PERFORMANCE OF P/M COMPONENTS DURING DYNAMIC LOADING Worcester - - PowerPoint PPT Presentation

PERFORMANCE OF P/M COMPONENTS DURING DYNAMIC LOADING

Worcester Polytechnic Institute October 22-23, 2003

Diana Lados & Diran Apelian

Morris Boorky Powder Metallurgy Research Center

OUTLINE

ß Background (and examples of fatigue studies from the literature) ß Objectives ß Our Approach … Experimental Plan ß Critical experimental details

BACKGROUND

General perspectives …

Two design concepts Defect intolerant Defect tolerant

BACKGROUND

Defect intolerant … s-N curve

e-N curve CSS curve

(Basquin eq.): (Coffin-Manson eq.): (cyclic stress-strain eq.):

( )

'

n p '

K e D ⋅ = s D

Ferrous Nonferrous Stress, s HCF (N > 105) LCF (N < 105) Strain range, Dep

( )

B a

N A⋅ = s

( )b

' f

N 2 2 ⋅ s = s D

( )c

' f p

N 2 2 ⋅ e = e D

( )

D p

N C⋅ = e D

Stress, s Strain, e Monotonic Cyclic

BACKGROUND

LCF – high plastic deformation/ low loading cycles HCF – quasielastic behavior/ very high loading cycles

LCF HCF

(r low) 10 < Nf

tr < 1000 (r high) PM steels (Ni-Mo)

Defect intolerant … contd.

BACKGROUND

Defect tolerant … Fatigue crack growth curve (LEFM)

(Paris eq.): (Forman eq.):

( )

m

K C dN da D =

( ) ( )

K K R 1 K C dN da

FT m

D

• ⋅
• D

= DKFT DKth

I II III

logDK

log(da/dN)

P/M iron

BACKGROUND

High cycle fatigue (HCF) … P/M iron

Region A:

• nucleation of microcracks;

Region B:

• appearance of slip bands on the specimen

surface; Region C:

• characteristic S-N curves where final

failure is caused by macrocracking;

BACKGROUND

High cycle fatigue (HCF) … P/M iron … contd. P/M iron

Region I:

• mostly closed porosity;
• cracking in the specimen interior;
• transgranular path between isolated pores;

Region II:

• transition from closed to open porosity;
• cracks nucleate @ specimen surface at

isolated pores and pore clusters;

• some broken sintering necks;

Region III:

• pores connected to each other (open)
• biphasic material: matrix phase + pore phase;
• simultaneous cracks @ specimen surface
• broken surface is smooth in both fatigue and

ductile fast fracture regions (broken sintering necks).

BACKGROUND

High cycle fatigue (HCF) … P/M iron … contd.

Water atomized Reduced sponge Tension- compression Plane bending Life (samples from reduced sponge powder) > Life (samples from water atomized powder)

• Axial testing – volume properties
• Bending – surface properties
• Fatigue limit (bending) < Fatigue

limit (axial loading)

BACKGROUND

High cycle fatigue (HCF) … P/M steels Fe-1.75Ni-1.5Cu-0.5Mo-0.6C (TM&S) Fe-2Cu-0.8C (P&F)

Fatigue life increases with increasing density and pore shape factors BUT density alone can not describe fatigue behavior of such PM materials pore/matrix interactions

BACKGROUND

High cycle fatigue (HCF) … P/M steels … contd. Fe-1.75Ni-1.5Cu-0.5Mo-0.6C Fe-1.75Ni-1.5Cu-0.85Mo-0.6C

• no significant difference between binder-treated and diffusion alloyed
• If Mo Fatigue life

BACKGROUND

Low cycle fatigue (LCF) … P/M steels Fe-1.75Ni-0.5Mo With decreasing density the differences between the strain life curves become smaller Increasing porosity reduces microstructural influence on fatigue life

BACKGROUND

Cyclic stress-strain … P/M iron

• I. Pure elastic response
• II. Microcracks opening (plastic strain-

softening)

