Magnesium

Resistance Welding Lesson Objectives When you finish this lesson you will understand: Learning Activities 1.View Slides; 2.Read Notes, 3.Listen to lecture 4.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 MgSO 4 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

Low Force No Higher Force Effect of Electrode Force Resistance Welder Manufacturers Association Bulletin # 22

Effect of Forge Force in Normally Cracked Region Forge Force Ratio > 1:1.5, 1:2, 1:3 Need Ratio 1:2 or greater applied before 4.6 cycle delay Crack 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 Crack 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 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 Welding Seam 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 cycles after upset Shielding atmosphere not usually required Alloys AZ31B, AZ61A, HM31A