Abstract
A novel approach to reach full density in powder metallurgy (PM) components is demonstrated in this work. Water-atomised Mo-prealloyed steel powder is utilised for manufacturing cylindrical and gear samples through double pressing and double sintering (DPDS) process route. The effect of sample geometry and powder size fraction on densification is investigated and it is found that the DPDS route enables a density level of > 95% which is sufficient to eliminate the surface open pores. Reaching such high density is necessary, in order to perform capsule-free hot isostatic pressing (HIP). After HIP, full densification is achieved for the cylindrical samples and only near full density is realised for the gears resulting in neutral zone formation due to the density gradient. In order to predict the densification behaviour during the compaction, FEM simulations considering the gear geometry are performed for both the pressing stages and HIP. The simulation predicted a similar densification behaviour with the formation of the neutral zone. The proposed DPDS route with capsule-free HIP in combination with FEM simulation is demonstrated as a potential route for manufacturing full-density PM steel components, e.g. gears, suitable for high-performance applications.
Highlights
Gear manufacturing through powder metallurgy (PM) route has an inherent advantage to form components with complex shapes and profiles from a single pressing operation in large volumes, compared to conventional gear manufacturing, where the typical process involves machining and gear cutting from the blank [1]
There is a need for continuous process improvement, in order to increase the density of PM steel component
The density of cylindrical and gear specimens after pressing, sintering and hot isostatic pressing (HIP) is given in Fig. 4a, b
Summary
Gear manufacturing through powder metallurgy (PM) route has an inherent advantage to form components with complex shapes and profiles from a single pressing operation in large volumes, compared to conventional gear manufacturing, where the typical process involves machining and gear cutting from the blank [1]. Porosity plays a significant influence on the properties of PM steel [8] and the pore size, pore structure and its distribution within the sample are known to affect the fatigue properties [9,10,11] by initiating the crack Post processing operations, such as surface rolling, will increase the load-bearing capacity of the PM gears, resulting in surface densification and improving the fatigue properties [12, 13]. Another approach is to increase the density levels of PM components to around 95%, which in turn enables to reach full density after performing capsule-free hot isostatic pressing (HIP) [14,15,16]. It provides the pycnometer density, from this value the closed porosity is estimated and by subtracting it from the total porosity, the amount of open porosity is obtained
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