Today’s powder metal market is dominated by the automotive industry, advancing from its early acceptance of bearings and suspension components into more demanding applications in the internal combustion engine and automatic transmissions. Lessons learned from the automotive sector, have transformed current and potential new users in the non-automotive and electric vehicle applications to look at PM for planetary gear set and gear reduction opportunities.
When most people think of powder metal, they think of pressed and sintered components. After compaction, a vital step in the PM process is sintering. The current norm for sintering is a temperature of 2050°F (1120°C) -- this has proven successful for most PM applications and has brought the industry to its currently highly regarded position of a cost-effective, mechanically proven metal forming technology.
In our opinion, the current sintering practice is good, but are there other sintering practices that can produce a better or perhaps best practice for the increasing demands being put on PM components? Ultra-high temperature sintering may be the solution.
The Automotive Industry: An Emerging Technology
With the ever-increasing “electrification” of transportation, many engineers are leaving the world of “good enough.” The increased torque of electric motors puts greater demands on components used in conjunction with these propulsion systems. Although the electrification of vehicles is inevitable, internal combustion engines will be a segway between the current market and full electrification. Internal combustion engines in this interim period will have increased demands for higher fuel efficiency, higher performance, and lighter weight (through the use of turbocharged combustion engines), thus placing even greater demands on existing sintered PM components.
This changing dynamics of the automotive industry will force a reevaluation of the role of sintering in powder metallurgy, specifically newer parts demanding higher tensile and fatigue strengths.
Sintering: Good, Better, & Best
This next leap in performance for powder metallurgy is not years away, but here today in the form of ultra-high temperature sintering (UHTS). UHTS opens the door to using non-traditional alloys to achieve greater mechanical properties.
Your design can achieve a combination of strength and ductility superior to current ferrous powder metallurgy material. Powder metallurgy now has the tools to take on applications previously thought unattainable with current PM.
That’s the short version (honestly!). Keep reading for a deeper dive into unique alloying additions, processing details, and the resulting mechanical property boosts UHTS delivers.
Sintering: Where the Magic Happens
We’ve already established that over 95% of powder metal parts are created through the art of sintering. Sintering, and more importantly sintering temperature, is key to creating interparticle bonds, partially alloying any elemental additives, and subsequently creating the mechanical properties that you see with these components.
The sintering temperature is a lot more than just a number. It’s a window of opportunity for part performance and customization.
First, let’s define terms. As in most things, there is good, better, and best.
- Conventional Sintering: 2050°F (1120°C) in a nitrogen hydrogen atmosphere (good)
- High-temperature sintering: 2125-2300°F (1160-1260°C) (better)
- Ultra-high-temperature sintering: In excess of 2500°F (1370°C) (best)
Conventional Sintering in Powder Metallurgy (Good)
Proper sintering of the compacted powder metal parts requires the following steps:
- Oxide reduction
- Formation of metallurgical bonds between pressed particles
- Complete or partial alloying
More than 80% of all structural powder metal components are sintered at 2050°F. These materials and processes have provided a low-cost metal-forming technology with adequate mechanical properties for a wide variety of powder metal parts in production today.
Despite this universal acceptance, limitations exist for sintering in this temperature range:
- Incomplete homogenization of premixed alloying elements, even copper
- When sintering at conventional temperatures, there are limited premix additives available to increase mechanical properties
- Oxides of alloying elements that provide greater mechanical properties may not necessarily reduce at 2050°F
- Limitations of conventional PM 2050°F sintering for structural components are reduced impact toughness, along with limited alloy selections
High-Temperature Sintering (Better)
High-temperature sintering is considered between 2125° and 2300° F.
Sintering non-stainless, ferrous materials at higher temperatures improves the performance:
- Higher Density
- Higher Tensile Strength
- Higher Fatigue performance
- Higher Modulus of elasticity
High-temperature sintering enables reduced time at temperature, thereby partially offsetting the increased cost associated with high-temperature sintering.
Despite the many benefits of high-temperature sintering, current market use for this technology is quite low. So, what’s the holdup?
- An assumed lack of dimensional precision
- Capital cost of equipment
- Cost (both fixed and maintenance) associated with the technology
Although these potential roadblocks exist, the advantage of using this better sintering, coupled with PMs shape and material efficiency, offers opportunities for existing and new PM parts designs with potentially lower costs.
Ultra-High-Temperature Sintering (in excess of 2500°F) (Best)
Ultra-high-temperature sintering enhances the many advantages of high-temperature sintering, with expanded mechanical performance and alloying options. At these ultra-high temperatures, the potential exists to use more compressible premixed metal powders, yet still achieve complete homogenization within the final part.
The result? New levels of strength of up to and greater than 200,000 psi tensile strengths and elongations approaching 2% are possible even at densities below 7.2 g/cm³.
Also of note’ there’s greater pore rounding with UHTS. This seemingly small feature has been shown to further improve:
- Tensile strength
- Fatigue strength
- Impact strength
One other advantage of conventional sintering is the ability to sinter-harden the PM part during the sintering process. Sinter hardening is a process that effectively merges sintering and heat treating into one operation. With the advent of ultra-high temperature sintering and its unique alloying opportunities, sinter-hardening can be achieved even after sintering at this high temperature. This creates a unique opportunity to have the best of both worlds: high levels of performance with the added advantage of no secondary heat treatment (win-win).
Aim for the Best with UHTS
To conclude, PM offers many advantages including little to no machining, material efficiency, and potentially low cost. The results of ultra-high-temperature sintering, specifically higher strength, higher modulus, and higher fatigue allow the designer to advance smaller, lighter-weight components for their high-demanding applications.
Editor's note: This blog was originally published on 06/25/2021 and was updated on 1/30/2023.