As compacted metal powders coalesce, and this is before the sintering process, the compressed material is imbued with what’s known as “Green strength.” Remember, the metal particles haven’t bonded yet, haven’t fused with each other. However, the unsintered product is mechanically tough enough to handle machining operations. Those supplementary stages won’t damage the compacted workpiece, nor will they remove the part’s surface details.
What Is Green Strength?
Just like it says in that opening paragraph, unsintered powder metals can be mechanically robust. The powdered grains won’t be hard enough to satisfy a stated application, of course. That hardened strength is added when the powder metal is sintered. No, the metal particles are still unbonded, but the compaction process has made the metal stable enough, solid enough to be handled and worked upon. That, in a nutshell, is what Green Strength is all about, pure unsintered, mid-processed tensile strength.
Green Strength Machining
Think of the “Green” factor as the pre-thermal treatment phase. The powder metal hasn’t become plastic, hasn’t fused together into a single mass, which is full of pores. Rather, the powder is uniformly distributed. The grain boundaries are there, the particle size is consistent, and the lubricant base is trapped inside the newly die-compacted product. But what if those discrete particle boundaries weaken the unsintered metal? Then it’ll fall apart when it’s clamped and machined. Conversely, if the particles exhibit a healthy measure of Green toughness, then that component, although unsintered, can be clamped and machined.
Using That Mid-Processing Advantage
Sometimes the heat, pressure, and particle morphology used on a product run causes problems. The additives and lubricants in the mix further complicate matters. Wouldn’t it be nice if we could put a P/M based production line on pause and machine those tiny inconsistencies away? Dimensionally accurate, the machined component would then enter the sintering phase. Unfortunately, several process factors exist as Green Strength attenuating influences. Among them, pressing oils and ejection lubricants can make pre-sintering operations completely untenable.
To maintain green strength, there are compaction warming cycles and heat treatment methods available. As for the trouble caused by those processing oils, new polymeric substitutes are maturing at an exceptional rate. They enhance Green Strength. That’s something of a major breakthrough. With the strength compromising lubricants out of the picture, low-cost machining operations can be established before the metal toughening sintering stage. After all, once the powder metal fuses, it’ll take a great deal of energy to make an impact on a finished component. It is better all-around if the fabrication tuning is done after the compaction stage and before the sintering.