• III. Pronounced opening of microcracks
• verrides matrix hardening
• IV. Growth of macro-cracks / final failure

Changes in hysteresis loop indicate hardening/softening of the material

BACKGROUND

Cyclic stress-strain … P/M iron … contd. P/M iron: cyclic softening

K’

• r
• n’ unaffected by density
• high density materials K’~K’fully-dense

BACKGROUND

Cyclic stress-strain … P/M steels Fe-1.75Ni-0.5Mo: low strain - softening & high strain - hardening

Homogeneous P Inhomogeneous P+F+M

Fe-2Cu-2.5Ni: work hardening Fe-1.5Cu-0.6C: softening Fe-0.8P (F&P): cyclic hardening

BACKGROUND

Fatigue crack growth studies, da/dN vs. DK …

ß Near threshold PM/C&W

similar behavior;

ß Higher DK, PM inferior to C&W,

cracks grow one order of magnitude faster;

ß Pseudo fracture toughness DKc

much lower in PM, 20-50 MPa m1/2

(compared to 80-130 MPa m1/2 for quenched and tempered steels).

BACKGROUND

Fatigue crack growth studies, da/dN vs. DK … contd. Fe-1.75Ni-0.5Mo-0.5C

(homogeneous - Divorced P)

Higher density Enhanced resistance to fatigue crack growth Uniform shifts Density/porosity dominates FCGR over the microstructure of the matrix

ß Many investigations on various P/M materials, but little knowledge on fatigue mechanisms and fracture; ß Pore/matrix interactions and how the presence of pores influences/changes the behavior of the matrix are not understood; ß Characteristic microstructural features as well as inhomogeneities need to be individually understood and further correlated to the pore structure (deconstruct/reconstruct); ß Fatigue life data corroborated with a fundamental understanding of the alloys behavior predictive abilities; ß There are no systematic studies to provide “knowledge based recipes” to optimize material characteristics and processing parameters for enhanced fatigue and fatigue crack growth

SUMMARY OF THE LITERATURE REVIEW

ß Study the effects of density/porosity on the fatigue initiation and propagation in P/M components; ß Investigate the porosity/microstructure interactions; ß Understand the effects of different microstructural phases on dynamic properties – mechanisms; ß Create guidelines for fatigue design corroborated with the fundamental understanding of the alloys behavior; ß Optimize the material characteristics and processing parameters for enhanced fatigue response.

OBJECTIVES

EXPERIMENTAL APPROACH

Materials selection … Pre-alloyed (QMP ATOMET 4601 Ni-Mo pre-alloyed powder) Admixed (QMP ATOMET 4001 Mo pre-alloyed powder admixed with Ni)

Molding grades particles (50-75 mm)

0.6 ~0.1 0.15-0.20 0.50-0.55 1.8 ~0.003

[%] Graphite additions O Mn Mo Ni C Chemical composition

EXPERIMENTAL APPROACH

Phases … Phase I (a): Phase I (a): Mechanistic understanding of the effects of pore amount/type on fatigue behavior;

ß Find the relationship density-open/closed

porosity ratios for our composition-processes;

ß Pore/Microstructure (matrix) interactions;

Phase I (b): Phase I (b): Microstructure effects on fatigue response;

ß Microstructure 1 vs. Microstructure 2;

Phase II: Phase II: Is fatigue resistance a state function ???

ß Effects of pore size/shape on fatigue.

