Defects.pdf

FILE: MET 492-082 Week 6 Rev NewREVISION DATE: 01 October 2003PAGE: 1

W. J. Arbegast(605)-394-6924

E-Mail: [email protected] Dakota School of Mines and Technology

AMPSDSM&T

Advanced Materials Processing CenterAdvanced Materials Processing Center

Week 6 Friction Stir Joining:

Characteristic Defects

William J. ArbegastDirector, Advanced Materials Processing CenterSouth Dakota School of Mines and Technology

Rapid City, South Dakota

MET 492-Special TopicsFriction Stir Processing

October 8, 2003


W. J. Arbegast(605)-394-6924


AMPSDSM&T


OutlineQuestions from previous week

Review of Quiz #3

Definition of a DefectSource of DefectsDefect TypesEffect of Defects on Lap Joint PropertiesShort Quiz #4


W. J. Arbegast(605)-394-6924


AMPSDSM&T


What Is a Characteristic Defect in a Friction Stir Weld?

• A “Characteristic Defect” is defined as a Microstructural or Geometric Anomaly peculiar to a FSW that adversely affects Form, Fit or Function.

• An “Indication” is defined as a Microstructural or Geometric Anomaly that DOES NOT adversely affects Form, Fit or Functionof a friction stir weld- BUT MAY NOT BE DESIRABLE

• A “Defect” may adversely affect any or all of the following properties

• Mechanical Properties (Ultimate, Yield, Elongation)• Dynamic Properties (Fracture and Fatigue)• Corrosion Resistance• Formability and Ductility


W. J. Arbegast(605)-394-6924


AMPSDSM&T


What Is a the Source of Defects in a FSW?

• Defects in Friction Stir Welds are related to Processing Temperatures, Metal Flow Patterns, and Joint Geometry

• Both Processing Temperatures and Metal Flow Patterns are a Function of Processing Parameters and Pin Tool Design

• Joint Geometry is a function of Fit-up and Alignment

Begin TipRadius

Bottom OfTool

DISPLACED DOWNAND BEHIND

PULLED INTOFLOW ARM

THREADS ALTERNATELY FILL FROMLEADING THEN FLOW ARM MATERIAL IN

INITIAL INTERLEAVING ZONE

LAMINARINTERLEAVING

VORTEX SWIIRLZONE

FLOW ARM MATERIALPRIMARILY FROMRETREATING SIDE

ADVANCING SIDE MATERIALINTERLEAVES WITH LEADING

SIDE MATERIAL

INITIAL FILL ZONE

VORTEX SWIRL ZONE AROUND PIN RADIUSSHOWS VERY LITTLE VERTICAL FLOW MOTION

Based on Metallographic analysis of David Braun, LMMSS

Longitudinal View

Upper Material FeedsShoulder Zone

Initial Deformation Zone

Extrusion Zone

Vortex Swirl Zone

Pmax

Vf

Shoulder Zone

Pre- Heat Zone

Forging Zone

Initial Deformation Zone

Extrusion Zone

Vortex Swirl Zone

Pmax

VfVf

Shoulder Zone

Pre- Heat Zone

Forging Zone

FRONTREAR

t

ht

he

hs

Vf


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AMPSDSM&T


* Defects in BLUE are easily controlled by RPM and IPM parameters.** Defects in RED are related to penetration (LOP) and seam tracking (LOF).

FSW Processing Maps Can Be Used toDetermine Points of Optimum Metal Flow and

Points of Defect Formation

IPM

RPM

Faying Surface*

Scalloping*

Collapsed Nugget*

Wormhole*

Chip LOF*

Root Flow*

Surface Galling*

Surface LOF*

Ribbon Flash*

COLD

HOT

Lack of Penetration (LOP) **

Lack of Fusion (LOF) **

Process Envelope

OperatingWindow

NominalSchedule

Excessive Indentation**

What are the Types of Defects in a FSW?• Cold Processing Defects

• Wormhole• “Chip” Lack of Fill• Scalloping• Cold Lap (lap joints)

• Hot Processing Defects• Ribbon Flash• Surface LOF• Nugget Collapse• Surface Galling• Faying Surface• Root Flow Defect• Sheet Thinning (lap joint)

• Geometry Related Defects• Lack of Penetration (LOP)• Lack of Fusion (LOF)• Excessive Indentation


W. J. Arbegast(605)-394-6924


AMPSDSM&T


What Is a “Wormhole” Defect in a FSW?Description

An advancing side tunnel of inadequately consolidated and forged material running in the longitudinal direction.

