Producing metal parts by machining is inherently wasteful. Your vendor expends energy turning metal into chips, which get scrapped and perhaps remelted. Clearly, a net shape or near-net shape process like powder metallurgy has many advantages -- not the least of which is saving on material costs. However, there are situations where die constraints mean parts need secondary machining.
Admittedly, machining powder metal parts presents challenges. Some manufacturers use these as an excuse for employing more wasteful manufacturing methods (such as copper infiltration or resin impregnation). The reality is, these limitations can be overcome with a thorough understanding of powder metal materials and the forming process.
Net Shapes Minimize the Need for Machining Powder Metal
When making powder metal parts, your vendor drops a precise quantity of powder into a mold or cavity. A punch then compacts the powder, ensuring every iota of the die is filled. After release, the compressed part goes into a furnace for sintering. This fuses the powder particles, creating a hard finished or semi-finished part.
Net-shape forming has at least four advantages over conventional processes, but there is also a limitation. The axial compression delivered by a punch moving into the die means some features can’t be formed. Either the powder won’t be moved into die, or if it does, release of the part afterward becomes difficult or even impossible. These features include:
Cross holes (These can be formed with some of the newer compaction presses, but as a general statement this is usually true.)
Design engineers often specify powder metal parts to leverage the unique capabilities of the material. While most powder metal parts are made from an iron-carbon-copper mix, there are other options available that offer enhanced properties. Additives include:
Chromium (a staple of stainless steels)
Typically these metals add corrosion resistance, strength, and hardness, but there are other benefits. Nickel, for example, is used in heat-treated PM components because nickel promotes better mechanical properties as heat-treated. Chromium improves both tensile and fatigue properties, and niobium and vanadium refine grain size.
Harder materials are more difficult to machine (i.e. stainless steel vs., say, aluminum), but powder metal brings its own challenges too. Bulk hardness may not appear especially high, but don’t underestimate the abrasive effects of porosity within the part. Low levels of porosity are inherent (and often desirable) in powder metal parts, however, this results in a microscopic interrupted cut that result in chatter, diminishing cutting and machine tool life.
In summary, when machining powder metal parts, cutting forces are often higher and tool life shorter. In addition, chatter can be bad for surface finish, particularly if aesthetics matter with your components.
Improving Powder Metal Part Machinability
There are three avenues to follow. See if your powder metal components vendor is using best practices like these:
1. Optimize Cutting Tool Geometry
In general terms, you want your vendor to use sharp tools and light cuts. More specifically, negative rake angles can improve toughness while tight radii help reduce forces.
2. Select Appropriate Cutting Tool Materials
Cermets (cemented carbide with hard titanium particles) are a cost-effective choice. Cubic boron nitride is often used for high-volume and high-precision applications. Carbide is a good choice for interrupted cuts and threading.
3. Powder Metal Material Modification
Two approaches are possible in machining powder metallurgy parts. Using a resin to infiltrate the porosity improves lubrication and can reduce chatter. In addition, some material additives, notably manganese sulfide, can reduce cutting forces and wear.
Secondary Machining: Rarely Needed, Usually Possible
Powder metallurgy has many advantages over machining. It’s a fast and efficient process and shrinks:
At the same time, it opens up many more materials possibilities. However, a result of compacting powder is that some features cannot be formed. When this is the case, secondary machining operations are a must if you want those features.
To those lacking powder metallurgy expertise, this can seem daunting. The reality is that powder metal parts can be machined with a little elbow grease and foresight. The only downside is the inherent material waste.