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Metal Casting Technologies : June 2008
TECHNICAL FEATURE Some aspects of titanium alloy castings I Dr. John Pearce. n recent years Titanium Alloys have become more available and cost competitive in both wrought product forms and as near net shape castings [1-5]. Titanium and its alloys are now generally well known for their excellent resistance to corrosion, competitive strength to weight ratios, and useful elevated temperature properties leading to their widespread use in aerospace applications, for marine and chemical plant corrosion service, and for medical prostheses. Titanium Alloys can be as strong as Steels but half the weight. Thanks to the highly protective nature of its oxide film, Ti is more resistant to pitting attack than Super Duplex Stainless Steel. Ti Alloys also have superior corrosion fatigue and stress corrosion cracking resistance in seawater. The relatively high melting temperature of 1668o C of Titanium contributes to its high temperature properties with superior creep resistance to Aluminium Alloys. BASIC METALLURGY Titanium is classified as a light metal having a relative density of 4.54 compared to that of 2.71 for Aluminium and 7.87 for Iron. Titanium can exist in 2 allotropic forms: ¦ alpha (a) low temperature form with a hexagonal close packed (HCP) structure ¦ beta (ß) high temperature form with a body centred cubic (BCC) structure. In pure Titanium the alpha phase is stable up to 883o C: this called the “beta transus” temperature. Some alloying elements stabilize the alpha structure raising the beta transus. The alpha formers include the substitutional elements - Aluminium, Gallium & Germanium, and the interstitial elements – Oxygen, Nitrogen and Carbon. Aluminium is the most common alloying addition since it reduces density and improves ductility. In contrast other alloying elements stabilize the higher temperature beta phase down to lower temperatures. Additions of Vanadium or Molybdenum give rise to isomorphous systems. Other beta formers, notably Iron, Chromium, Manganese, Nickel, Copper and Silicon, give rise to eutectoid systems [3,5]. Titanium Alloys can be classed as 3 main types: ¦ alpha alloys – these are generally non heat treatable, having medium strength, good notch toughness, and good creep resistance. ¦ alpha - beta alloys – mixtures of alpha and beta giving to medium - high strengths. ¦ beta alloys – these are heat treatable to high strengths but have relatively low ductility and higher densities. Commercial purity (CP) Ti Grades 1-4 are the most important alpha alloys. The main elements present are Iron and the 32 www.metals.rala.com.au Figure 1: Microstructure of cast Ti-6Al-4V alloy showing grain boundary alpha phase (B) and plate-like colonies of alpha phase (C). The structure has been subjected to hot isostatic pressing to remove porosity . interstitials C, O, N, and H. Proof stress values for CP Ti range from 170MPa for Grade 1 (99.5%Ti) to 480MPa for the higher interstitial Grade 4 (99%Ti). Castings in CP grades are not normally heat treated and are used for their corrosion resistance. Small additions of Palladium (0.05-0.15%) or Ruthenium (0.08- 0.14%) can be made to CP and other Ti alloy grades to further improve resistance to cavitation, erosion, pitting and crevice corrosion. The alpha alloy Ti-5Al-2.5Sn (grade 6) used for low temperature toughness has also been produced as castings. In some alloys small amounts of beta phase can be present: these are termed near alpha alloys. The most popular alpha-beta alloy for casting is Ti – 6%Al – 4%V. On cooling after solidification, below the beta-transus, at around 995oC, the cast dendritic beta structure begins to transform to alpha. The alpha phase forms at the beta grain boundaries and as plate-like colonies within the beta grains leaving residual beta between the plates of alpha giving a typical Widmanstatten structure (Figure 1). Castings can be heat treated by annealing at 730-850o C under vacuum to minimize oxidation and to allow any dissolved Hydrogen to diffuse out giving tensile strengths around 900-950MPa. Ti-6Al-4V alloy has been used for some time for body implants such investment cast hip prostheses but concerns over possible damaging effects of Al and V on body tissue have lead to the development of newer Ti-Mo-Nb alloys which are V free and have very low Al levels. Ti, Mo and Nb do not have any known adverse effects on the body. Based mainly on Ti-10V and Ti-15V the beta and near-beta alloys have developed as high strength materials having tensile strengths up to 1500 MPa. They have good hot formability for use as springs and fasteners, and as air frame forgings.