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.