powder compact sintering, induction heating.

The possibility of synthesizing bulk nanostructured or ultrafine structured metallic materials by consolidation of powders using high strain powder compact forging

Abstract This paper assesses the possibility of synthesizing bulk nanostructured or ultrafine structured metallic materials by consolidation of powders using high strain powder compact forging. In the work, inter-particle boundaries were categorized into three types: Type I-fresh surface/fresh surface boundaries, Type II-fresh surface/oxide covered surface boundaries and Type III-oxide covered surface/oxide covered surface boundaries. The possibility of turning each of these types of inter-particle boundaries into grain boundaries or interphase boundaries without causing significant grain/particle growth through plastic deformation, and the amount of plastic deformation needed were discussed. The related experimental findings were also reviewed. It was concluded that by using high strain powder compact forging the possibility of producing bulk nanostructured or ultrafine structured metallic materials is very high.

Fabrication of TiAl Target by Mechanical Alloying and Applications in Physical Vapour Deposition Coating

The research involves the development of a powder metallurgical route for producing good quality TiAl targets for making physical vapour deposition (PVD) coatings. Mixtures of elemental titanium and aluminium powders were mechanically milled using a novel discus milling technique under various conditions. Hot isostatic pressing (HIP) was then employed for consolidation of the mechanically alloyed powders. A cathodic arc vapour deposition process was applied to produce a TiAlN coating. A microstructural examination was conducted on the target material and PVD coatings, using X-ray diffractometry (XRD), X-ray photoelectron spectrometry (XPS) and scanning electron microscopy (SEM). It has been found that combining mechanical alloying and HIP enable us to produce a fairly good quality of TiAl based target. The PVD coatings obtained from the TiAl target showed very high microhardness values.

Characterisation of Ti(Al,O)-Al2O3 Composite Powders and Thermally Sprayed Coatings

A feedstock of Ti(Al,O)-Al2O3 composite powders was produced by high energy milling of a mixture of Al and TiO2 powders followed by a thermal reaction process. The feedstock was then thermally sprayed using a high velocity oxygen-fuel (HVOF) technique on H13 steel substrates to produce Ti(Al,O)-Al2O3 composite coatings. The performance of the coatings was assessed in terms of thermal fatigue behaviour and reaction with molten aluminium (soldering). The composite powders and coatings were characterised using scanning electron microscopy (SEM) and X-ray diffractometry (XRD). This paper reports the experimental observations and discusses characteristics and potential applications of the composite coatings.

Preparation of Titanium Alloy Parts by Powder Compact Extrusion of a Powder Mixture and Scaled up Manufacture

Blended Elemental Powder Metallurgy (BE-PM) is a very attractive method for producing titanium alloys, which can be near-net shape formed with compositional freedom. However, a minimization of oxygen pick-up during processing into manufactured parts is a big challenge for powder metallurgy of titanium alloys. In this paper, different approaches for preparing titanium alloy parts by powder compact extrusion with 0.05-0.1wt.% of oxygen pick-up during manufacturing are discussed. The starting materials were a powder mixture of HDH titanium powder, other elemental powders and a master alloy powder. Different titanium alloys and composites, such as Ti-6Al-4V, Ti-4Al-4Sn-4Mo-0.5Si, Ti-5Al-5V-5Mo-3Cr, and Ti-5Al-5V-5Mo-3Cr-5vol%TiB, with different profiles such as round and rectangular bars, a wedge profile, wire and tubes have been successfully manufactured on a laboratory and pilot-plant scale. Furthermore, a possible route for scaling up the titanium processing capabilities in the University of Waikato has also been discussed.

Tensile properties of Ti-6Al-4V discs produced by open die powder compact forging of pre-alloyed HDH powders

Open die powder compact forging was employed for consolidation of Ti-6Al-4V(wt.%) powders which were produced by a hydride-dehydride (HDH) process. The powders were initially warm-compacted into cylindrical shapes. Induction heating was used for preheating the compacts prior to forging into plates. The effect of the forging parameters such as temperature and pre-sintering time on the microstructure and tensile properties of the forged plates were studied. Porosity and the alpha-beta phase distribution were investigated to enable a better understanding of their effect on the fracture behaviour of the tensile tested specimens. The fracture surfaces of the broken tensile specimens were analysed using Scanning Electron Microscopy (SEM) to find the correlation between microstructure, residual porosity and fracture behaviour. A combination of high ultimate tensile strength of over 1200MPa and a good ductility reflected by an elongation to fracture of around 10% was achieved in some of the forged discs.