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Fatigue Performance Evaluation of Forged Versus Competing Process

Technologies Study

George F MochnalDirector of Research and Education

Forging Industry AssociationForging Industry Educational and Research Foundation


Forging Industry Educational and Research Foundation

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Education1. To support programs at a number of universities where there is

exceptional interest in forging and substantial course enrichment in forging process and application.

2. To offer to selected universities instructional materials on forging for incorporation into metallurgy and materials courses.

Research and Development1. To sponsor research and development projects aimed at:

• Easing or solving technical problems facing the forging industry;

• Advancing technology in design, metallurgy, manufacturing and processing of forgings.


Joint Industry AllianceCompany Logo

Steel, Forging and Heat Treating Industry Pact

Heat Treating Society

JointIndustry Alliance

(JIA)


Dimensional Inspection of Elevated Temperature Parts

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Measurement of a turbine blade

Monitoring of a 5-station vertical press Measurement of a stem pinion

Monitoring of a 4-station horizontal press


Study of Comparative PropertiesCompany Logo

Dr. Ali Fatemi and Research Assistant Mehrdad Zorouficonducted an experimental and analytical comparative study of forged steel, cast iron and cast aluminum steering knuckles at the University of Toledo.

This research was funded by Forging Industry Educational and Research Foundation (FIERF) in cooperation with the American Iron and Steel Institute (AISI)


Results of StudyCompany Logo

1. Yield strength of forged steel to be 140% higher than cast aluminum and 85% higher than cast iron;

2. Ductility of forged steel to be 270% higher than cast aluminum and 48% higher than cast iron;

3. Fatigue strength (at 106 cycles) of forged steel to be 190% higher than cast aluminum and 40% higher than cast iron;

4. Forged steel to be superior with respect to cyclic plastic deformation - a major concern for automotive suspension components;

5. Forged steel knuckle gives about 100 times longer fatigue life than cast aluminum knuckle at the same stress level.


Connecting Rod and Steering Knuckle Optimization Technologies

Adila Afzal, Pravardhan Shenoy, Mehrdad Zoroufiand

Ali Fatemi, ProfessorThe University of Toledo

Funded by:

and


Steering Knuckle• Forged Steel: 2.5 kg, 11V37 Steel• Cast Aluminum: 2.4 kg, A356-T6• Cast Iron: 4.7 kg, 65-45-12

OVERALL OBJECTIVES

- Durability Comparison of Competing Manufacturing Technologies- Optimization Study

Example Components: Engine (Connecting Rod)Suspension (Steering Knuckle)

Connecting Rod• Forged steel: 0.93 lb,

150 HP @ 5700 rpm• Powder metal: 1.2 lb

150 HP @ 5200 rpm


DURABILITY COMPARISONSConnecting Rod: Specimen Tests

2.54

76

6.985

9.53

2.5

24.5R1

2.7

0100200300400500600700800900

1000

0.0% 0.5% 1.0% 1.5% 2.0%

True Strain (%)

Tru

e St

ress

(M

Pa) Forged Steel

Powder Metal

O Forged steelX Powder metal

1E+4 1E+5 1E+6 1E+7200

1000

1E+3

Tru

e St

ress

Am

plitu

de, Δσ/

2 (M

Pa)

Reversals to Failure, 2Nf

0.10%

1.00%

1E+3 1E+4 1E+5 1E+6 1E+7 1E+8Reversals to Failure, 2Nf

Tru

e St

rain

Am

plitu

de, Δε/

2, %


1E+4 1E+5 1E+6 E+70.10%

1.00%

1E+3Reversals to Failure, 2Nf

1E+8

Tru

e St

rain

Am

plitu

de, Δε/

2, %

Tensile Tests

Fatigue Tests Fatigue Tests


DURABILITY COMPARISONSConnecting Rod: Component Tests

Sa = 852 (Nf)-0.0572

Sa = 971 (Nf)-0.0895


R = - 1.25

(2)

1E+4

(2)

1E+5 1E+6 1E+7200

600

Cycles, NfSt

ress

Am

plitu

de (S

a ), M

Pa (M

Pa)

Forged Steel Connecting Rod: 0.93 lbPowder Metal Connecting Rod : 1.2 lb


DURABILITY COMPARISONSSteering Knuckle: Specimen Tests

2.54

76

6.985

9.53

2.5

24.5

R12.

