The forming process of powder metallurgy parts is a crucial step in transforming metal or alloy powders into green bodies with predetermined geometric shapes and sufficient green strength.Its quality directly determines the subsequent sintering effect and the performance of the finished product. This process, centered on pressure, enables loose powder to achieve high-density arrangement and shape fixation within a mold, offering advantages in near-net-shape forming and mass production, and thus plays an important role in modern manufacturing.
The basic process of powder metallurgy forming is compression forming. First, a fixed amount of powder is uniformly filled into the mold cavity. A stable pressure perpendicular to the pressing direction is applied using a mechanical or hydraulic press, causing the powder particles to approach each other under pressure and undergo plastic deformation or breakage, thereby forming a dense green body. At this stage, the particle size distribution, morphological characteristics, and lubricant content of the powder significantly affect the frictional resistance between particles and the uniformity of filling, thus determining the uniformity of the green body density and demolding performance. Reasonable pressing pressure and holding time can reduce dimensional springback caused by elastic aftereffects and improve shape retention.
In compression forming, mold design is a crucial prerequisite for process success. The cavity dimensions must comprehensively consider the loose density, compression ratio, and sintering shrinkage of the powder to ensure the dimensional accuracy of the finished product after sintering. The mold material should possess high wear resistance and sufficient strength to withstand wear and fatigue caused by repeated high-pressure loading. Furthermore, for complex structural parts, multi-step, multi-directional pressing, or combined molds can be used to reduce density gradients and forming dead zones.
To further improve molding quality and adapt to complex shape requirements, the industry has developed various special molding processes. Isostatic pressing uses liquid or gas media to apply uniform pressure to the powder in all directions, resulting in a green body with a more consistent density distribution, suitable for large or irregularly shaped parts. Injection molding mixes powder with a polymer binder to form a paste, which is then injected into the mold cavity through an injection molding machine. It is suitable for small, intricate, and high-volume production of small parts; the binder can be removed before sintering to obtain a near-net-shape green body. Warm pressing involves pressing at a moderately elevated temperature to improve powder plasticity and reduce friction, thereby increasing the density and strength of the green body and reducing subsequent sintering shrinkage fluctuations.
Controlling process parameters during the molding process is equally crucial. The pressure required needs to be optimized based on the material system and product shape. Excessive pressure can lead to mold wear and powder cold welding, while insufficient pressure results in insufficient density and easy deformation during sintering. Pressing speed affects powder filling and venting efficiency; too fast a speed can trap air, creating internal pores, while too slow a speed reduces production efficiency. The type and amount of lubricant must balance ease of demolding and sintering cleanliness to avoid defects caused by residue.
The completed green body must undergo demolding, inspection, and debinding (for injection molding) before entering the sintering stage. At this stage, although the green body has a certain strength, it has not yet formed a metallurgical bond, and its shape stability still depends on the density and uniformity of the initial molding. Therefore, the molding process not only determines the product's outline but also lays the foundation for sintering densification and performance development.
Overall, the molding process for powder metallurgy parts primarily uses pressing, combined with mold optimization and various special forming technologies, to achieve early-stage control over shape, density, and performance. Scientifically setting and controlling pressing pressure, holding time, mold structure, and auxiliary process parameters is the fundamental way to ensure the quality of green blanks and improve the consistency and reliability of finished products. It is also the core support for powder metallurgy to achieve high-precision and high-efficiency production.