influence of compaction strategy on dimensional and
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Influence of compaction strategy on dimensional and geometrical stability of a low alloy Mo steel
M. Zadra1, L. Girardini1, G. Pederzini2, G. Patuelli2, M. Piva2,
S. Bordin3, L. De Mitri3, A. Popolizio4, I. Cristofolini4, A. Molinari4
1K4Sint Srl, PergineValsugana, (TN), Italy
2Powder Metal B.U. – Sacmi Imola S.C., Imola (BO), Italy
3TFM SpA, Lissaro di Mestrino (PD), Italy
4University of Trento, Povo di Trento, Italy
Life project LIFE16 ENV/IT/000231 (LIFE4GreenSteel)
Introduction
Mechanical properties
Fraction of the load bearing section
PorosityPore morphology
Maximum pore size
Green density Sintering shrinkage
Metallic matrix
Sintering temperaturePowder / additives
Heat
treatments
Introduction
Cold compaction
Warm compaction
Warm die compaction
Die wall lubrication
Green density
Powder compressibility
Lubricant
Compaction pressure
Compaction strategy
B. A. James, Powder Metallurgy 30(4)(1987)273-280
Aim of the work
Preliminary tests on die wall lubrication compaction in comparison to bulk lubrication and warm die compaction of a commercial prealloyed 1.5%Mo iron powder with 0.5% graphite.
High compaction pressure (up to 1200 MPa).
High temperature sintering (1250°C)
Focus on: density, ejection forces, sintering shrinkage
dimensional and geometrical issues
microstructure
Die wall lubrication
Potentiality demonstrated by James in 1987 (Powder Metallurgy 1987), with trials performed in an industrial environment.
Advantages: density
ejection force
small sintering shrinkage
mechanical properties
Crititical issue: process capability, linked to the efficiency and the reliability of methodology adopted to deposit the lubricant on the die and punches surface minimizing the impact on the press productivity.
Experimental procedure
• Powder: 1.5%Mo prealloyed iron powder with 0.5% graphite.
• Lubricant: Acrawax in bulk lubrication; an aqueous suspension of lubricants that is sprayed on the die cavity surface and the punches surfaces every stroke used in die wall lubrication.
• Specimens: 25 mm diameter, 20 mm height.
• Compaction press: laboratory hydraulic press working in force control with a manual filling system.
Experimental procedure
Compaction strategies (in force control, and with a constant filling height):
- bulk lubrication with 0.7% Acrawax C
at 30°C and 800 MPa (reference) – 30°C b.l.
warm die at 90°C and 800, 1000, 1200 MPa – 90°C b.l.
- die wall lubrication with the proprietary lubricant
warm die at 90°C and 800, 1000, 1200 MPa – 90°C d.w.l.
The compaction temperature
Experimental procedure
• Delubrication: at 500°C in a vacuum furnace with argon backfilling.
• Sintering: at 1250°C, 30 minutes isothermal holding, in a vacuum furnace with argon backfilling and forced cooling in a pressurized nitrogen flux.
• Density, microstructure.
• Measurements: by CMM, the same specimen was measured in the green and sintered state; the surfaces were measured by continuous scan: dimensions were derived from the surfaces; local height was measured as the distance between two points on the two surfaces in various positions.
Green density
30°C b.l.
90°C b.l.
90°C d.w.l.
Diameter of the green parts and springback
The ejection curve
Sintered density
Sintering shrinkage
P (MPa) P (MPa)
eH (%) eD (%)
Diameter along the axial direction
Height
Cylindricity
Flatness
Microstructure: porosity800 MPa 30°C b.l.
1200 MPa 90°C b.l.
1200 MPa 90°C d.w.l.
Pore size: Deq
800 MPa 90° d.w.l. 1000 MPa 90°C d.w.l. 1200 MPa 90°C d.w.l.
800 MPa 90°C b.l. 1000 MPa 90°C b.l. 1200 MPa 90°C b.l.
800 MPa 30°C b.l.
Microstructure: the metallic matrix800 MPa 90° d.w.l. 1000 MPa 90°C d.w.l. 1200 MPa 90°C d.w.l.
800 MPa 90°C b.l. 1000 MPa 90°C b.l. 1200 MPa 90°C b.l.
800 MPa 30°C b.l.
Microstructure: the metallic matrix800 MPa 90° d.w.l. 1000 MPa 90°C d.w.l. 1200 MPa 90°C d.w.l.
800 MPa 90°C b.l. 1000 MPa 90°C b.l. 1200 MPa 90°C b.l.
800 MPa 30°C b.l.
Conclusions
Warm die compaction with bulk lubrication:
- increases green density
- slightly reduces springback and the ejection force
- slightly reduces the sintering shrinkage
- increases the sintered density
Die wall lubrication with a warm die
- further increases green density but slightly reduces relative green density
- strongly decreases springback and the ejection force
- improves the stability of the ejection force
- greatly reduces the sintering shrinkage
- increases the sintering density
Conclusions
The combination of springback and sintering shrinkage results in a better
dimensional stability in case of die wall lubrication.
Cilindricity and flatness of the compaction surfaces are excellent in all the cases
both in the green state that after sintering at high temperature.
The median pore size and the microstructure of the metallic matrix are not
significantly modified by warm die wall lubrication compaction.
Pore shape: fshape800 MPa 90° d.w.l. 1000 MPa 90°C d.w.l. 1200 MPa 90°C d.w.l.
800 MPa 90°C b.l. 1000 MPa 90°C b.l. 1200 MPa 90°C b.l.
800 MPa 30°C b.l.
Pore shape: fcircle800 MPa 90° d.w.l. 1000 MPa 90°C d.w.l. 1200 MPa 90°C d.w.l.
800 MPa 90°C b.l. 1000 MPa 90°C b.l. 1200 MPa 90°C b.l.
800 MPa 30°C b.l.
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