Open pores

EXPERIMENTAL APPROACH

Phase I … Density –closed/open porosity relationship ß Produce samples of our composition in both pre-alloyed and admixed conditions; ß Adjust compaction (conventional press, warm compaction, powder forging, etc.) to get the full range of densities:

Set 1 Set 2 Set 3

100% Closed 100% Open

Porosity 7.75

• r

highest possible 7.0 <6.5 Density [g/cm3]

Closed pores

EXPERIMENTAL APPROACH

Phase I … Density levels selection Micro- structure

Low level of closed porosity 30% open porosity & 70% closed porosity 70% open porosity & 30% closed porosity

Pore amount/ type Set 3 7.75+ Set 2 7.2-7.25 Set 1 6.8-6.9 Density [g/cm3]

EXPERIMENTAL APPROACH

ß Compaction:

‘ low densities (Set 1): normal compaction; ´ intermediate densities (Set 2): controlled temperature compaction (warm compaction 145° F );

” high densities (Set 3): powder forging. ß Sintering: · temperature:T=2050° F ;

6 time: t=30 min;

ÿ T and t invariant for phase I. Phase I … Compaction +Sintering

EXPERIMENTAL APPROACH

ß Post sintering heat treatment: ÿ austenitize @ 1600-1700° F for 30 min (similar austenitic grains) ÿ quench to 2 microstructures (for both pre-alloyed and admixed): ÿ temper @ 350-450° F for 30 min-1 hr (similar matrix micro-hardness) Phase I … Heat treatment Martensite + ~10% R.A. Martensite + Bainite Martensite + Pearlite + ~10%R.A. Martensite + Bainite + Pearlite

EXPERIMENTAL APPROACH

ß Two microstructural considerations are relevant: Phase I(a): pores vs. matrix

• How porosity interacts with the matrix and when microstructure

becomes cause of failure

• Two microstructures (M / M+X) will be analyzed at three porosity

levels and the pore-to-matrix transition will be investigated for all the 12 cases (6 for pre-alloyed and 6 for admixed)

Phase I(b): matrix 1 vs. matrix 2

• How different microstructures influence fatigue behavior
• Two microstructures will be studied and their effects on fatigue

initiation and propagation will be assessed for both pre-alloyed

Phase I … Effects of pores and microstructures

EXPERIMENTAL APPROACH

Phase I … Two microstructural considerations Low High density density Pore Pore/Matrix Matrix control control control

A. ? ? B.

Cooling Fatigue rate 2 behavior 2 Cooling Fatigue rate 1 behavior 1 Microstructure 1 Microstructure 2

EXPERIMENTAL APPROACH

Fatigue testing … Specimens and equipment Dog-bone specimens for pull-pull/push-pull CT specimens for FCGR

[Courtesy of Westmoreland]

• +

smin smax smean sa

EXPERIMENTAL APPROACH

Fatigue testing …

• The experiments will be

conducted by the WPI team in collaboration with FTA;

• 1 sample for each of the 12

conditions (prealloyed+admixed, 3 density levels, 2 microstructures)

• The tests will be done at an
• utside testing facility in parallel

with the fatigue crack growth work;

• 3 failed samples at 4 life levels

for each of the 12 conditions): * 103-104

* 104-105 * 105-106 * 106-107

• 2. Fatigue crack growth

tests (E647)

• 1. Pull-pull / Pull-push tests

(E466)

One density level is selected (~7.2 g/cm3, same as Set 2 in Phase I) and 4 ways

• f achieving are investigated in parallel (we choose the most attractive alloy

from fatigue point of view [of the 12 combinations] and concentrate our attention on how pore size/shape will influence its behavior):

1. Compaction - coarser powder, 100-105 mm; 2. Normal compaction to 7.0 g/cm3, followed by a different temperature/time sinter; 3. Double press/Double sinter 4. Surface densification (7.0 g/cm3 - core and 7.2 g/cm3 -

• uter shell)

EXPERIMENTAL APPROACH

Phase II … Is Fatigue Limit a State Function ? Pore size/shape effects on the fatigue behavior

EXPERIMENTAL APPROACH

Phase II … Various pore sizes/shapes

Pore morphology (size/shape)

4.

Surface densified (core 7.0 g/cm3 and

• uter layer

7.2 g/cm3)

3.

DP/DS

2.

Press to 7.0 g/cm3 and different sinter to 7.2 g/cm3

1.