7

Tensile Tests

0

100

200

300

400

500

600

700

800

900

0.0% 0.2% 0.4% 0.6% 0.8% 1.0%

True Strain (%)

True

Stre

ss (

MP

a)

Tensile Tests

Cast Iron

Forged Steel

Cast Aluminum


DURABILITY COMPARISONSSteering Knuckle: Specimen Tests

0.001%

0.010%

0.100%

1.000%

1E+2 1E+3 1E+4 1E+5 1E+6


True

Pla

stic

Strai

n Am

plitu

de (%

)

Forged Steel 11V37Cast Iron 65-45-12Cast Aluminum A356-T6

0.10%

1.00%

1E+2 1E+3 1E+4 1E+5 1E+6 1E+7 1E+8


True

Str

ain

Am

plitu

de (%

)

Forged Steel 11V37Cast Aluminum A356Cast Iron 65-45-12

0.1

1.0

10.0

1E+2 1E+3 1E+4 1E+5 1E+6 1E+7 1E+8


ε aσ a

(MPa

)

Forged Steel 11V37Cast Iron 65-45-12Cast Aluminum A356-T6


DURABILITY COMPARISONSSteering Knuckle: Component Tests

0.5

1.5

0 0.2 0.4 0.6 0.8 1

Normalized Number of Cycles, N/Nf

Dis

plac

emen

t Am

plitu

de (m

m) Forged Steel Knuckle

Cast Aluminum Knuckle

crack nucleates


REFERENCE PUBLICATIONSSteering Knuckle

• “Fatigue Life Comparison of Competing Manufacturing Processes: A Study of Steering Knuckle”, M. Zoroufi and A. Fatemi, SAE Technical paper 2004-01-0628, SAE World Congress 2004, Detroit, MI, March 2004.

• “Fatigue Performance Evaluation of Forged vs. Competing Process Technologies: A Comparative Study”, A. Fatemi and M. Zoroufi, 24th Forging Industry Technical Conference, Cleveland, OH, October 2002.

• “Durability Comparison and Life Predictions of Competing Manufacturing Processes: An Experimental Study of Steering Knuckle”, M. Zoroufi and A. Fatemi, 25th Forging Industry Technical Conference, Detroit, MI, April 2004.

• “A Comparative Study of Forged Steel 11V37, Cast Aluminum A356-T6, and Cast Iron 65-45-12; Monotonic Properties, Cyclic Deformation, and Fatigue Behavior”, M. Zoroufi and A. Fatemi, Technical Report to FIERF and AISI (available through www.forging.org), March 2003.

• “Fatigue Performance Evaluation of Forged versus Competing Manufacturing Process Technologies: A Comparative Analytical andExperimental Study”, M. Zoroufi and A. Fatemi, Technical Report to FIERF and AISI (available through www.forging.org), September 2004.


REFERENCE PUBLICATIONSConnecting Rod

• “A Comparative Study of Fatigue Behavior and Life Predictions of Forged Steel and PM Connecting Rods”, A. Afzal and A. Fatemi, SAE Technical paper 2004-01-1529, SAE World Congress 2004, Detroit, MI, March 2004.

• “Connecting Rod Optimization for Weight and Cost Reduction”, P. Shenoyand A. Fatemi, SAE Technical paper 2005-01-0987, SAE World Congress 2005, Detroit, MI, April 2005.

• “Fatigue Behavior and Life Predictions of Forged Steel and Powder Metal Connecting Rods”, A. Afzal and A. Fatemi, Technical Report to AISI(available through www.autosteel.org), May 2004.

• “Dynamic Load Analysis and Optimization of Connecting Rod ”, P. Shenoyand A. Fatemi, Technical Report to AISI (available through www.autosteel.org), May 2004.


OPTIMIZATION STUDY

Forged Steel Connecting Rod• Dynamic load analysis• FE modeling (elastic)• Stress analysis• Optimization

– Geometry constraints– Manufacturing processes– Material alternatives– Cost

Forged Steel Steering Knuckle• FE modeling (inelastic)• Stress analysis• Optimization



OPTIMIZATION STUDYSteering Knuckle FE Model

• Primary loading is bending• Spindle 2nd step fillet is the critical location• Local yielding occurs• Material model

– Elastic-plastic– Cyclic deformation properties


DESIGN VARIABLES

• Material

• Manufacturing process– Forging steps– Machining– Grinding

• Part processing– Heat treatment– Surface treatment– Inducing compressive

residual stress– Fillet rolling– etc.

