·Mechanical behavior
Porous structure is one of the widely used characteristics of PM parts. Most of the properties of PM parts, including machinability, are related not only to their alloy chemistry, but also to the porosity of the porous structure. Many structural parts have porosity as high as 15% to 20%, and parts used as filter devices may have porosity as high as 50%. Whereas forged or HIP (Thermal Ion Die Casting) parts have a porosity of 1% or less. HIP materials are suitable for applications in automobiles and aircraft because they can achieve higher strength levels.
The tensile strength, toughness and elongation of PM materials will increase with the increase of density, but because the harmful effect of PM material porosity on the tool tip is reduced, its machinability is improved instead. Increasing the porosity of the material improves the sound insulation properties of the part, and the damping oscillations common in standard parts are reduced in PM parts, which is important for machine tools, air conditioning blowpipes and air tools. In addition, high porosity is also necessary for self-lubricating gears.
·Processing difficulties
Although PM parts only require a small amount of machining, it is extremely difficult to machine PM parts, which is mainly caused by the porous structure of PM materials, which reduces the service life of the tool.
Porosity causes microscopic fatigue of the cutting edge. The tool tip is continuously impacted as the tool reciprocates from the hole to the solid particles. Continued small impacts can cause small cracks on the cutting edge, and these fatigue cracks grow until the cutting edge microchips. This chipping is generally very small and usually manifests itself as normal abrasive wear.
Porosity also reduces the thermal conductivity of PM parts. The temperature on the cutting edge of the tool during cutting is high and can cause crater wear and deformation. The interconnected porous structure provides a pathway for cutting fluid to drain from the cutting area and can cause thermal cracking or deformation, which is especially severe in drilling.
The increased surface area caused by the inherent porous structure also allows oxidation and/or carbonization during heat treatment, and these oxides and carbides are very hard and wear-resistant.
Due to the existence of pores, the hardness value also fluctuates in a small area. Even if the measured macro hardness is HRC20~35, the particle hardness of the component parts will be as high as HRC60, and these hard particles will cause severe and sharp edge wear.
Many PM parts are harder and stronger after heat treatment. Sintering and heat treatment techniques, as well as the gases used, can cause PM parts to contain hard and wear-resistant oxides and/or carbides.
The presence of inclusions in parts is also detrimental. During machining, these particles are pulled up from the surface, creating a scratch or scratch on the surface of the part as they pass from the front of the tool. These inclusions are usually large and leave visible holes in the surface of the part. In addition, uneven carbon content leads to inconsistencies in workability. For example, the FC0208 alloy has a carbon content of 0.6% to 0.9%, and the material with a carbon content of 0.9% is relatively hard and has a low tool life; while cutting a material with a carbon content of 0.6%, the tool can get a higher service life.