Titanium

Resistance Welding

Lesson ObjectivesWhen you finish this lesson you will understand:•

Learning Activities1. View Slides; 2. Read Notes, 3. Listen to lecture4. Do on-line workbook

Keywords

Donachie, Titanium,ASM, 1988

Donachie, Titanium,ASM, 1988

Unique Properties• Light Weight• Excellent Corrosion Resistance• Strong tightly adherent oxide• High strength-to-weight ratio

Applications• Chemical Industry• Aerospace Industry• Marine Fields• Medical Applications

T i t a n i u mM e l t i n g

p o i n t3 0 3 5 F1 6 7 0 C

B o i l i n gP o i n t

5 9 0 0 F3 2 6 0 C

C o e f f i c i e n to f

E x p a n s i o n

F/in/in67.4

D e n s i t y 3in/lb163.0

U T S 3 0 k s i P u r e T i2 0 0 k s i A l l o y s

D u c t i l i t y 1 5 – 2 5 %

Phases in Pure Titanium• Alpha {hcp} < 1620F (880C)• Beta {bcc} > 1620F

Classification of Titanium Alloys• Alpha & near Alpha Alloys• Alpha-Beta Alloys• Metastable Beta Alloys

AWS Welding Handbook

AWS Welding Handbook

Can Lead to BrittlenessEspecially in welds (see below)

AWS Welding Handbook

Commercially Pure Titanium• Yield Strengths 35-80 ksi (interstitial O, N, C)

Alpha Titanium Alloys• Not heat treated to increase strength• Used for moderate elevated-temp strength and creep resistance

Near-alpha Alloys• Contain small amount of Beta stabilizers• Marginally heat treatable• Most contain Si for improved high temp properties

Alpha-Beta Alloys (e.g. Ti-6Al-4V)• Mixture of Alpha & Beta• Annealed or Solution heat treated and aged• Outstanding strength to density ratio in heat treated condition• Excellent fracture toughness when annealed

Metastable Titanium Alloys• Contain high percentage of Beta stabilizing elements• Slow transformation - air cooling gives 100% Beta• Aging precipitates of fine Alpha can occur• Aging give strength but ductility and fracture toughness sacrificed• Frequently used for fasteners and springs

Applications Using Titanium Alloys

For more information about applications see:http://www.titanium.org/GIsec2a.htm

General Weld Problems

Oxygen Contamination Cracking• Oxygen > 3000ppm = WM & HAZ Cracking• Alpha (hcp) particularly susceptible• Removal of surface oxide film recommended • Gas shielding generally recommended (especially for seam welds)

Degree of Oxidation• Silver (mild oxidation)• Straw• Blue• White (severe oxidation - weld problems likely)

General Weld Problems

Iron Contamination CrackingIron particle dissolve into surface causing loss of :• Corrosion• Embrittlement (when sufficient iron)

Prevention• Avoid steel fabrication near titanium fabrication• Avoid airborne dust• Avoid tools used for both Fe and Ti• Scratch Brush just prior to welding• Use clean gloves for handling

Http://www.twi.co.uk/bestprac/jobknol/jk24.html

General Weld Problems

Hydrogen Embrittlement• Precipitation of titanium hydrides

• Hydrogen > 200ppm can cause problems• Use low moisture shielding gas

AWS Welding Handbook

Example of Hydride Formation in Resistance Seam Weld

% H

ydri

de

Co

atin

g

Days

100

0500

2 days– no hydrides

43 days-at toe

227 days-1/2 around

450 days-Full around

25 ppm Hydrogen

2056 ppm hydrogenH

Toosky, R. “Evaluation of Titanium Hydride Formation in Resistance Seam Welded Dissimilar Titanium Joints”, Boeing Co., 1998

Toosky, R. “Evaluation of Titanium Hydride Formation in Resistance Seam Welded Dissimilar Titanium Joints”, Boeing Co., 1998

Seam Weld Start, Nugget Pull-Out

General Weld Problems

Porosity• H & O partitioned between solidifying dendrites• Micropores form when diatomic gas overcomes head pressure

AWS Welding Handbook

• Mircopores coalesce to form macropores• To keep O & H out of weld pool, clean & degrease

AWS Welding Handbook

General Weld Problems

Ductility Dip• Alpha-beta alloys containing Nb rather than V more susceptible• Alpha films at prior beta boundaries more susceptible

Niobium Containing

• Material in HAZ with these peak Temp can experience Dip•Cracks are intergranular• Believed to be caused by volumetric difference during transformation

Resistance Spot Welding• Surface Cleaning - Mechanical or Chemical {HF-HNO3}

• To lower surface resistance to below 50 microhms• To keep Titanium Oxide out of weld metal - embrittlement

• Handle with gloves• Store in low-humidity less than 48 hours• Higher resistivity for Alloys than steel = lower current

• Pure Ti can use SS Schedule•Class 2 Electrodes with 3 in radius spherical face• Gas shielding not required for spot welds

AWS Welding Handbook

Same Scheduleas SS for Pure Titanium, But Not For alloys

AWS Welding Handbook

Schedule for Two Typical Alloys

AWS Welding Handbook

Equiaxed Grains

Columnar Grains due to Effect of Alloys

Resistance Seam Welding

AWS Welding Handbook

• Same Cleaning procedures as spot welding• Class 2 Electrode Wheels with radius face = wheel radius• Inert shielding gas recommended

• Electrode Force and Welding Current slightly higher than for spot welds in the same material

Flash Welding Titanium

• Use machine capacities similar to that for steel• Gas Shielding not absolutely necessary but recommended

(Titanium Oxidizes rapidly)• Use fast flashing rates & shorter flashing times than steel

(minimize weld contamination)• Lower upset force than for steel• Flashing and upset distances slightly less than for steel

Flash Welding Has Been Done on Following:• Commercially Pure Ti• Ti-6Al-4V• Ti-8Al-1Mo-1V• Ti-5Al-2.5Sn

• Ti-6Al-2Sn-4Zr-2Mo• Ti-6Al-2Sn-4Zr-6Mo

Info from AWS and Titanium Industries Inc.

SomeTitanium Welding

Applications

Froes, FH, et al, “Non-Aerospace Applications of Titanium” Feb 1998, TMS

SAAB 340 Aero Engine Shroud constructed from commercially pure titanium using resistance, spot and TIG welding. http://www.swantec.com/industry.htm

Welding Battery Connectors for Implantable Medical Device

Electrodes

Shield Gas (Ar) Tube

Battery Case

Battery Case Top Feed Through Contacting Pin(glass sealed in case top)

Battery Tab

Weld Conditions1000 amps40 msec3.5 kgrams

Weld Combinations (inch)0.018 Ti tab to 0.035 Nb pin0.018 Ti tab to 0.020 Mo pin0.018 Ti tab to 0.020 Ta pin0.004 Ni tab to 0.020 Mo pin

Berkowitz, F, et al, “Implantable Medical Device With High Reliability Electrical Connection using Reactive Metals” US Patent 5,712,462 Jan 27, 1998

Resistance Weld Ti Alloy HoneycombStructures for High Speed Civil Transport

(Requirement 72,000 hours @ 350F)(Al-Li max use temp = 225F)

Weld

Heat Treat1675-1825 F

15 min – 4 hours

Weld between Ti 6242S face sheet and Beta 21S honeycomb low heat treatment – Low mechanical Boundary Present

Weld with heat treatment – good properties

El-Soudani, S, “Process for Enhancing the Bond Strength of Resistance Welded Joints Between Titanium Alloy Articles” US Patent 5,830,289 Nov 3, 1998