Low-Cost Powder Metallurgy Titanium Alloy Shape Control Integration

Titanium alloy has excellent properties such as light weight, high strength, and corrosion resistance, and plays an irreplaceable role in many fields. However, the poor processing performance of titanium leads to high cost of parts produced by traditional casting and forging + machining processes, which limits its application. Powder metallurgy technology is to directly sinter powder into parts, which can fundamentally solve the problem of high titanium processing costs.

At present, powder metallurgy titanium alloy parts have two major problems: poor performance and easy deformation during sintering. Among them, low sintering density and high oxygen content are the main reasons for poor performance, and uneven shrinkage caused by uneven particle mixing is the root cause of deformation of sintered products. Therefore, it is urgent to develop low-cost powder metallurgy titanium alloy shape control integrated technology to promote the engineering application and development of powder metallurgy titanium alloy.

The "Functional Modification and Advanced Manufacturing of Special Powders" research team established by Yang Yafeng, a researcher at the Institute of Process Engineering, Chinese Academy of Sciences, has carried out a series of research work around the problem of shape control of powder metallurgy titanium alloys. Through years of research, a series of high-efficiency sintering aids and oxygen impurity adsorbents have been developed, which have greatly improved the sintering density and performance of powder metallurgy titanium alloys. On this basis, a new type of coated titanium powder with high homogenization characteristics has been developed, which has achieved precise shape control of complex-shaped titanium alloy parts. More than 10 papers have been published on related research results, which have attracted widespread attention in related fields.

Article Highlights
The design principle of powder metallurgy titanium alloy sintering aids and its mechanism for strengthening sintering densification behavior are proposed.

The law of oxide film dissolution and diffusion behavior during sintering was explored, and an efficient oxygen impurity adsorbent was designed, which significantly improved strength and plasticity;

Graphical analysis：

Figure 1：

Difficulties and solutions faced by powder metallurgy titanium alloys

Figure 2：

Sintering microstructures of (a) Ti–3Fe–1Si and (b) Ti–3Fe–2Si samples, and (c) the changing trends of sintering density and properties with Si addition.

The addition of Fe-Si sintering aid increases the sintering density of titanium alloy to more than 99%

Figure 3：

(a) Expansion curve of Ti powder compact, high-resolution XPS depth profile spectra of original Ti powder (b) and heat-treated Ti powder (c) at different etching times.

The results of thermal expansion curve and photoelectron spectroscopy both indicate that the oxide film on the titanium surface begins to diffuse into the matrix at 670°C. Based on this, the design criteria for oxygen impurity adsorbents are established (reacting and removing the oxide film before it dissolves).

Figure 4：

Trends in titanium alloy properties with type and amount of oxygen absorbent added

By adding 0.3wt% NdB6 high-efficiency oxygen absorber, the elongation of titanium alloy is increased from ~12.5% ​​to more than 17%.

Figure 5：

(a) SEM photo of coated powder, (b) thermal expansion curves of coated powder and mixed powder, (c) dimensional accuracy measurement photo of automobile shock-absorbing parts sintered by mixed powder and coated powder

The alloy composition of the coated powder is more uniform, and the cold-pressed green body exhibits uniform linear shrinkage during the sintering process, solving the problem of easy deformation of complex-shaped parts of powder metallurgy titanium alloy during sintering.

Figure 6：
(a–d) morphology and element distribution of coated powder, (e) photos of automotive parts after pressed green body and sintered coated powder

The surface of titanium powder is uniformly coated with sintering aids and oxygen absorbers, which not only achieves high-density sintering and high performance, but also realizes the one-time sintering forming of complex parts.

It was revealed that the uneven composition of mechanically mixed powders is the intrinsic cause of sintering deformation, and a highly homogenized coated titanium powder was developed, which strengthened the linear shrinkage behavior of the product during sintering and achieved precise forming.

This paper introduces the new strategy proposed by the team to strengthen the sintering densification of powder metallurgy titanium alloy by "reaction-induced formation of low-temperature liquid phase". The new Fe-Si auxiliary agent developed produces transient liquid phase and permanent liquid phase during the sintering process. The two liquid phases increase the sintering density to more than 99% through synergistic strengthening. Then, in order to solve the brittleness problem caused by high content of dissolved oxygen impurities, the team first revealed that the initial dissolution temperature of the oxide film on the surface of titanium powder is 670ºC, and established the design criteria of oxygen impurity adsorbent of "reacting and removing the oxide film before it dissolves".

Then, 0.3wt% of NdB6 oxygen adsorbent was introduced to achieve a significant improvement in the plasticity of titanium alloy. Based on the above research work, the uniform coating of sintering aids and oxygen impurity adsorbents on the surface of titanium powder was achieved through the fluidized bed powder surface functional modification technology, which promoted the rapid homogenization of the composition of the green body during the sintering process, strengthened the uniform linear shrinkage ability, and solved the sintering deformation problem of powder metallurgy titanium alloy. Based on the above theories and research results, the team cooperated with enterprises to form a new integrated powder metallurgy titanium alloy shape and property control technology, established a low-cost production line for powder metallurgy titanium alloy, and promoted the development of powder metallurgy technology and engineering applications of titanium alloys.