Research progress of titanium alloy binder system in titanium alloy injection molding

Research progress of titanium alloy binder system in titanium alloy injection molding

Brief introduction of powder injection molding technology

Powder injection molding technology is developed on the basis of powder metallurgy technology. Combined with plastic injection technology, it realizes the utilization rate of raw materials of nearly 100%. It is a near net forming technology. The general operation process is as follows: first, the prepared powder and binder are mixed and granulated to prepare a granular feed, then the feed is molded into a product green of a specific shape on an injection molding machine, and then the product with the required performance is obtained through degreasing and sintering. The advantages of titanium alloy powder injection molding are:

① It can realize the batch preparation of small 3D complex shape parts;

② Uniform composition, fine structure and excellent mechanical properties;

③ It is easy to add alloying elements to prepare required materials;

④ It is easy to control the material microstructure.

In the injection molding process of titanium alloy powder, the design of the binder is a core link, which is responsible for making the titanium alloy powder smoothly complete the injection in a fluid state in the entire injection molding process, and can maintain the shape until the pre sintering stage after the green billet is formed. However, the added binder has also become one of the most likely sources of pollutants in the entire injection molding process. In addition, higher content of binder will reduce the powder loading, which will not only lead to the shape collapse of the body after degreasing, deformation, cracking and other defects, but also increase the sintering shrinkage, seriously reducing the product size accuracy; Although the binder with low content can ensure a high powder loading, it is difficult to prepare a feed with good fluidity and complete the injection smoothly. Ensuring the balance between the binder content and the powder loading greatly increases the difficulty in the binder research process. It can be seen that although the final composition of sintered products is not determined by the binder, its selection and use will directly affect the subsequent degreasing, sintering and other processes, thus affecting the product quality. Therefore, in the research work of powder injection molding titanium alloy technology, the focus is on the binder technology, and the key problem is grasped. In this paper, the research status of different binder systems for powder injection molding of titanium alloy is introduced, and the improvement measures for the existing problems are proposed.

Qinhuangdao Zhongzhong produces powder injection molding titanium alloy products for precision parts: (a) engineering application parts prepared by German TiJet Company; (b) German TiJet company prepares biomedical parts; (c) Ti ❑ 6Al ❑ 7Nb alloy bone screws; (d) CP Ti artificial stapes; (e) Titanium alloy spectacle frame; (f) Ti ｜ 6Al ｜ 4V real case

China titanium alloy eyeglass frame is designed and manufactured by Zhongwei

Some research progress

Titanium metal has high activity. When the temperature is close to 400 ℃, it is easy to carbonize, nitride and oxidize, forming impurities such as titanium carbide, titanium nitride and titanium oxide, which reduces the relative density of sintering and worsens the mechanical properties of materials. Among impurities such as carbon, hydrogen, oxygen and nitrogen, it is usually more difficult to control the oxygen content than other impurities. The influence of oxygen content (mass fraction) on the mechanical properties of titanium alloys is shown in the figure. With the increase of oxygen content, the strength of titanium alloys increases, but the plasticity deteriorates significantly. Therefore, the following three points should be achieved when selecting the binder for powder injection molding titanium alloy:

① High powder loading shall be ensured as far as possible to improve the dimensional accuracy of products;

② The feeding material should have sufficient fluidity to ensure that the entire cavity can be filled smoothly during injection;

③ The binder components used do not react with highly active titanium materials, and there is no residual decomposition and removal.

At the initial stage of research, most of the binders used in powder titanium alloy injection molding followed the binder system of other metals. With the deepening of scientific research, new binders such as water-soluble and polyacetal based binders appeared. At present, the binder systems widely used in powder injection molding titanium alloys are thermoplastic wax based adhesives, plastic based adhesives, and environmentally friendly water-based adhesives.

Effect of oxygen content on mechanical properties of titanium alloys

Conclusion and prospect

The further expansion of the application market of powder injection molding titanium alloy faces two challenges. First, the spherical titanium powder with relatively mature powder injection molding technology has high cost, and its products are difficult to be used in 3C and automobile fields on a large scale; The other is the lack of binder system suitable for titanium alloy powder injection molding. The emergence of hydrogenated dehydrogenated titanium powder ushered in the dawn of cost reduction. Compared with spherical titanium powder, its cost can be reduced to about 20%. However, most of the binder systems used in powder injection molding titanium alloys are still used from other metals, and the characteristics of titanium alloy materials have not been fully considered, so the research and development process was once in a bottleneck. Although the titanium alloy binder system independently developed in China has broken the technical blockade of companies such as BASF, its research and development is still in the stage of large-scale trial and error due to the lack of systematic theoretical guidance, and the process of practicality is still relatively slow. Based on the research status of binder system, the author puts forward some suggestions on the existing problems of powder injection molding titanium alloy at this stage for the reference of researchers in the same industry, and jointly promote the industrialization process of powder injection molding titanium alloy.

(1) In view of the low dimensional accuracy and poor plasticity of wax based binder powder injection molded titanium alloy products, the research on partial replacement of PW components by PEG can be further deepened. Compared with PW, PEG has better wettability and lower decomposition temperature, which helps to increase the loading capacity of the feed and reduce the content of impurities in the degreased billet, thus improving the dimensional accuracy and mechanical properties of titanium alloy products.

(2) In view of the problem that POM, the main component of plastic based binder, is easy to react with low-cost and highly active hydrogenated dehydrogenated titanium powder, first, a special atmosphere mixer for titanium alloy can be used to prepare feed to isolate oxygen and improve the thermal oxygen stability of POM; Second, in the research and development of new plastic based binder system, continue to optimize the ratio of antioxidants to improve the stability of the feed.

(3) In view of the problem that the green body injected with water-based adhesive is easy to soften, firstly, the strength of green body can be improved by adding low oxygen or even oxygen free skeleton agent components; Second, we can continue to deepen the research of PEG molecular weight on the formability and shape retention of water-based feed, and select PEG molecular weight according to the shape complexity of injection parts.