Cost is king for commodity goods. Production costs can be most easily reduced using production technologies that allow for high volume production in a short time frame. That is why injection molding of polymers has taken the world by storm, making the use of cast metal parts obsolete for most consumer goods. For components that need to withstand greater stress or wear, high volume production technologies using metals or ceramics have become abundant. A significant portion of these are based on die pressing technology, in which powdered metal or ceramic materials are poured into pre-machined molds and compressed to produce large numbers of identical “green” preforms that are then densified by furnace sintering.
While the results of this large-scale production are often hidden from the view of the average consumer, these parts are often the true heroes of modern conveniences. Some examples include the water heating block and grinder burrs in our bean-to-cup espresso machines, the thermostatic valve components in our anti-scald shower faucets, the mechanical gears in handheld power tools, a plethora of parts in modern cars, and carbide cutting tools such as masonry drills, to name just a few.
Currently these processes use single component powders or homogeneous powder mixtures as the dies are filled completely in one go. Even for high-end applications the entire pressed part is made from the same bulk material, meaning that the one material selected must be a compromise to satisfy the often-different mechanical requirements of the surface and the bulk of the part. One common example is mechanical parts and cutting tools that benefit from hard wear surfaces made from brittle materials combined with a tough body made from a ductile material. In this case, if an adequate compromise material can’t be found, joining techniques such as brazing must be used to physically attach the wear surface material to the body material. In addition to time cost, these post-processing steps can introduce a potential weakness in the completed product.
With a selective powder deposition technology like Aerosint’s, material compromises and additional post-processing could become unnecessary. By careful selection and co-deposition of two or more compatible but functionally different materials in a single die in a predetermined pattern, the composition (and thus the properties) of the final part can be locally controlled. After all, once the die is filled, the consolidation process — be it pressing followed by sintering, or pressure assisted sintering — remains largely unchanged. It should therefore be possible to optimize parts for longevity, strength, cost and more with a minimal change in the production process itself.
One simple but industrially useful example would be to employ different steel grades to create local zones of various hardness within a part. Other, more exotic combinations of metals and metal alloys could yield high value parts with optimized thermal, mechanical, electrical, and/or magnetic properties. Various ceramics (alumina, zirconia, etc.) might be combined to optimize surface hardness or thermal/chemical resistance, etc. while retaining interior toughness for mechanical integrity.
An added advantage of the Aerosint deposition system is that the contour of the deposit can be changed on the fly for every layer. In this manner, even complex multi-level dies can be filled accurately, allowing for the same degree of part geometric complexity possible in today’s single-material powder compaction processes.
The disadvantage of this unconventional approach is the fact that the pressing cycle time must be increased, because successive deposition of multi-material powder layers in a die will always be slower than single-material, at-once die filling. As in any series production method, the increased cycle time directly translates to an increased cost of the final components. It’s for this reason that multi-material die filling will be best suited to demanding applications where part performance and quality requirements are high and production volumes are relatively low.
Still, the potential to create parts containing two or more materials with locally varying properties represents an enormous opportunity: to boost the capabilities of an established manufacturing technique with an innovation in just one step of the overall process.
by Bram Neirinck, Ph.D., Senior R&D / Applications Engineer @ Aerosint