Powder Manufacturing Processes Machinery's Handbook, 31st Edition
1540
Fluid
Mold Seal Plate
Pressure Seal Cover
Pressure Vessel
Metal Mandrel
Wire Mesh Basket
Rubber Mold (bag)
Pressure Source
Powder
Fig. 16. Cold Isostatic Pressing Good dimensional accuracy is difficult to achieve in isostatic pressing, due to the flex - ible mold. Normally, this technique is only used for roughly shaped components, all of which require considerable secondary operations to produce the final, accurately dimen - sioned component Hot Isostatic Pressing: Hot isostatic pressing (HIP) is a manufacturing process that uniquely combines pressure and temperature to produce materials and parts with substantially better properties than other techniques. The pressure medium is an inert gas, usually argon. Powders are usually encapsulated in a sheet metal container (mold), which is assembled of steel sheets and optionally pipes and metallic inserts by TIG weld- ing. The container is tested to make sure it is leak-free and is designed to give the part its full density and desired shape. Once the container is leak-free, the powders are filled into it via a fill tube. In order to achieve maximum and uniform packing of the powders, which is necessary to ensure a predictable and consistent shrinking, a vibration table is used. In special cases such as critical aerospace applications, the filling operation is done under inert gas or vacuum to minimize contamination of the powders. The next step is outgas- sing to remove adsorbed gases and water vapor. After outgassing, the fill tube is welded to seal the container. The absence of leaking is critical. Otherwise, when the HIP vessel is pressurized, argon will enter the container and become entrapped in the powder mass, creating argon-filled pores, with damaging effects on the mechanical properties. The container is then placed in a hot isostatic vessel. The HIP vessel is evacuated to elim inate the air. Then, during heating, argon gas pressure is increased in the vessel. After the calculated pressure is reached, further increase in pressure will occur through gas ther- mal expansion. During the holding time, gas pressure and temperature are kept constant. After this, a rapid cooling takes place, with decreasing pressure and temperature. The hot isostatic pressing parameters of pressure, temperature and time are predeter mined to give the material full density. Chosen temperatures are below solidus (approxi mately 0.8 T solidus) to avoid a liquid phase. Modern HIP systems can feature uniform rapid cooling (URC) that circulates a lower temperature gas to cool the part at a controlled rate of up to 212° F/min (100° C). The HIP quenching technique cuts cycle time dramatically by shortening the cooling stage by as much as 80 percent. It also provides the benefit of combining heat treatment with HIP in a single step. The uniform rapid cooling restricts grain growth and thermal distortion of the parts and avoids surface contamination by using high purity argon gas. A HIP unit consists mainly of a pressure vessel, a heating system and an argon gas sys tem. Various HIP constructions are available. Fig. 17 schematically illustrates one type of hot isostatic pressure unit. Molybdenum furnaces are used for temperatures up to 2462° F (1350° C) and 14500 to 43511 psi (100 to 300 MPa). The sheet metal container remains on the part after the hot isostatic pressing and heat treatment, and it is removed by machining or by chemical milling. After the container has been removed, depending on the type of material and the application, the PM HIP part will be heat treated, machined, and subjected to various types of quality control, such as ultrasonic inspection, dimensional control, and testing of mechanical properties.
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online