For hot pressing, a controlled sequence of pressure and temperature is used. Frequently, the pressure is applied after some heating has occurred because applying pressure at lower temperatures could have adverse effects on the part and tooling. Hot pressing temperatures are several hundred degrees lower than regular sintering temperatures. And nearly complete densification occurs rapidly. The speed of the process as well as the lower temperature required naturally limits the amount of grain growth.
A related method,spark plasma sintering (SPS), provides an alternative to external resistive and inductive modes of heating. In SPS, a sample, typically powder or a precompacted green part, is loaded in a graphite die with graphite punches in a vacuum chamber and a pulsed DC current is applied across the punches, as shown in Figure 5.35b, while pressure is applied. The current causes Joule heating, which raises the temperature of the specimen rapidly. The current is also believed to trigger the formation of a plasma or spark discharge in the pore space between particles, which has the effect of cleaning particle surfaces and enhancing sintering. The plasma formation is difficult to verify experimentally and is topic under debate. The SPS method has been shown to be very effective for densification of a wide variety of materials, including metals and ceramics. Densification occurs at lower temperature and is completed more rapidly than other methods, frequently resulting in fine grain microstructures.
Hot Isostatic Pressing (HIP). Hot isostatic pressing is the simultaneous application of heat and hydrostatic pressure to compact and densify a powder compact or part. The process is analogous to cold isostatic pressing, but with elevated temperature and a gas transmitting the pressure to the part. Inert gases such as argon are common. Powder is densified in a container or can, which acts as a deformable barrier between the pressurized gas and the part. Alternatively, a part that has been compacted and presintered to the point of pore closure can be HIPed in a “containerless” process. HIP is used to achieve complete densification in powder metallurgy. and ceramic processing, as well as some application in the densification of castings. The method is particularly important for hard to densify materials, such as refractory alloys, superalloys, and nonoxide ceramics.
Container and encapsulation technology is essential to the HIP process. Simple containers, such as cylindrical metal cans, are used to density billets of alloy powder. Complex shapes are created using containers that mirror the final part geometries. The container material is chosen to be leak-tight and deformable under the pressure and temperature conditions of the HIP process. Container materials should also be nonreactive with the powder and easy to remove. For powder metallurgy, containers fashioned from steel sheets are common. Other options include glass and porous ceramics that are embedded in a secondary metal can. Glass encapsulation of powders and preformed parts is common in ceramic HIP processes. Filling and evacuation of container is an important step that usually requires special fixtures on the container itself. Some evacuation processes take place at elevated temperature.
The key components of a system for HIP are the pressure vessel with heaters, gas pressurizing and handing equipment, and control electronics. Figure 5.36 shows an example schematic of a HIP set-up. There are two basic modes of operation for a HIP process. In the hot loading mode, the container is preheated outside of the pressure vessel and then loaded, heated to the required temperature and pressurized. In the cold loading mode, the container is placed into the pressure vessel at room temperature; then the heating and pressurizing cycle begins. Pressure in the range of 20–300 MPa and temperature in the range of 500–2000°C are common.
Post time: Nov-17-2020