Metal Powder Injection Molding (MIM) is a new type of powder metallurgy production process developed on the basis of traditional powder metallurgy and plastic injection processes. The process involves mixing metal powder and organic binder to obtain a feed with certain fluidity at a certain temperature, injecting it into the mold cavity on the injection machine for molding, and then bonding and sintering the product blank to obtain the product; Then, polishing, heat treatment, and surface chemical treatment are carried out as needed to obtain the final product.
In traditional hard alloys, compression molding is generally used for molding. Due to the limitations of molds and molding processes, only products with simple shapes and single structures can be produced. For products with complex structures, shapes or special shape requirements, they can only be completed through subsequent processing. Due to the high hardness, strength, and wear resistance of hard alloys, it brings great difficulties to the subsequent processing of hard alloys and greatly increases processing costs.
The application of MIM technology in the production of hard alloys has greatly changed the limitations of hard alloys in forming. This method not only inherits the advantages of conventional powder metallurgy production processes with few, no or few cuts, but also overcomes the defects of low material density, uneven material, difficulty in forming structures, complex shapes, and thin-walled products. At the same time, it can control product accuracy well, obtain final or near final shaped products with good product consistency and balanced performance, thereby significantly reducing production costs. Especially suitable for producing products with large quantities, complex structures, and special requirements.
1、 Hard alloy MIM process
1. Raw materials:
(1) Powder: Due to the particularity of MIM technology, the particle size of alloy powder is 20 μ Below m, in order to improve the fluidity of the feed injection and make the raw material uniform.
(2) Organic binder: MIM requires alloy powder particles to have fluidity and lubricity during feed injection, and also requires alloy product blanks to have sufficient strength. Therefore, it is required that the binder has good lubricity, appropriate viscosity, lower melting point, wider pyrolysis temperature range, is easy to degrease, and does not react chemically with alloy powder.
2. Process flow:
Metal powder and binder → mixing → feeding → injection molding → extraction → debonding, sintering → post-treatment → finished product.
The uniformity of feed composition is the main factor affecting feed performance. It not only affects the flowability of feed, but also has a significant impact on the material uniformity and mechanical properties of alloy products. Therefore, feeding is an important process in MIM. Through mixing equipment, hard alloy powder is uniformly mixed with organic binders to form a well flowing feeding.
The influence of injection molding temperature, injection pressure, mold accuracy, and feed shrinkage coefficient (by adjusting the binder content and injection process parameters) on the accuracy of the blank is significant. Therefore, controlling these factors is crucial for obtaining high-precision finished products.
The binder content of hard alloy blanks produced by MIM technology is generally between 4.2% and 7.2%. Before sintering, it is necessary to remove the organic binder from the blank through a certain debonding process to prevent defects such as bubbles and cracks that may occur during sintering and cause waste. Sintering is one of the important processes in the production of hard alloys, which has a significant impact on the structure and properties of hard alloy products. In the MIM sintering process, different sintering processes should be adopted according to different grades, binder systems, and alloy compositions.
2、 Performance of Hard Alloy MIM Products
The MIM process is applied to produce hard alloy products, and its mechanical properties meet the corresponding metallurgical standard requirements for hard alloy performance (YS/T400-94), meeting the customer's usage requirements. The physical and mechanical properties of some common grades of hard alloy MIM products are shown in Table 1.
3、 Application examples
1. Nozzles, spray guns:
The nozzle and spray gun for oilfield use using WC Ni Co series hard alloy powder injection molding are shown in Figures 1 and 2. High carbon WC powder is used, and the binder system adopts an oil-based multi-component binder system.
2. Mobile phone parts:
The mobile phone parts formed by tungsten alloy powder injection molding are shown in Figure 3, which reduces processing time, lowers costs, and has significant benefits.
3. Table parts:
The integral hard alloy case, pendulum, and buckle are shown in Figure 4. The use of WC TiC Ni Co and other additives, along with the use of paraffin based multi-component binders, not only meets the material performance requirements well, but also ensures the shape and dimensional accuracy of the product.
4. Conclusion
The application of MIM technology in hard alloy production has greatly changed the limitations of traditional hard alloy production in terms of molding, making it possible to mass produce complex shaped, structurally complex, and thin-walled hard alloy products. Due to MIM being a near net forming production technology, it not only greatly reduces the subsequent processing volume of products and lowers the production cost of hard alloys, but also brings opportunities for expanding the application fields of hard alloys. MIM technology, as an active cutting-edge technology in the new manufacturing industry, will receive people's attention and wide application.