Novel uses of electric fields and electric currents in powder metal (P/M) processing
Introduction
Since 1995, IAP has had significant experience in applying electric fields and electric current to enhance the performance of powder metal compaction processes. The two most promising results from our work have been that we have demonstrated the feasibility of using electric fields to reduce the surface porosity of powder metal compacts and that in-die electric current processing can significantly increase the green density of powder metal compacts. This paper describes the results of various research efforts, conducted over the past several years, with a goal of applying electric current and electric field processing to P/M part production made from iron powders commonly used in the industry.
Section snippets
Technical background of electric field sintering
The use of electric field assisted sintering in P/M processing resulted from previous research performed on electro-plastic effects in commercial metal forming operations [1] and initial work by Conrad [2]. In Conrad’s pioneering work he found that the application of a DC electric field during the superplastic deformation of the 7475 alloy decreased the flow stress at all strain levels, reduced the rate of strain hardening and increased the strain rate hardening exponent. In addition, the
Technical background of electric current consolidation
The consolidation of metal powders involves the elimination of porosity from the part to be made. Work in electro-plastic effects has shown that the application of current during metal forming operations can enhance the dislocation mobility in wrought materials. Applying the premise that porosity in powder compaction processes can be related to dislocation mobility, the application of current during compaction should reduce porosity. Combined with improved plasticity resulting from resistive
Description of electric field-sintering experiments
The material used for all the electric field sintering specimens was ATOMET4401 iron powder with the following nominal chemical composition (wt%): C (0.003), O (0.08), S (0.007), P (0.01), Mn (0.15), Mo (0.85), Ni (0.07), Si (0.003) Cr (0.05), Cu (0.02), and Fe (>98).
The test apparatus that was used in our Electric Field Sintering experiments is shown in Fig. 2. The sintering chamber is fabricated from a 5 cm diameter 304 stainless schedule 40 pipe. Threaded stainless steel pipe flanges were
Description of electric current consolidation experiment
The electric current consolidation experiments were conducted using a modified hydraulic press that is shown in Fig. 4. We retro-fitted a 100 T hydraulic press to work with a DC power supply capable of delivering a 50 kA peak current for up to 30 s. The current was introduced into the system via the top punch, which is insulated from the rest of the tooling set and the press. The bottom punch was tied to the other polarity of the DC supply. Although the current output of the supply was as high
Metallographic image analysis
The change in surface porosity was measured by performing image analysis of mounted circular cross-sections of the electric field sintered specimens and comparing them to a specimen compacted to the same pressure and sintered without an electric field. After each sample was sintered, the specimens were mounted and carefully polished. An image analysis using the Beuhler OMNIMET Advantage Image Analysis System was then performed to determine the percent porosity in the region just below the
Results of electric current consolidation experiments
We conducted a series of experiments on 4401 powder with an addition of 0.5% graphite using the experimental set-up described above. The typical current pulse for these tests is shown in Fig. 7.
The DC current pulse is applied to the pre-compacted P/M specimen over a specified time period. In this case the pulse time was just over 0.25 s. The peak current rises during the pulse as the particle contact spots grow and the total axial electrical resistance drops. As the temperature of the specimen
Commercial benefits of electric field sintering
Application of the vacancy mobility phenomena to the commercial sintering process is the reduction of surface porosity in sintered metal parts. Currently, P/M gears are difficult to harden with carburization techniques because of connected porosity throughout the part accelerates carbon diffusion causing complete diffusion of carbon through the part. This is not desirable since only a small case depth of approximately 0.5 mm is required in the teeth area, while a more ductile microstructure is
Commercial benefits of electric current consolidation
P/M parts makers are actively seeking new consolidation technologies that will yield green densities greater than 7.5 g cc−1 with near net shape features. This would result in the insertion of high-density net shape P/M processes into markets currently dominated by forging processes. The payoff for high-density net shape P/M parts is that this type of processing can be done at significantly lower processing costs than machined forgings. Wrought forged products are generally produced at $3–5/lb,
Summary and conclusions
The experiments conducted on the use of electric fields to sinter common 4401 iron powder material and electric currents to consolidate 4401 iron powder have shown great promise in improving key processing characteristics. Following is a summary of the key results for each experiment.
References (4)
- D.C. Newman, Y. Fahmy, H. Conrad. Electric field sintering for improved surface finish of powder metal parts, National...
J. Mater.
(1990)
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