Transcript No Slide Title

Titanium
Resistance Welding
Lesson Objectives
When you finish this lesson
you will understand:
•
Keywords
Learning Activities
1. View Slides;
2. Read Notes,
3. Listen to lecture
4. Do on-line workbook
Donachie, Titanium,
ASM, 1988
Donachie, Titanium,
ASM, 1988
Unique Properties
• Light Weight
• Excellent Corrosion Resistance
• Strong tightly adherent oxide
• High strength-to-weight ratio
Melting
point
Boiling
Point
Coefficient
of
Expansion
Density
UTS
Ductility
Titanium
3035 F
1670 C
5900 F
3260 C
4.67  in/ in/ F
0.163lb / in 3
30 ksi Pure Ti
200 ksi Alloys
15 – 25%
Applications
• Chemical Industry
• Aerospace Industry
• Marine Fields
• Medical Applications
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
Can Lead to Brittleness
Especially in welds (see below)
AWS Welding Handbook
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 Cracking
Iron 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
25 ppm Hydrogen
2056 ppm hydrogen
H
% Hydride Coating
100
450 days-Full around
2 days– no hydrides
227 days-1/2 around
43 days-at toe
0
Days
500
Toosky, R. “Evaluation of Titanium
Hydride Formation in Resistance Seam
Welded Dissimilar Titanium Joints”,
Boeing Co., 1998
Seam Weld Start, Nugget Pull-Out
Toosky, R. “Evaluation of Titanium Hydride Formation in Resistance
Seam Welded Dissimilar Titanium Joints”, Boeing Co., 1998
General Weld Problems
Porosity
• H & O partitioned between solidifying dendrites
• Micropores form when diatomic gas overcomes head pressure
• 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
AWS Welding Handbook
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
Same Schedule
as SS for Pure
Titanium, But
Not For alloys
• 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
Schedule for Two Typical Alloys
AWS Welding Handbook
Equiaxed Grains
Columnar Grains due to
Effect of Alloys
AWS Welding Handbook
Resistance Seam Welding
• 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
AWS Welding Handbook
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-2Sn-4Zr-2Mo
• Ti-6Al-4V
• Ti-6Al-2Sn-4Zr-6Mo
• Ti-8Al-1Mo-1V
• Ti-5Al-2.5Sn
Info from AWS and Titanium Industries Inc.
Some
Titanium
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
Shield Gas (Ar) Tube
Electrodes
Battery Case Top
Weld Conditions
1000 amps
40 msec
3.5 kgrams
Feed Through Contacting Pin
(glass sealed in case top)
Battery Case
Battery Tab
Weld Combinations (inch)
0.018 Ti tab to 0.035 Nb pin
0.018 Ti tab to 0.020 Mo pin
0.018 Ti tab to 0.020 Ta pin
0.004 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 Honeycomb
Structures for High Speed Civil Transport
(Requirement 72,000 hours @ 350F)
(Al-Li max use temp = 225F)
Weld
Heat Treat
1675-1825 F
15 min – 4 hours
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
Weld between Ti 6242S face
sheet and Beta 21S honeycomb
low heat treatment – Low
mechanical Boundary Present
Weld with heat treatment –
good properties