A Green Technology

PM is an energy-efficient and eco-friendly metal-forming technology

Powder Metallurgy—Intrinsically Sustainable

The Sustainability Role of Powder Metallurgy

For many years, powder metallurgy has been delivering sustainable value as an industry. We have just not defined ourselves or compared our products and processes to competing metal-forming process alternatives in those terms. The balance of this discussion will compare and contrast PM’s sustainable value with other metal forming processes. Addressing manufacturing processes, PM’s sustainable value is primarily derived from its net-shape capabilities and its very high material-utilization factor, which minimizes all energy inputs. In general, any metal component can be manufactured by any of several manufacturing technologies. A simple gear can be produced by machining a cylindrical piece of solid bar stock, forging a steel blank in forging dies, in some cases stamping it from sheet or roll stock, possibly casting it and machining features, or in the case of PM compacting powder in tooling dies that result in the product’s final shape. The trick to evaluating the sustainability of a product’s manufacture will be found in comparing the process steps, resources, and economic costs that go into the manufacture of that product.

Material Sustainability

As mentioned above, the net-shape capability of PM is the primary advantage in the process. When evaluating a component, such as the gear shape in the figure to the left, you can quickly realize that there is no waste in the shaping of the component. Production of the gear by chip-generating machining would result in perhaps 40% of the material being machined away and discarded. While the machining waste can be recycled, it is extraneous to the final component and is a net loss to the material and energy eco-efficiency of the final component. It is estimated that 85% of all PM powders are produced from recycled material. Most metals can be repetitively recycled in collectable quantities. The predominant metal powder used, iron/steel, nearly a half million tons per year, is nearly always produced via atomization of electrically melted steel scrap. Particle-size distribution in a given sample or lot of powder is controlled by sieving operations, and if the particle size needs to be adjusted, the powders can be milled to avoid waste of oversized particles

Energy Sustainability

The net-shape nature of PM similarly influences the energy demand per component. All manufacturing processes require the use of thermal, chemical, or mechanical energy to achieve product form. Some processes require several heating and re-heating steps to achieve final form. The only time metal for powders is melted is in the atomization step; all other thermal operations are undertaken below melting temperature, conserving energy while achieving the final shape and developing the necessary material properties/mechanical performance. In addition, there is little if any finishing to final product specifications necessary, further conserving the energy necessary to achieve final product characteristics. The chart above compares the energy consumption per kilogram of completed components for the various processes; only casting and deep-drawing operations are in the general range of PM’s energy consumption. A comparative analysis of per-piece energy consumption demonstrates 44% less energy is required to produce the PM version than the forged and machined part

Environmental Sustainability

The net-shape characteristics of PM components generally result in a finished component ready for packaging and shipment after sintering. However, PM components can be given a wide assortment of secondary finishing operations such as machining to exacting tolerances, plating, or coating. On a per-kilogram of-finished-component basis, PM final machining operations are minimal, in most cases resulting in minimal use of cutting oils per pound produced. Also, most plants only emit cooling water to public water/septic systems, minimizing the likelihood of toxic releases due to these point-source contaminants. The PM process does require the use of graphite or other lubricants in the compaction stage, but recent improvements in both binder and lubricant technologies has resulted in most companies shifting from chemical systems using listed constituents such as zinc sterates to more innocuous materials in order to minimize or eliminate the potential for gaseous toxic releases from sintering operations. Gaseous emissions are also minimized by the net-shape conservation of product mass affecting releases. With increased automation, more customers are deploying returnable shipping containers and crates in an effort to shrink the volume of cardboard that finds its way to landfills. Since nearly all scrap produced is metallic, it is routinely recycled, thus minimizing the industry’s contribution to landfills. Compared with other manufacturing processes, PM displays few environmental hazards. Again, the low energy intensity per pound of product minimizes overall environmental impact.

(credit:) Metal Powder Industries Federation