FATIGUE MECHANISMS IN P/M COMPONENTS Worcester Polytechnic Institute April 28-29, 2004 Diana Lados & Diran Apelian M orris B oorky P owder M etallurgy R esearch C enter
OUTLINE I. Impact of Porosity on Fatigue and Fatigue Crack Growth Behavior of P/M Components (examples from the literature) Relevance of open/closed porosity � Open/closed porosity measurement techniques � II. WPI project … Objectives and Experimental Plan
BACKGROUND Factors controlling fatigue behavior … 1. Porosity/density: Amount (% Porosity) � Type (Open/Closed) � Morphology (size/shape - due to initial powder � morphology and sintering conditions) Distribution (initial powder size) � 2. Microstructural phases: Amount (% Phase) � Type (Martensite, Bainite, Pearlite, Ferrite, � + Cementite, Ni-rich areas, etc.) Size (Fine/Coarse) �
BACKGROUND Fatigue life vs. density/porosity … Fatigue limit Porosity % for both: � P/M iron � P/M steels in: � as-sintered � heat treated (quench and tempered)
BACKGROUND Fatigue life vs. density/porosity … sintered P/M iron Water atomized 4 hrs @ 2050°F Reduced sponge 30 min @ 2280°F Sintering treatment affects fatigue Life (samples from reduced sponge powder) > life at intermediate porosity ranges Life (samples from water atomized powder) (water atomized iron powder, single or (single, double, triple sequence double pressed, and sintered @ sintering @ 2050°F or 2200°F) 2050°F (30 min) and 2280°F (4 hrs))
BACKGROUND Fatigue life vs. density/porosity … sintered P/M steels Bimodal Large 30 min @ Small 2340°F Base 20 min @ 2050°F ABC (atomized) Fe-1.5Cu-1.75Ni-0.5Mo-0.5C (SE) and Fe-2Cu-0.8C (30 min @ 2050°F) MH (sponge) Fe-1.5Cu-4Ni-0.5Mo-0.5C (AE)
BACKGROUND Fatigue life vs. density/porosity … sintered P/M steels Fe-1.5Cu-0.6C (30 min @ 2050°F) Fe-2Cu-2.5Ni (60 min @ 2280°F)
BACKGROUND Fatigue life vs. density/porosity … Q&T - P/M steels S Q&T Fe-1.5Cu-1.75Ni-0.5Mo-0.5C (sponge) Fe-1.5Cu-1.75Ni-0.5Mo-0.5C (atomized) Fe-1.5Cu-1.75Ni-0.5Mo-0.6C Fe-1.5Cu-4Ni-0.5Mo-0.5C (atomized)
BACKGROUND ∆ K th vs. density/porosity … sintered P/M iron and steels S Q&T Fe-1.5Cu-1.75Ni-0.5Mo-0.5C (sponge) Fe-1.5Cu-1.75Ni-0.5Mo-0.5C (atomized) Fe (atomized) Fe-1.5Cu-4Ni-0.5Mo-0.5C (atomized) Fe-1.5Cu-1.75Ni-0.5Mo-0.6C Fe-1.75Ni-0.5Mo-0.5C (atomized)
BACKGROUND FCGR and ∆ K FT vs. density/porosity … Fe-2Cu-2.5Ni (60 min @ 2280°F) Fe-1.5Cu-0.6C (30 min @ 2050°F)
BACKGROUND ∆ K FT vs. density/porosity … Fe-Cu-Ni-Mo-C alloys Fe-0.8Cu-1.8Ni-0.5Mo-0.2Mn-0.4C as-sintered
BACKGROUND ∆ K FT vs. density/porosity … FL 4605 FL 4605 heat treated as-sintered FL = pre-alloyed FLN = pre-alloyed + elemental Ni blend FN = elemental blended
BACKGROUND ∆ K FT vs. density/porosity … Homogeneous Inhomogeneous Fe-1.75Ni-0.5Mo-0.5C as-sintered
BACKGROUND Total porosity vs. Open/Closed porosity … Open porosity Total Porosity: Open + Closed + Isolated Open porosity: continuous pore channels intersecting the surface of the specimen (and each other) Closed porosity: closed gaps between powder particles resulting from compaction and/or sintering (not accessible to the surface BUT can be connected to each other !!) Closed Isolated porosity: pores present in the initial porosity Isolated powder particles (not affected by compaction porosity and sintering)
BACKGROUND Open porosity … the controlling parameter ? Tension- compression Plane bending • Axial testing – volume properties P/M iron • Bending – surface properties
BACKGROUND Open porosity … the controlling parameter ? P/M iron � Region I: closed porosity ( ∆ K th ≈ ct.) Fe-1.75Ni-0.5Mo-0.5C � Region II: ∆ K th porosity (no increase in threshold � Region III: open porosity ( ∆ K th ≈ ct. above ~7.5 g/cm 3 ) and low)
BACKGROUND Open porosity level … function of density only ? Literature: Open porosity is assumed to gradually increase with density Is this correct ? Or the “path to density” prevails over the “density” itself …
BACKGROUND Open porosity measurement techniques … Open porosity Open porosity Closed unresolved by penetrated by He porosity penetrating oil Open porosity penetrated by oil Isolated porosity Gas-impregnation Oil-impregnation (ASTM B328) More accurate measurements of Calculate the interconnected open porosity due to increased porosity from the volume of oil that has impregnated the pore penetration ability of gases compared to oils - RECOMMENDED specimen
