magnesium. resistance welding lesson objectives when you finish this lesson you will understand:...
TRANSCRIPT
Magnesium
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
•{hcp} crystal structure •High strength to weight ratio• Formability increases with temp• Magnesium oxide forms rapidly when heated in air• Corrosion resistance similar to aluminum• Galvanic corrosion can be severe• Some alloys susceptible to SCC• Melts at 1200F (similar to Al)• Boils at 2025F (low)• Coefficient of thermal expansion 15x10-6 (same as Al)• Electrical Resistivity about 1.7 times Al• Latent heat of fusion 2/3 Al, 1/5 Steel• Notch sensitive in fatigue• Alloys supplied in castings, forging, sheet, plate,extrusions
AWS Welding Handbook
Resistance Welding Manual, RWMA
Sheet Supplied with:• Oiled Surface
• Chrome Coating
Must be removed before Resistance Welding
Clean with:Chromic Acid & small amount of MgSO4 to Avoid pitting RWMA
AWS
Limit time after cleaning to 24 hours
or
Weld Same as Aluminum• Class 1 or 2 Electrodes• Higher Current Than Steel• Shorter time Than Steel• Lighter Force Than Steel (because softer)• Rapid electrode follow-up to Maintain pressure• Dual force not required but often used
Spot Weld
• High Thermal & Electrical Conductivity
Machine Types Used• AC• DC• Frequency Converter• Capacitor Discharge
Mg & Mg Alloys, ASM 1999
Resistance Welding Manual, RWMA
Single Phase AC
Mg & Mg Alloys, ASM 1999
(second source)
Welding Handbook, AWS
When Dual Force Technique is Used, Timing is Important• Too Late - Temp too low to consolidate nugget• Too Soon - Nugget Size too small
Excessive indentation
Electrode Force• Insufficient
• Weld metal expulsion• Internal discontinuities• Surface burning• Excessive Electrode sticking
• Excessive•Indentation• Sheet separation, distortion• Unsymmetrical weld nuggets
Cracking @ Low ForceNo Cracking @ Higher Force
Effect of Electrode Force
Resistance Welder Manufacturers Association Bulletin # 22
NoCrack
Crack
3.3 3.6 4.6
Forge Delay (Cycles)
Weld Force1200
NoCrack
Crack
3.3 3.6 4.6
Forge Delay (Cycles)
Weld Force1600
Effect of Forge Force in Normally Cracked Region
NoCrack
Crack
3.3 3.6 4.6
Forge Delay (Cycles)
Weld Force1300
Forge Force Ratio > 1:1.5, 1:2, 1:3
Need Ratio 1:2 or greater applied before 4.6 cycle delayNeed Ratio 1:2 or greater applied before 4.6 cycle delay
Cra
ckC
rack
Resistance Welder Manufacturers Association Bulletin # 22
Welding Handbook, AWS
Resistance Welder Manufacturers Association Bulletin # 22
Effect of Post Heat in Sound Weld Region
Postheat• Softens Structure• Enlarges HAZ
Resistance Welder Manufacturers Association Bulletin # 22
Cra
ckC
rack
Effect of Post Heat and Simultaneous Forge Force in Crack Region
Note Beneficial Effect
Mg & Mg Alloys, ASM 1999
Electrodes
• Class 1 or 2• Domed Shaped (radius @ thickness - see next slide)• Face kept clean, electrode redress tool (280 grit abrasive)• Electrode life limited by transfer of Cu to part (sticking)
• Electrode Alloying• Cooling efficiency• Method of BM cleaning• Mg Alloy Composition• Welding Conditions
• Copper pick-up increases corrosion of part• Cu on part use 10% acetic acid - Turns black
Effect of Sheet Cleaning on Electrode Life
Seam WeldingSeam welds have been successfully made in M1A & AZ31B alloys.
Flash Welding• Techniques similar to Aluminum Flash Welding• High Current Density• Extremely rapid flashing & Upset rates• Upset current for 5 - 10 cycles after upset• Shielding atmosphere not usually required• Alloys AZ31B, AZ61A, HM31A