What Is Hot Isostatic Pressing?
Overview
Hot Isostatic Pressing (HIP) technology was proposed in the 1950s by Battelle Memorial Institute. It was originally used for diffusion bonding of nuclear reactor parts. It was soon discovered that it is an excellent technology for powder consolidation and elimination of porosity in cemented carbide. Hot isostatic pressing has developed into a recognized technology in industry for densification and alloy densification to improve its mechanical properties, smoothness and plating ability. Its main function is to remove voids inside the material. Most metal injection molding manufacturers outsource this process to professional MIM hot isostatic pressing service providers or certified HIP processing suppliers.
Because the process is simple and only requires high temperature and high pressure treatment of parts, hot isostatic pressing equipment is specially designed to achieve these conditions, and the equipment cost is high. During the hot isostatic pressing process, high temperature makes the material very soft and easy to deform, while high pressure compresses the micropores inside the material. The pores are eliminated through creep and diffusion mechanisms. The yield strength (YS) of the material decreases as the temperature increases. When oxygen is applied to metal materials, the warm protons will not diffuse into the metal materials. The pre-deformation mechanisms in the early stage of hot isostatic pressing include Nabarro-Herring creep (diffusion through the interior of the grain), Coble creep (grain boundary creep) and dislocation creep.
The final stage of hot isostatic pressing includes diffusion bonding of the closed pore walls to each other. Because the sintered density of metal injection molded parts is high enough and the internal pores are closed (not interconnected) and compressible, they are very suitable for hot isostatic pressing technology. If the metal injection molded parts have exposed and unclosed pores, the vented compressed gas will fill these pores without compressing them. After preparing titanium alloy by powder hot isostatic pressing technology, the metal powder needs to be put into a hot isostatic pressing furnace and vacuumed, and then the furnace door is closed, so that the close bonding between the powders can be achieved under the action of pressure and temperature. Fortunately, the sintered density of metal injection molded parts usually reaches more than 95%, and in most cases exceeds 98%, while the minimum density of hot isostatic pressed materials is between 92% and 94%. Table 9.1 shows the minimum density of metal injection molded parts after hot isostatic pressing. By marking on metal injection molded parts with a high-temperature marker can test whether it can be hot isostatically pressed.
If the mark is not recorded clearly and does not diffuse into the interior of the part, hot isostatic pressing can be used to obtain higher density. If it enters the interior of the part, hot isostatic pressing cannot be used to obtain higher density. Many global metal injection molding HIP densification suppliers now offer one-stop post-processing services to help customers achieve full-density MIM components.
Table 9.1 Minimum Density of Metal Injection Molded Parts After Hot Isostatic Pressing (g/cm³)
| Alloy | Theoretical Density | Minimum Density After HIP |
|---|---|---|
| Ti–6Al–4V | 4.43 | 4.1 |
| F2886 (F75) | 8.4 | 7.8 |
| 17–4PH SS | 7.8 | 7.2 – 7.10 |
| 316L SS | 8.0 | 7.4 |
| Low-alloy steel | 7.6 – 7.9 | 7.1 – 7.3 |
| S7 | 7.83 | 7.2 |
Hot Isostatic Pressing Process
The hot isostatic pressing process uses compressed inert hot gas to apply pressure to the workpiece. For metal injection molded parts, the hot isostatic pressing temperature is usually 100~200℃ lower than the sintering temperature, and the pressure is usually in the range of 15 000~20 000 psi (105~140 MPa). The preferred gas for hot isostatic pressing is argon because its atomic size is large. Nitrogen can also be used, but the effect is not as good as argon. Figure 9.1 shows a schematic diagram of hot isostatic pressing. The process is a batch processing process and usually lasts 4~10 h. The steps are as follows:
(1) Load the material into the furnace and close the furnace door;
(2) Vacuum and fill with inert gas;
(3) Heat up and pressurize at the same time;
(4) Cool down and depressurize at the same time;
(5) Exhaust;
(6) Take out the parts.
This sequence represents an independent hot isostatic pressing process and is usually used to manufacture dense billets for casting and metal injection molded parts. In the cemented carbide industry, a process called hot isostatic pressing sintering or pressure sintering is the process is used to densify cemented carbide/cobalt powder based components. Since sintering and hot isostatic pressing are completed in one step, the total processing time and cost are reduced. The pressure is usually 1.5~10 MPa, which is much lower than the standard hot isostatic pressing, but it is enough to eliminate pores in cemented carbide. Leading China MIM parts hot isostatic pressing vendors and international metal injection molding full-density solution providers widely adopt this integrated technology to deliver higher fatigue performance components for aerospace, medical and automotive customers.
