By: Xin LIN, Mocong YANG, Xiaojing XU, Haiou YANG, Jing CHEN, Weidong HUANG

State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, P.R.China

With the increasing demand of the aviation and aerospace industries, high temperature titanium alloys are widely used for their excellent combination of high strength-to-weight ratio, excellent high-temperature strength and high oxidation resistance. Especially, the development of high temperature titanium alloys has contributed significantly to the spectacular progress in thrust-to-weight ratio of the aero engine. For most of the titanium alloys components used in the supersonic aircraft and aero-engine et al., only local area of the entire component will encounter extreme high temperature, and therefore they need not be made of mono-composition high temperature titanium alloys, and compositionally or functionally graded materials (FGMs) could be appropriate. In so doing, a significant saving in material cost and weight (especially for Ti₂AlNb-based alloys), together with a possible reduction of stress concentration in critical areas, can be achieved. In recent years, combining laser cladding with rapid prototyping (RP) technologies, laser solid forming (LSF) has been shown to be a promising manufacturing technology for FGMs. In this paper, a thin-wall Ti60-Ti₂AlNb alloy (Ti60 and Ti₂AlNb alloys are positioned for service in 600°C and 850°C, respectively), with continuous composition gradient along the deposited direction, was fabricated by laser solid forming (LSF). The phase morphological evolution and microstructure evolution along the deposited direction were investigated. With the increase of aluminum and niobium contents, a series of phase evolutions along the compositional gradient occurred: a+b ® a+a’ ® a’ ® a+b → a+b/B2+a₂ → b/B2+a₂ → b/B2+a₂+O → B2+O → B2. There is a large composition range from Ti60 to Ti60-60wt.%Ti₂AlNb for the existence of a phase. The hardness of the graded material increases with the increase of aluminum and niobium contents, and reaches the maximum with the formation of B2+O phases, then decreases sharply as obtaining the whole B2 phase at the top position of Ti₂AlNb parts. Based on the non-equilibrium phase diagram of the Ti-rich corner, the phase morphological evolution during laser solid forming of the graded materials is explained on combining with the analysis of the influence of the Al and Nb on the stabilities of a, a₂, b/B2 and O phases in titanium alloys and the effects of recurrent tempering/annealing and heat accumulation in laser solid forming.

Figure 1 The XRD patterns of the graded material measured along the compositions gradient.

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here

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