Fatigue Mechanisms in PM Components

Diana Lados and Diran Apelian

Introduction

At present, the fatigue behavior of many sintered and heat treated Fe-based PM alloys is commonly inferred from the data developed for wrought steels due to the absence of a fundamental knowledge and comprehensive databases directly associated with PM materials. This does not provide reliable design for PM components, it restricts the use of current gear design, and it impedes the potential extension of these materials to more demanding, fatigue critical and structural applications.

Objectives

The objectives of the project are:

Develop the crack initiation and crack propagation mechanisms of porous Fe-based PM steels considering the roles of both porosity and microstructure
Corroborate the fatigue crack growth mechanisms with the fatigue life behavior of the materials
Generate fatigue and fatigue crack growth databases for enhanced (optimized) design.

Methodology

The objectives will be achieved through a judicious selection of the experimental framework including all critical material and processing parameters, performing representative fatigue and fatigue crack growth testing, and conducting an advanced and thorough analysis and interpretation of the results. Two PM materials (a pre-alloyed and an admixed Fe-1.8Ni-0.5Mo-0.6C) and two heat treated structures (as-sintered and fully hardened), Figure 1, are investigated at three density levels (6.9, 7.2, and 7.83 g/cm³, Figure 2. The importance of porosity type (open vs. closed) and pore morphology and distribution will also be investigated by proper selection of the sintering conditions, Figure 3.

Figure 1. Structures of the pre-alloyed (a, c) and admixed (b, d) materials in as-sintered conditions (a, b) and fully hardened conditions (c, d); the as-sintered structures contain divorced eutectoid and pearlite (+ Ni-rich areas in the admixed material) and the fully hardened structures are martensitic (+ Ni-rich areas in the admixed material).

Figure 2. Density levels selected for the study: 6.9 g/cm³ (a), 7.2 g/cm³ (b), and 7.83 g/cm³ (c).

Figure 3. Differences in porosity type and morphology: open and irregular (a), closed and round (b), open and round (c).

Publications

D.A. Lados and D. Apelian, "Key Issues Affecting the Performance of P/M Components During Dynamic Loading", Proceedings of the 2003 International Conference on Automotive Fatigue Design & Applications, Novi, Michigan, October 2003, Edited by: Russell A. Chernenkoff & William F. Jandeska, published by MPIF, Princeton, NJ, 2003, pp. 49-67 (invited paper).
D.A. Lados and D. Apelian, "Response of Open/Closed Porosity to Density Variations in Pre-alloyed and Admixed Ni-Mo P/M Alloys", Proceedings of the 2004 International Conference on Powder Metallurgy & Particulate Materials (PM2TEC 2004 - Chicago), published by MPIF, Princeton, NJ, Part 10, pp. 67-87, 2004.
D.A. Lados and D. Apelian, "Porosity And Microstructure in P/M Alloys: Critical Review of Their Effects on Fatigue and Fatigue Crack Growth", Proceedings of the 2005 International Conference on Powder Metallurgy & Particulate Materials (PM²TEC 2005 - Montreal), published by MPIF, Princeton, NJ, Part 10, pp. 90-110, 2005.
D.A. Lados, D. Apelian, and F.J. Semel "Open and Closed Porosity in P/M Materials - Measurement and Variation with Density Levels and Sintering Conditions", Proceedings of the EURO PM2005 (2-5 October 2005, Prague, Czech Republic), published by EPMA, Shrewsbury, UK, Vol. 3, pp. 9-17, 2005.
F.J. Semel and D.A. Lados, "Porosity Analysis of P/M Components by Helium Pycnometry", Powder Metallurgy, vol. 49, no. 2, pp. 173-182, 2006.
D.A. Lados and D. Apelian "Effects of Porosity and Microstructure on the Fatigue Crack Growth Behavior of Pre-alloyed and Admixed Fe-Ni-Mo P/M Alloys", presented at the 2006 International Conference on Powder Metallurgy & Particulate Materials (PowderMet 2006), San Diego, CA, June 2006, and to be published in the conference proceedings.

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Last modified: October 23, 2007 08:32:37