CauseInsufficient metal flow into the Zone I interleaving area on the advancing side above the swirl Zone IV. Created by excessive travel speed (ipm) for given rotational speed (rpm) Cold weld Too low weld pitch

Effect on mechanical propertiesSevere wormhole creates a reduction in mechanical properties

Inadequate fill of Advancing Side above Swirl Zone

Advancing Side


W. J. Arbegast(605)-394-6924


AMPSDSM&T


What Is a “Chip Lack of Fill” Defect in a FSW?Description

Similar to wormhole defect. The void contains small shards of metal that were sheared off the parent material (chips)

CauseInsufficient heating results in shearing and cutting of the parent metal leaving small shards of metal within the tunnel (wormhole). Excessively cold weldToo low weld pitch

Effect on mechanical propertiesSevere chip LOF creates a reduction in mechanical properties

Advancing Side

Chip or Shards of Metal in

Void


W. J. Arbegast(605)-394-6924


AMPSDSM&T


What Is a “Scalloping” Defect in a FSW?Description

A series of small voids located in the advancing side interleaving Zone I along the length of the weld.

CauseInsufficient flow and metal forging in the forging zone. Lower limiting case of the wormhole defectModerately cold weldInsufficient forge pressureToo low weld pitch

Effect on mechanical propertiesSevere scalloping will show a reduction in mechanical propertiesModerate scalloping may not show effect on properties

Fracture Surface of Tensile Specimen

Longitudinal View

Welding Direction

Interlayer Voids


W. J. Arbegast(605)-394-6924


AMPSDSM&T


What Is a “Ribbon Flash” Defect in a FSW?Description

Excessive expulsion of material on the top surface leaving a corrugated or ribbon-like effect along the retreating side

CauseExcessive forge load or plunge depthThickness mismatch between advancing side and retreating sideExcessively hot weldToo high weld pitch

Effect on mechanical propertiesExcessive heating may result in reduction in propertiesExcessive flash may reduce fatigue life


W. J. Arbegast(605)-394-6924


AMPSDSM&T


What Is a “Surface Lack of Fill” Defect in a FSW?Description

A continuous or intermittent top surface void on the advancing side

CauseInsufficient flow arm formation across top surfaceInsufficient forge pressureImproper backside supportInsufficient plunge depth.Separation of the plates

Effect on mechanical propertiesExcessive surface lack of fill may result in reduction in properties

Advancing SideAdvancing SideAdvancing Side


W. J. Arbegast(605)-394-6924


AMPSDSM&T


How Does Pressure Distribution Affect Surface Fill?

Anvil

Pin Diameter Lead Trail

1Anvil

Pin Diameter Lead Trail

2

Pin Diameter

Anvil

Lead Trail

3Anvil

Lead TrailPin Diameter

4

Shoulder Zone/ Flow Arm

Shoulder Zone/ Flow Arm

Retreating SideExtrusion Zone

Advancing SideExtrusion Zone

Swirl / VortexZone

Advancing Retreating Advancing Retreating

Advancing Retreating Advancing Retreating


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AMPSDSM&T



BP BP

NBP NBP

NBP BP

BP NBP

1 2

3 4NBP = No Back Pressure BP = Back Pressure

Retreating SideFlow Arm

shoulder affectedzone


Shoulder affectedzone


Material


Material


shoulder affectedzone


Material

Advancing Retreating


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AMPSDSM&T



RetreatingSide ExtrusionZone Material

Flow ArmShoulder

Affected Zone

Swirl orVortex ZoneBeneath Pin

Tip

Cavity on Advancing side to befilled with shoulder zone and

Advancing Side material

Ripple pitch same as weldpitch- ie, rpm/ipm

Sample prepared with no backside pressure on

retreating side


W. J. Arbegast(605)-394-6924


AMPSDSM&T


What Is a “Nugget Collapse” Defect in a FSW?

DescriptionImproper formation of DXZ nugget shape

CauseExcessive flow arm formationExcessive material flow into Zone I advancing sideExcessively hot weldToo high weld pitch

Effect on mechanical propertiesExcessive heating may result in reduction in propertiesExcessive nugget collapse may reduce properties

Excessive Flow Arm Formation and injection of material into

Advancing Side (Zone I)

Collapse of DXZ Nugget

Advancing Side


W. J. Arbegast(605)-394-6924


AMPSDSM&T


What Is a “Surface Galling” Defect in a FSW?

DescriptionGalling and tearing of the metal on the top surface of the weld beneath the pin tool

CauseSticking of metal to pin toolExcessively hot weldToo high weld pitch

Effect on mechanical propertiesSevere surface galling may reduce mechanical properties


W. J. Arbegast(605)-394-6924


AMPSDSM&T


What Is a “Faying Surface” Defect in a FSW?Description

Discontinuous / continuous line of second phase particles @ the location remnant of the original faying surface

CauseOriginal joint faying surface not broken up during FSWExcessively hot weldToo high weld pitchContamination of faying surface.Improper seam tracking (too far offset to advancing side)

Effect on mechanical propertiesExcessive faying surface defect may result in reduction in properties1000X


W. J. Arbegast(605)-394-6924


AMPSDSM&T


What Is the Point of Divergence?

