Developments in Friction Stir Welding of Thin Automotive Sheet Steels:A Microstructural Comparison

Center for Welding, Joining and Coatings ResearchDepartment of Metallurgical and Materials Engineering

Colorado School of Mines

Golden, Colorado

Scott Gordon and Stephen Liu

Current Developments in Ferrous Alloys, MTGN 5497 December, 2009

Outline of Presentation

‣Background Information‣Purpose of Study‣Experimental Program‣Results and Discussion‣Summary‣Acknowledgments

Grey and silver are standard steel and aluminum, respectively. Blue is high-strength steel, orange is very-high-strength steel, yellow is extra-high-strength steel, and red is ultra-high-strength steel.

(www.autoblog.com/media/2006/04/Volvo-safety-1.JPG) (Speer et al. 2002)

Applications of Automotive Steels

‣ Solid-state welding process‣ Final weldment can have significant grain refinement depending on

initial microstructure with favorable mechanical properties‣ High equipment cost limiting its use to selected industrial

applications, e.g. in some European trains, rockets components, jets, race car frames, armor plating, etc…

‣ This is a “green” welding process lending itself to little environmental impact

‣ Process is still little known to the overall welding industry, therefore it has not been accepted by the industrial community as a viable process

Background

Process Description

•A rotating tool is plunged into a material to create both heat and a clean surface near the tool free of oxides and contamination •By traveling, this causes the tool to displace the material in front of the tool and atomically bond it’s surface to the circumferential plasticized material and deposit it behind•Under proper steady state operating conditions, there is a ideal temperature gradient with a relatively equal amount of strain over a semi-symmetrical volume will yield steady state conditions•Defects such as worm holes, surface galling, lack of fusion, scalloping, etc., occur outside operating window

Different Zones of the Weld‣ Stir Zone (SZ)

– Characterized by the most mechanical working caused by sticking and direct contact with the pin‧ Considerable grain size reduction is observed within this zone.

‣ Thermo-Mechanically Affected zone (TMAZ)– Experiences both thermal and mechanical effects

‧ Reduction in grain size is observed that is caused by the mechanical working because of the close proximity to the tool. The tool has no direct contact with this region.

‣ Heat Affected Zone (HAZ)– Region in which process heat caused microstructural changes such as grain

coarsening and precipitation‧ Mechanical properties may be deteriorated in this region

‣ Base Material (BM).

PinShoulder

Purpose of Work

To show that FSW is a viable process to join advanced automotive steel sheets, but that proper precautions and setup need to be taken in order to assure a quality joint

To design a tool that is versatile for usage and service

To develop process parameters for friction stir welding of thin steel sheets

To examine the macro & microstructures of welds produced in similar and dissimilar joints of advanced automotive steels

2 HP Bridgeport vertical mill.

Shoulder and pin fabricated from WRe alloy.

Fixture for restraining samples.

Experimental Program

Pin and Shoulder Design

Manually positioning the tool plunge depth controls the amount of material expelled from the shoulder.

Semi-Automatic Friction Stir Welding (SA-FSW)

Experimental Materials

‣ As-Received Advanced High Strength Steels (AHSS) – IF EDDS (Interstitial Free Extra Deep Drawing) Steel

‧ Easily Formable ‧ Ideal For Painted Body Panels

– TRIP-780 (Transformation Induced Plasticity) Steel‧ Ideal for High Strength Internal Structural Components‧ Under Strain or Heat the Retained Austenite transforms to Martensite

‣ Welding of the Advanced Automotive Sheet Steels – IF EDDS

‧ RSW‧ FSW

‣ Strain Rate Sensitivity ‣ Micro and Macro-Structure

– Dissimilar TRIP-780 Steel to IF EDDS Steel‧ RSW ‧ GTAW‧ FSW

Sample Preparation and Characterization

‣ Samples were cross sectioned at the midpoint of the weld to assume the most steady state condition.

‣ Grinding and polishing with SiC paper; Suspended diamond for rough and fine polishing steps.