CONSTRAINTS

• Equivalent or longer life• Geometry:

– Strut mounting bolt-hole size and location

– Suspension connection bolt-hole size and location

– Spindle diameters and length

OBJECTIVE

Minimize Weight and Cost

OPTIMIZATION STUDY Steering Knuckle


OPTIMIZATION STUDYSteering Knuckle Manufacturing Process

FORGING MACHINING

RAW MATERIAL INPUT

RAW MATERIAL INSPECTION

CUT TO SIZE OF PART

INDUCTION OR FURNACE HEATING

IMPRESSION-DIE FORGING

TRIMMING

INSPECTION & DIMENSIONAL CHECK

TURN SPINDLE

DIMENSIONAL CHECK

MILL STRUT JOINT HOLES

CLEANING, FINAL INSPECTION, SHIPPINGSHOT CLEANING

MAKE STRUT JOINT CHAMFER

MAKE HUB MOUNTING HOLE CHAMFER

MILL TENSION STRUT STEPS

DRILL HUB MOUNTING HOLES

CUT SPINDLE THREADS

CUT HUB MOUNTING HOLE THREADS

MAKE LATERAL LINK HOLE

SPINDLE DIAMETER AND THREADS INSPECION

SAMPLE EDDY CURRENT INSPECTION


FORGRED STEEL STEERING KNUCKLE OPTIMIZATION ALTERNATIVES

NO CHANGE IN ATTACHMENT

GEOMETRY

LIMITED CHANGE IN ATTACHMENT

GEOMETRY

FOCUS:Minimize spindle’s mass with limited attachment changesModify manufacturing process

FOCUS:Minimize mass while maintaining the overall shape and attachment dimensionsModify manufacturing process

STAGE I

STAGE II

OPTIMIZATION STUDY Steering Knuckle


Stress distribution under primary loading at the vicinity of top strut attachment

OPTIMIZATION STUDY Steering Knuckle: STAGE I

Higher stress in the optimized area, but still lower than the critical location


Redesigned spindle

Tapered-bore bearing

Stress contour

Final design

OPTIMIZATION STUDY Steering Knuckle: STAGE II


OPTIMIZATION STUDYSteering Knuckle: Process Modifications and

Alternative Materials

Material Alternative Criteria• Superior mechanical

properties & fatigue strength

• Equivalent or better machinability

• Microalloyed• Cost?

Material AlternativesSAE 15V24SAE 15R30VSAE 1522 MoVTiSAE 1522 MoVTiSSAE 1534 MoVTiSAE 1534 MoVTiSi

Very limited weight saving is achieved, due to geometrical constraints

Process ModificationsPrecision forging vs. conventional forgingWarm forging vs. hot forgingReduction of machining steps

(pierced mounting holes versus machined)

4 HUB MOUNTING

HOLESSTRUT MOUNTING HOLES

Improving Fatigue PerformanceSurface hardeningSurface rollingCost?


OPTIMIZATION STUDYSteering Knuckle: Improving Fatigue Performance

Example ofSurface Hardening

Cost?

Example of Surface Rolling

Cost?

Effect of induction hardening on truck stub axle and depth of induction hardened zone (Schijve, 2001)

Effect of rolling of the notch root on rotating bending fatigue of 37CrS4 steel (Kloos et al., 1987).


5% + material saving

At least 12%Spindle redesignedStage II

5% + material saving

At least 9%Strut holes pierced

Hub mounting bolt holes pierced

Material removed from bodyHub mounting thickness optimizedLateral link joint thickness optimized

Stage I

Cost Reduction

Weight Reduction

Process ChangeGeometry Change

OPTIMIZATION STUDYSteering Knuckle: Summary

Limitations• Many attachment compatibility constraints

• More comprehensive change on constraints can result in major design alterations in suspension system

• Significant thickness reduction in this relatively small part results in distortion of the component during forging

• Emphasis on optimization process


OPTIMIZATION STUDYConnecting Rod

Forged Steel Connecting Rod• Dynamic load analysis• Stress analysis (elastic FEA)• Optimization


0

5

10

15

20

25

30

35

40

0 100 200 300 400 500 600 700Crank Angle - degree

Cyl

inde

r Pre

ssur

e - b

ar Compressive: Max gas load

-250

-200

-150

-100

-50

0

50

100

150

200

250

Crank Angle - deg-150000

-100000

-50000

0

50000

100000

150000

2

Angular Velocity

0 360 720

Angular AccelerationTensile: Inertia load at 360o

Design loads:• Compressive: Max gas load• Tensile: Inertia load at 360o

at max. speed


- Forces at the Crank and Pin Ends at Max. Speed(5700 RPM)

- Max Fx at each end is different.- Fy is significant (i.e. bending).