Faying Surface is ContainedWithin Thread Extrusion Zone

And is Consumed

Vf

w CL

ADVANCING

RETREATINGExtrusion Zone

Initial DeformationZone

Xdc2 rs

Vf

w CL

ADVANCING

RETREATINGExtrusion Zone

Initial DeformationZone

Xdc2 rs

Faying Surface is at Peripheryof Thread Extrusion Zone and is

Partially Broken Up

POD POD

The Point of Divergence (POD) is that point within the Initial Deformation Zone ahead of the pin tool where the original Faying Surface begins to

transition around the pin tool

Too Hot

A Point of Divergence (POD) too far ahead of the pin tool results in the faying surface defect


W. J. Arbegast(605)-394-6924


AMPSDSM&T


What Is a “Root Flow” Defect in a FSW?Description

Chevron shaped flow patterns visible on backside (root) surface

CauseExcessive metal flow within swirl zone beneath pin tip (Zone IV) and “breakout” of flow patterns to back surfaceExcessive pin lengthImproper pin tip radiusExcessively hot weldToo high weld pitch

Effect on mechanical propertiesExcessive Root Flow Defect may result in reduction in properties

320 Demo Panel4.5X

Advancing Side Standard 320 Tool

0.320” 2195 (Root Flow)

Advancing Side

MP-1-M24.5X0.250” 2195 (Root Flow)

Flat Nosed Tool

Bottom View of

Weld

Good Root

Structure

Root Flow

Defect


W. J. Arbegast(605)-394-6924


AMPSDSM&T


What Is a “Lack of Penetration” Defect in a FSW?Description

Remnant of original faying surface on root side of weld beneath DXZ

CauseInadequate recrystallization of Swirl Zone IV on back surfaceInsufficient metal flow in Swirl Zone (IV)Inadequate pin lengthImproper pin tip radiusCold weldToo low weld pitch

Effect on mechanical propertiesExcessive Lack of Penetration Defect may result in reduction in properties

LOP

Faying Surface

Remnant

DXZ

Insufficient Nugget Penetration


W. J. Arbegast(605)-394-6924


AMPSDSM&T


LOF

What Is a “Lack of Fusion” Defect in a FSW?Description

Remnant of original faying surface on root side of weld beneath DXZ

CauseImproper seam trackingWeld seam is missed and consequently one portion of a weld joint is not fused to an adjacent portion of a weld joint

Effect on mechanical propertiesExcessive Lack of Fusion Defect may result in reduction in properties

Faying Surface

Remnant

DXZ

Sufficient Nugget Penetration but Joint is Missed


W. J. Arbegast(605)-394-6924


AMPSDSM&T


What are the Defects in a FSW Lap Joint?Lap welds have two additional defect types in addition to those observed in butt joints

SHEET THINNING DEFECTCOLD LAP DEFECT

Cold Lap Defect

Effective Shear Area

Retreating

Sheet Thinning Defect

Advancing

50x original

Cold Lap Defect

Advancing

EffectiveShear Area


Retreating

Faying Surface


W. J. Arbegast(605)-394-6924


AMPSDSM&T


200X

TMZ DXZ

Sheet Thinning

Defect

200X

TMZ DXZ

Sheet Thinning

Defect

What is a Lap Joint Sheet Thinning (ST) Defect?Description

Up-turning (or down-turning) of original joint line faying surface resulting in reduction of sheet thickness in a lap weld

CauseHot processing parameters setting up excessive vertical flow patternsFaying surface is “pulled” up or down in the TMZ as a result of these flow patterns

Effect on mechanical propertiesIncreasing the Sheet Thinning (ST) defect may decreases the lap shear strengthIncreasing the ST defect reduces fatigue life

Advancing Side

Joint Line/ Faying Surface

Top Sheet

Bottom Sheet


W. J. Arbegast(605)-394-6924


AMPSDSM&T


What is a Lap Joint Cold Lap Defect (CLD)?Description

A “Dragging” of the original joint line into the DXZ on the retreating side without mixing or consolidation

CauseCold processing parameters resulting in inadequate vertical mixing within DXZFaying surface is “pulled” around pin and into the DXZ as a result of these flow patterns

Effect on mechanical propertiesIncreasing the Cold Lap Defect (CLD) may reduces the lap shear strength

Retreating Side

Joint Line/ Faying Surface

Top Sheet

Bottom Sheet


W. J. Arbegast(605)-394-6924


AMPSDSM&T


What is a the Effective Shear Area (ESA)?Description

The effective Shear Area is that area where the faying surface in a lap joint is CONSUMED AND CONSOLIDATED to produce a SOUND JOINTIt is the remaining area within the DXZ when the Cold Lap Defect (CLD) is subtracted from the width of the DXZ along the original joint line