‣ Two-Stage Color Tint Etch Process: The polished surface was immersed in a 2% Picral with 1-3 drops of hydrochloric acid in a Petri dish for 12-15 seconds, rinsed with ethanol, followed by water, then submersed in a 7.5% sodium metabisulfite and water solution for 6 seconds. The samples were rinsed in water followed by ethanol, and then dried.

‣ Light Microscopy was used to examine the surface.

0.0110.0130.0361.631.590.19

NPAlSiMnC

0.00290.0090.032<0.0030.0830.0028

NPAlSiMnC

•~ 100% Ferrite• Ferrite• Bainite

IF EDDS Trip-780

•Retained Austenite •Martensite

Bainite

Retained

Martensite

Initial Sheet Steel Microstructures

IF EDDS RSW

Re-crystallized Columnar Ferrite

Fusion Line

As Solidified Columnar Ferrite

Weld Nugget

Weld Nugget HAZ

HAZ

•Large As Solidified and Re-crystallized Columnar Grains•Dissolution, Partitioning and Segregation of Alloying Elements Due to Melting

RSW – Current Industrial Practice - Reference

2200 rpm 200 mm/min

2075 rpm 100 mm/min

2380 rpm 250 mm/min

2200 rpm 100 mm/min

Effects of Process Parameters: Rotational Speed and Travel Speed

Increased Rotational SpeedSlip negligible

Low Rotational Speed,

Low Strain Volume

Increased Rotational Speed,Increased Strain VolumeSlip Sticking T

Increased Strain VolumeSlip Sticking T

2075 rpm 100 mm/min

Insufficient oxide particle break up and dispersion

ASRS

RS TMAZ/SZ Boundary

AS TMAZ/SZ Boundary

SZ

Low Rotational Speed,Low Strain Volume

2200 RPM 100 mm/min

Wide Spacing Between Columnar Ferrite

ASRS

RS TMAZ/SZ Boundary SZ

AS TMAZ/SZ Boundary

Normal Rotational Speed,Low Strain Volume

2200 RPM 200 mm/min

Highly Refined TMAZ and SZ

AS RS

TMAZ

SZ

TMAZ TMAZ

Narrow Spacing Between Columnar Ferrite

RS SZ

Normal Rotational Speed,Normal Strain Volume

2380 rpm 250 mm/min

With a small 2 Phase region (IF), during heating this results in an instantaneous release of energy upon a phase transformation due to strain rate sensitivity change because the system is constantly under strain. The increase in T causes slipping condition and local cooling. This turns into a cyclic phenomenon

Strain Rate Sensitivity Changes in Single Phase Materials

ASM Vol 3, 1990

Cyclic Sticking and Slipping

into the phase region

2000

2100

2200

2300

2400

2500

0 100 200 300 400

Travel Speed (mm/min)

Rot

atio

nal S

peed

(rpm

)

2200 rpm 200 mm/min

2200 rpm 280 mm/min

2200 rpm 100 mm/min

2075 rpm 150 mm/min2075 rpm 100 mm/min

2380 rpm 210 mm/min 2380 rpm 250 mm/min

2075 rpm 200 mm/min

2380 rpm 300 mm/min

2380 rpm 400 mm/min

Establishing the Operating Window via Surface Defect and Microstructure Characterization

Process Parameters and Microstructural Development In High Strength Steels

JIS S70Cequivalent to UNS G10700, SAE-AIEI 1070,0.72%C–0.2%Si–0.7%Mn–0.016%P–0.004%S–0.01%Cu

Rotational S

peed

Travel Speed (Cui et al, 2006)