BACKGROUND Gas-impregnation measurement techniques … pycnometry Gas displacement pycnometer: a sample of known weight (a solid, a powder, or a porous material) is placed in one of the chambers and the change in pressure needed to balance the two chambers is used to calculate the volume of the sample (P 1 V 1 =P 2 V 2 ) bulk density pore free density pycnometric density
BACKGROUND Calculation of open/closed porosity using He pycnometry … ρ = − V 1 bulk % Total porosity = 100 x V total total ρ − pore free ρ = − V 1 bulk % Open porosity = 100 x V open open ρ pycnometri c V closed = V total – V open % Closed porosity = 100 x V closed * The amount of isolated porosity was assumed insignificantly small
BACKGROUND Open/closed porosity results … P/M iron 25 Danninger et al. [18] Ledoux and Prioul [1] Total porosity Lados and Apelian (pycnometry) Lados and Apelian (oil) 20 15 Porosity (%) Open porosity 10 Closed porosity 5 0 5.50 6.00 6.50 7.00 7.50 8.00 Density (g/cm 3 )
BACKGROUND Open/closed porosity results using He pycnometry … … P/M steels in sintered conditions Sintered conditions 1.25 1 Closed porosity (%) 0.75 A4601 A4001 0.5 A4001 A4601 0.25 0 6.8 6.9 7 7.1 7.2 7.3 7.4 Density (g/cm 3 )
BACKGROUND Open/closed porosity results using He pycnometry … … P/M steels in quenched and tempered conditions Quenched and tempered conditions 1.25 A4601 after oil quench 1 Closed porosity (%) 0.75 A4601 A4001 0.5 A4001 after gas quench 0.25 0 6.8 6.9 7 7.1 7.2 7.3 7.4 Density (g/cm 3 )
BACKGROUND Open/closed porosity results using He pycnometry … … P/M steels in high temperature/long time sintering Sintered conditions 7 A4601 @ 2350 ° F for 6 hrs 6 oil pycnometry 5 Closed porosity (%) 4 3 A4601 @ 2050 ° F for 30 min 2 (pycnometry) 1 0 6.8 6.9 7 7.1 7.2 7.3 7.4 Density (g/cm 3 )
BACKGROUND Sources of errors in evaluating open porosity … 1. Open porosity measurement technique: Higher closed porosity level is measured using oil- � impregnation (the segments of open porosity channels that are not reached by oil are incorrectly assumed to be closed porosity); It is more accurate at high level of closed porosity; � Pycnometric measurements are more accurate; � 2. Correlating open/closed porosity to density alone: Simple correlations open porosity level – density are � incorrect; The path followed to reach the density is critical for � correct interpretations of open/closed porosity; No closed porosity is achieved through compaction and � regular sintering T and t.
OBJECTIVES Study the effects of density/porosity on the fatigue � initiation and propagation in P/M components; Investigate the porosity/microstructure interactions; � Understand the effects of different microstructural � phases on dynamic properties – mechanisms; Create guidelines for fatigue design corroborated with � the fundamental understanding of the alloys behavior; Optimize the material characteristics and processing � parameters for enhanced fatigue response.
EXPERIMENTAL APPROACH Materials selection … Molding grades particles ( 70-85 µ m ) Chemical Sintered Ni Mo Mn composition C [%] 0.6 1.75-1.8 0.50-0.55 0.15-0.18 Ni Graphite Graphite Admixed Pre-alloyed (QMP ATOMET 4001 (QMP ATOMET 4601 Mo pre-alloyed powder Ni-Mo pre-alloyed powder) admixed with Ni)
EXPERIMENTAL APPROACH Phases … Phase I (a): Phase I (a): Mechanistic understanding of the effects of pore amount on fatigue behavior; � Pore/Microstructure (matrix) interactions; Phase I (b): Microstructure effects on fatigue response; Phase I (b): � Microstructure 1 vs. Microstructure 2; Phase II: Is fatigue resistance a state function ??? Phase II: � Effects of pore size/shape/type on fatigue.
EXPERIMENTAL APPROACH Phase I … Two microstructural considerations Low High density density A. Pore Pore/Matrix Matrix control control control Cooling Fatigue Microstructure 1 rate 1 behavior 1 ? ? B. Cooling Fatigue Microstructure 2 rate 2 behavior 2
EXPERIMENTAL APPROACH Phase I … Density levels selection Produce samples of our composition in both pre-alloyed � and admixed conditions; Adjust compaction (conventional press, warm compaction, � powder forging, etc.) to get the full range of densities: Set 1 Set 2 Set 3 Density [g/cm 3 ] ~6.8 7.2-7.3 7.8+ Micro- structure
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