Effect on mechanical propertiesThe Effective Shear Area (ESA) is that area that carries the load during a lap shear test of a lap jointIncreasing the ESA increases the lap shear strength of a joint

50x original

Cold Lap Defect

Advancing

EffectiveShear Area


Retreating


W. J. Arbegast(605)-394-6924


AMPSDSM&T


How Do Processing Parameters Affect ST and CLD?The Sheet Thinning and Cold Lap Defect are “Competing” defectsHOT welds INCREASE Sheet Thinning and DECREASE Cold Lap DefectCOLD welds DECREASE Sheet Thinning and INCREASE Cold Lap Defect

50 x

EffectiveShear Area

Sheet Thinning

Cold Lap DefectLeading

5.0 x

Leading

FSW Direction

0.080”Sheet

50 x

EffectiveShear Area

Sheet Thinning


5.0 x

Leading

FSW DirectionFSW Direction

0.080”Sheet

Advancing

50 x

EffectiveShear Area

Sheet Thinning


5.0 x

Leading

FSW Direction

0.080”Sheet

50 x

EffectiveShear Area

Sheet Thinning


5.0 x

Leading

FSW DirectionFSW Direction

0.080”Sheet

Advancing


W. J. Arbegast(605)-394-6924


AMPSDSM&T


How Do ST and CLD Affect Joint Strength?By controlling the direction of welding, the ST Defect can be “included” (Type I Joint) or “excluded” (Type II Joint) from the load path in a lap shear test.

Type I

Sheet Thinning in Load PathCold Lap Defect

Effective Shear AreaFSW Direction

Pin Tool Rotation

Free Edge on Trailing Side+ST: reduce strength-ST: no effect on strength

Sheet Thinning in Load PathCold Lap Defect

Effective Shear AreaFSW DirectionFSW Direction

Pin Tool Rotation

Free Edge on Trailing Side+ST: reduce strength-ST: no effect on strength

Type IIFree Edge on Leading Side

+ST: no effect on strength

-ST: reduce strength

Cold Lap Defect in Load Path

Type IIFree Edge on Leading Side

+ST: no effect on strength

-ST: reduce strength

Cold Lap Defect in Load Path


W. J. Arbegast(605)-394-6924


AMPSDSM&T


How Do ST and CLD Affect Joint Strength?Lap Shear Strengths is generally higher for Type II Joints

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0.050 / 0.050 0.050 / 0.063 0.063 / 0.063 0.050 / 0.080 0.063 / 0.080 0.080 / 0.080

Welded Thickness (top / bottom sheet - inches)

Load(lbs)

Type II

Type I

CCW Pin Tool Rotation

Type II - Free Edge on Leading Side

Type I - Free Edge on Trailing Side

Lap Shear Strength (pounds per inch of weld) for various sheet thickness combinations- 2024 aluminum


W. J. Arbegast(605)-394-6924


AMPSDSM&T


How Do ST and CLD Affect Fatigue Life?Placing the Sheet Thinning Defect in the Load Path can reduce fatigue life for lap joint tests and “peel” tests

y = 5089x-0.179

R2 = 0.997

y = 5389.9x-0.1793

R2 = 0.9999

0

200

400

600

800

1000

1200

1400

1600

1000 10000 100000 1000000

Cycles

Load

(lbs

)

Type II

Type I

CCW Pin Tool Rotation

Type II - Free Edge on Leading Side

Type I - Free Edge on Trailing Side

y = 528.71x-0.1944

R2 = 0.9988

y = 737.12x-0.2699

R2 = 0.95150

50

100

150

200

250

100 1000 10000 100000 1000000

Number of Cycles to Failure

Load

(lbs

)

Type IV

Type III

Type IV

FSW Direction

Type III

Fatigue Properties for 0.080”/0.080” 2024 aluminum Friction Stir Lap Welds


W. J. Arbegast(605)-394-6924


AMPSDSM&T


SummaryDefects in Friction Stir Welds are primarily related to Metal Flow PatternsPin Tool Design and Joint Geometry can also induce defectsCold Welds can show defects unique to cold processing parametersHot Welds have defect unique to hot processing parametersLap Joints have two additional defects over those seen in butt joints (ST and CLD defects)A defect is NOT A DEFECT UNLESS it adversely affects FORM, FIT, or FUNCTION


W. J. Arbegast(605)-394-6924


AMPSDSM&T


References

W.J. Arbegast, E.R. Coletta and Z. Li, "Characterization of Friction Stir Weld Defect Types", presented at the TMS 2001 Annual Spring Meeting, New Orleans, LA, February 11-15, 2001

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