Fy

Fx

Crank End

-2.0E+04

-1.5E+04

-1.0E+04

-5.0E+03

0.0E+00

5.0E+03

1.0E+04

1.5E+04

2.0E+04

0 200 400 600

Crank Angle- deg

Forc

e-N

FxFy

Piston Pin End

-1.5E+04

-1.0E+04

-5.0E+03

0.0E+00

5.0E+03

1.0E+04

1.5E+04

0 90 180 270 360 450 540 630 720

Crank Angle- deg

Forc

e- N

FxFy

OPTIMIZATION STUDYConnecting Rod: Load Analysis


OPTIMIZATION STUDYConnecting Rod: Dynamic Analysis

-150.0

-100.0

-50.0

0.0

50.0

100.0

0 100 200 300 400 500 600 700

Crank Angle- deg

Stre

ss -

MPa

8-XX9-XX8-von Mises9-von Mises

1

2

3

4

56

7

8

9

10

11

-100

-50

0

50

100

150

200

0 100 200 300 400 500 600 700

Crank Angle- deg

Stre

ss -

MPa

10-XX10-YY10-XY11-XX11-YY11-XY10-von Mises11-von Mises

• Multiaxial stress state at some locations

• R ratio varies along rod.• Bending stress is significant


OPTIMIZATION STUDYConnecting Rod: Static vs Dynamic Analysis

1

2

3

4

56

7

8

9

10

11

0

50

100

150

200

250

300

350

400

450

1 2 3 4 5 6 7 8 9 10 11Location on the Connecting Rod

Equi

vale

nt S

tress

Am

plitu

de -

MPa

StaticQuasi-dynamic



Objective: Optimize weight and cost.– Weight: Optimize geometry– Cost: Use C70 “crackable” steel.

Expected cost reduction 25% (Repgen, 1998).

Focus mainly on the shank region.

Constraints: Protect against:• Maximum Tensile Load: Yielding in tension• Maximum Compressive Load: Yielding in compression or buckling• Maximum Load Amplitude: Fatigue failure• Maximum Bending Stress: Deflection in bending

• Side constraints (Dimensions compatible with existing geometry)

• Manufacturing constraints (Forging feasibility: i.e. distortion, draft angle)


Optimized connecting rod

Existing connecting rod

Existing Connecting Rod

Optimized Connecting Rod


• The optimized geometry is 10% lighter than the original in spite of lower strength of C-70 steel.

• With higher strength facture crackable materials such as micro-alloyed steels, the weight can be further reduced.

• Weight reduction in the shank region is limited by manufacturing constraints (i.e. forging distortion).



Manufacturing steps eliminated by using C-70 crackable steel:

• The need to separately forge the cap and the body of the connecting rod • Heat treatment• Machining of the mating faces of the crank end• Drilling for the sleeve

Cost Savings by using C-70 crackable steel

• Machining cost comprises 62% of the total cost for the conventional forged steel connecting rod (Clark et al., 1989)

• Machining cost reduction from utilization of the fracture splitting process, results in 23% total cost savings (Clark et al., 1989)

• “The development of fracture splitting the connecting rods achieves a total cost reduction up to 25% compared to conventionally designed connecting rods and is widely accepted in Europe” (Repgen, 1998, SAE Paper 980882 )


CONCLUSIONSConnecting Rod

• Tensile and fatigue strengths of forged steel were higher than those for the powder metal, based on specimen and component fatigue tests.

• There is considerable difference in the structural behavior of the connecting rod between static loading and dynamic loading (i.e. operating condition).

• The optimized geometry is 10% lighter than the existing rod. With higher strength crackable materials the weight can be further reduced.

• Reduction in machining operations achieved by using the fracturesplitting process reduces the production cost by about 25%.

• The unique fracture surface from the fracture splitting process prevents the rod and the cap from relative movement. This results in an increase in the stiffness and reduction of stresses at critical locations.


CONCLUSIONSSteering Knuckle

• Forged steel is considerably stronger and more ductile than castaluminum and cast iron. Fatigue strength of forged steel is also considerably higher than cast aluminum and cast iron.

• The material undergoes plastic deformation at stress concentrations during overload cycles. The use of linear elastic FEA is not sufficient for reliable life predictions and optimization.

• Overall weight reduction of at least 12% and cost reduction of at least 5% are estimated for the forged steel steering knuckle.

• Much higher potential for weight and cost savings exists for larger components and, if a more comprehensive change to the suspensionsystem can be made.

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