Effect of particle size and manufacturing technique on the properties of the PM Ti-5Fe alloy

Highlights

• The Fe particle size significantly affects the porosity of PM Ti-5Fe.

• Hot forging is effective to reduce and eliminate the residual porosity.

• Elongated prior β grains with a fine α + β lamella sub-structure is achieved.

• The mechanical behaviour strongly depends on powder type and processing condition.

Abstract

In this study the low-cost Ti-5Fe alloy was fabricated by means of the press and sinter powder metallurgy route considering two particle sizes and alternative processing techniques. The primary aim is to reduce the high costs associated with the production of Ti alloys. Thermomechanical processing by means of forging in the β region was also analysed aiming to enhance the mechanical behaviour. The effect of the processing conditions and of the particle size on the microstructure and the mechanical properties was thus investigated. It has been found that the use of larger Fe particles leads to the formation of larger pores due to the dissolution of the Fe powder particles. Consequently, lower mechanical behaviour, tensile properties and Vickers hardness, resulted due to the presence of larger pores. Refinement of the microstructure, texturing, and sealing of the porosity due to forging permits to enhance the mechanical properties with respect to the sintered Ti-5Fe alloy.

Introduction

Titanium (Ti) is a strong and light transition non-ferrous metal that exhibits excellent corrosion resistance properties. Ti alloys have several advantages over both other non-ferrous and ferrous materials, including high specific strength, high heat resistance, and outstanding mechanical properties. The maximum working temperature for Ti alloys is approximately 550 °C [1]. Over the past decades, commercially pure (CP) titanium and Ti alloys have been increasingly employed in aerospace, aviation, shipbuilding, and chemical applications [2,3]. Moreover, the biocompatibility of Ti makes Ti-based materials attractive candidates for various biomedical applications [4] and for the development of new metallic biomaterials [5]. Generally, Ti alloys are classified into the categories of α alloys, β alloys [6], and α + β alloys [1]. Dual-phase α + β Ti alloys are generally employed in applications that require good toughness, high strength, excellent fatigue behaviour, and corrosion resistance. The relative volume fraction of the α and β phases, as well as the phase morphology, mainly depends on the composition of the alloy, the manufacturing method and the process conditions [7]. In Ti alloys, the β phase is stabilised by alloying elements that form substitutional solutions and they are classified as: β-isomorphic (Mo, V, Ta, Nb), β-pseudo-isomorphic (Rh, Re, Ru, W, Ir, Os), and β-eutectoid (Fe, Cr, Co, Mn, Ni, Cu, Au, Ag) [8,9]. Even though among all β-eutectoid stabilisers Fe has the highest β-stabilising strength, its use as an alloying element in wrought Ti has generally been avoided, mainly because of the segregation of Fe during the casting process as a result of its relatively high density. Moreover, the Ti-Fe phase diagram indicates the formation of TiFe intermetallic compounds at 1085 °C [10]. Thus, the formation of the eutectic liquid should be avoided as the presence of intermetallics leads to embrittlement of Ti-Fe based alloys. The attention of many researchers has lately focused on the development of low-cost Ti alloys using elemental Fe as a β-stabiliser as Fe is an abundant non-toxic alloying element that can be added to titanium to produce α + β alloys with the typical lamellar structure [[11], [12], [13]]. Since Fe is the cheapest metallic element available, any addition of this metal to the composition of the alloy lowers the cost of the final material. However, the addition of Fe as β-stabiliser has to be carefully controlled if manufacturing of low-cost materials that exhibit mechanical properties suitable for specific engineering applications has to be considered [[14], [15], [16]].

Powder metallurgy (PM) is an effective process for the fabrication of commercial Ti alloys that overcomes the aforementioned limitations of using Fe as alloying element for Ti. The intrinsic solid-state nature of the PM approach permits to prevent the sedimentation of Fe as well as limit the formation kinetics of the TiFe intermetallic compounds [12] because the alloy does not reach the molten state. Apart being cost-effective, PM processing yields the advantage of a high degree of freedom in the selection of microstructural design and alloy composition [17,18]. Specifically, the blended elemental (BE) approach allows designing Fe-bearing low-cost Ti alloys by simply mixing Fe and Ti particles [19]. For example, Wei et al. [17] reported that the addition of Fe enhances the sintering behaviour of Ti alloys, and the most significant sintering shrinkage is achieved upon heating from a temperature of 950 °C to 1200 °C. The accelerated mobility of Ti atoms as a result of rapid diffusion of Fe is a factor explaining the improved sinterability of Ti-Fe alloys [17,20,21]. Chen et al. [22] investigated the influence of the parameters of the cooling process (holding temperatures of 740 °C, 640 °C, 550 °C) on the formation of α phase and the mechanical properties of Ti-3Fe, Ti-5Fe, and Ti-7Fe alloys produced by vacuum sintering at 1150 °C. The samples with higher values of hardness and tensile strength were obtained at holding temperatures of 740 °C and 640 °C. This can be explained by the presence of acicular α precipitates inside the β grains and the higher amounts of stabilised β phase. The strength of Fe as a β-stabiliser is demonstrated by the suppression of the eutectoid reaction at a temperature of 595 °C, as evidenced by the absence of TiFe intermetallic compounds even at slow cooling rates [22].

The present work therefore studies a cost effective way of fabricating the PM Ti5Fe alloy considering two processing routes: pressing plus vacuum sintering, and hot forging in the β-phase field. The work is carried out as a comparative study to understand the effect of the Fe particle size on the physical and mechanical properties on the fabricated alloy.