Coarser particles

100-105 mm

Molding grades particles

50-75 mm

Case study

7.0 / 7.2

ß Fatigue crack growth work will be conducted for one

selected microstructure and one density level

EXPERIMENTAL APPROACH

Other experimental considerations … ß Inclusion level is low shed light on pore and microstructure effects; ß Sintering atmosphere: synthetic dissociated ammonia; ß Lubricating additives: Acrawax C, AncorMaxD; ß Low residual stress levels are critical to understand the true behavior of the materials and have a fair comparison basis:

• stress relief is done during tempering
• additional stress relieving may be needed after machining.

IT IS ALL IN THE DETAILS …

ß

Prepare samples of both pre-alloyed and admixed alloys for the 2 microstructures with the whole range of densities from 6.5 to 7.86 g/cm3; ß Determine closed and open porosity levels for each alloy and microstructure; ß Develop relationships between density and closed to open porosity ratios for all the 2 microstructures in both pre-alloyed and admixed conditions; ß Select three densities corresponding to: closed porosity only, two controlled mixtures of closed and open porosity (70%C+30%O and 30%C+70%O) for each case; ß Perform static tensile tests to get YS, Young’s modulus, UTS for all the 12 cases in Phase I and 4 cases in Phase II (10 samples per case);

IT IS ALL IN THE DETAILS … contd.

ß Analyze microstructural results and microhardness for different heat treatments on both the homogeneous and non- homogeneous materials for each of the microstructures:

• Different quenching conditions

ß Do a study on the austenitic grain size and assess the possibility of constant austenitic grain size for all the cases; ß Check the residual stress level and decide if an additional stress relieving is needed after the post-sintering heat treatment; Adjust the stress relieving procedure to eliminate residual stress; ß Prepare samples for the life study (200 dog-bone samples) and the fatigue crack growth work (16 compact tension specimens); ß Machine all the samples; ß Start fatigue work.

AFGROW

ß AFGROW was developed by the Analytical Structural Mechanics branch, Air Vehicle Directorate, U.S. Wright-Peterson Air Force Research Laboratory ß Input data required: material properties, DKth, Paris coefficients, C, m, geometry and dimensions of the component, initial flaw characteristics, maximum applied load, stress ratio, choice of constant/variable amplitude; retardation/closure corrections, residual stress adjustments for known residual stresses, etc ß Output data: life predictions (cycles to failure) and failure modes for various applications

http://afgrow.wpafb.af.mil/downloads/afgrow/pdownload.php

AFGROW … CONTD.

D UA MA UE ME UA-T4 MA-T4 0.02 0.04 0.06 0.08 0.1 0.01 0.03 0.05 0.07 0.09

Maximum flaw size (in) s=20 ksi N=1 000 000 cycles

Case study I:

s = 20 ksi N = 1 000 000 cycles a = ?

W=0.5” T=4”

AFGROW … CONTD.

Case study III:

s = 20 ksi N = 10 000 cycles a = ?

D UA MA UE ME UA-T4 MA-T4 0.1 0.2 0.3 0.4 0.05 0.15 0.25 0.35

Maximum flaw size (in) s=20 ksi N=10 000 cycles

AFGROW … CONTD.

1 10

DK (ksi÷in)

10

DK (MPa÷m)

10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1

da/dN (in/cyc)

10-7 10-6 10-5 10-4 10-3 10-2 10-1 100

da/dN (mm/cyc)

R=0.1 Alloy

13%Si-M 13%Si-UM

AFGROW … CONTD.

D UA MA UE ME UA-T4 MA-T4 4000000 8000000 12000000 16000000 2000000 6000000 10000000 14000000

Number of cycles

s=17.4 ksi s=18 ksi s=21 ksi

Initial flaw size a=c=0.05 in

D UA MA UE ME UA-T4 MA-T4 4000000 8000000 12000000 16000000 2000000 6000000 10000000 14000000

Number of cycles

s=17.4 ksi s=18 ksi s=21 ksi

Initial flaw size a=c=0.07 in