Refined and

Globular Cementite

Ferrite Pearlite

High Cooling RateLow Cooling Rate

Max Tem

p M

in Temp

IF EDDS

Trip-780FSW GTAW

RSW

Cross section

Cross section

Cross section

AS

RS

Dissimilar Welding of 1.27mm TRIP-780 to IF EDDS Sheet Steel

60A 100mm/min2075 RPM 100mm/min

Surface Examination Shows Larger

HAZ with More Distinct Regions

1. Reference RSW2. GTAW3. FSW

RSW TRIP-780 to IF EDDS

FerriteMartensite

Columnar

BM

IF BM

TRIP HAZ

IF HAZ

HV 550

HV 150

HV 110HV 440

HV 270

HV 440

GTAW - HAZ TRIP-780 to IF EDDS

Columnar Ferrite

Ferrite

Martensite

TRIP BM

PartiallyTransformed TRIP BM

HV 515

GTAW - Weld Metal TRIP-780 to IF EDDS

Acicular Ferrite

Ferrite with Second Phase Aligned

Primary Ferrite

Acicular Ferrite

HV 515

HV 310

HV 215

AS RS

FSW - TRIP-IF BM & HAZ Microstructure

Refined TMAZ

TRIP BM IF BM

Partially Transformed TRIP

Mostly Transformed TRIP

Columnar RS TMAZ

HV 450

HV 715 HV 120

AS RS

FSW - TRIP-IF Stir Zone Microstructure

FSNA Ferrite

Martensite

Retained Austenite

Evidence of FSNA Transformation

Martensite Bainite or FSNAFerrite

HV 230

HV 270

HV 475

TRIP

IF

RSW

TRIP IFGTAW

FSW TRIP IF

110

270550

440

440

440440

515 310 305 215515

715453230 270

475

119

150

Comparison of Weld Vickers Hardness in the Three Welds

270

270

110

110119

Comparison between RSW, GTAW, and FSW of IF-TRIP Dissimilar Welds

‣Weld Metal– Lack of complete mixing in the GTAW and FSW– Non uniform composition in GTAW and FSW– Non uniform mechanical properties In GTAW and FSW due to

variations in microstructures– Composite (Layered) microstructure in FSW

‣HAZ– High Hardness and Hardness Gradient in FSW– No large columnar ferrite zone observed in FSW

Summary ‣Welding parameters must be appropriately selected to

perform welding on materials that have a phase change associated with heating

‣A wide range of final microstructures can be produced by varying the welding parameters, set up, and alloy selection

‣Less volume of transformations were observed in the base metal and HAZ microstructures in the FSW than the other two fusion processes

‣It will be interesting to conduct mechanical testing to characterize the mechanical properties and fracture behavior of the welds

Acknowledgments

‣George ParaskosAK Steel Research, Middletown, Ohio

‣Dr. Alex Landau Ben Gurion University, Ber Sheeva, Israel

‣Dr. Gerard MartinsColorado School of Mines, Golden, Colorado

2380/210Etching response (light and dark) shows the development of highly textured SZ and TMAZ as the rotational speed is increased

2200/200

Development of Crystallographic Orientation and Texture

Random Orientation

Textured Orientation

ASRS

AS RS

Specimen Al B C Ca Cr Cu Mn Mo

1 IF EDDS 0.032 <0.0003 0.0028 <0.0003 0.048 0.045 0.083 0.004N Nb Ni P S Si Sn Ti

0.0029 0.004 0.019 0.009 0.010 <0.003 0.005 0.044

V

<0.003

C Mn Si Al P N

0.19 1.59 1.63 0.036 0.013 0.011

 

(Arbegast 2008)

Defect Formation

Defect Formation

(Kim et al. 2005)

Three Defect types • Large mass of flash due to the

excess heat input;

• Cavity or groove-like defect caused by an insufficient heat input

• Cavity produced by the abnormal stirring

2000

2100

2200

2300

2400

2500

0 100 200 300 400

Travel Speed (mm/min)

Rota

tiona

l Spe

ed (r

pm)

2200/200

2200/280

2200/100

2075/1502075/100

2380/2102380/250

2075/200

2380/300

2380/400

By characterizing the surface defects, an acceptable operating window was established.

Carbon Saturated Austenite/ Ferrite mixture

Carbon Depleted Austenite/ Ferrite mixture

TRIP-780 IF EDDS

+ =

Bainite

If the TRIP is in close proximity of the IF and the temperature provided is favorable for the kinetics of carbon to diffuse into the a phase, then there is a driving force for the formation of bainite

Bainite Formation

+ Fe3C

c

c

c c

cc

c

cc

c

Future Work

‣Examine end microstructures using process parameters within the phase field

‣Examine process parameters bordering phase boundary for other alloy systems for welding of single crystal components, e.g. Ni super alloy turbine blades