Resistance Welding of Aluminum and Aluminum Alloys

PED16 - Second Year – Material Technology Project

Contents: • Aluminum in industry • Resistance welding (Definition ,Principle ,Types)• Spot (RSW) in automotive industry • Conditions and rules must be applied in Spot welding of AL• Contact-Resistance Measurement• Equipment for RSW of aluminum• Electrodes for (RSW) aluminum • Problems connected with electrodes by Spot (RSW) • Arc-Cleaning Method of Spot Welding Aluminum• Defects in spot welds • Conclusion • References

Aluminum In Industry

• According to the newest studies, automotive industry tends to use light alloys like aluminum and magnesium alloys. Companies more often use aluminum as an alternative material for vehicle body.

• The most popular method of joining body sheets is resistance spot welding. Aluminum and its alloys series 5xxx and 6xxx can be connected by this technique.

Resistance Welding (Principle)

• Electrical heating combined with pressure

(RSW) Principle cont.

• The general heat generation formula in (RSW) is Heat=(I^2)*R*t*K Where: I = Electrical current (Ampair) R= Electrical resistance (Ohm)

t = Weld time (HZ) K= thermal expansion coefficient

Resistance welding • Advantages:

(1) Very little warpage of the metal (Sheet metal).

(2) The process can accurately controlled and the weld constantly uniform.

(3) Suitable for all automatic production.

(4) Clean and environmentally friend.

(5) Operator safety because of law voltage.

Resistance Welding

• Types (all types based on the same principle )

(1) Spot welding (Most common in industry)

RSW (Types) cont. (2) Stud Welding

(3) Seam Welding (RSEW)

RSW (Types) cont.

(4) Projection welding (RPW)

(5) Resistance butt welding (RBT)

RSW (Types) cont.

(5) Flash welding

Spot (RSW) in automotive industry • Resistance Spot Welding (RSW) is widely used

in the high-volume manufacture of sheet metal products. The automotive industry has relied on RSW as a principal joining method for many decades. A typical car contains between 2000 and 3000 spot welds.

• General Motors Yukon (SUV), with lightweight spot welded all-aluminum rear door (tailgate). Manufactured at 500,000 units per year.

Spot (RSW) in automotive industry Cont.

• Advantages of RSW in high-volume automotive manufacturing

(1) Inexpensive (per-weld cost ~€ 0.05)

(2) Rapid process (< 1 second per weld)

(3) Ease of automation (flexible process)

(4) No per-weld consumables (i.e. no issues of piece-cost, inventory, additional weight or recycling difficulties)

(5) Low training costs (in manual operations).

Conditions and rules must be applied in Spot welding of AL

• Resistance Spot Welding of aluminum and its alloys involves applying high power welding guns because welding current must be 2–3 times higher than in case of steel but the welding time is 1/3 weld time of steel.

• The main of that is 3 times higher than in case of steel its thermal and electrical conductivity. It means the electric parameters (current and voltage) must be controlled more precisely in narrower window of time.

Contact-Resistance Measurement• Because of the resistance of the oxide layers and its effect

on resistance welding, it is quite important to know the contact and total resistance of the sheet samples to be welded. This device uses low currents through the weld while measuring the voltage drop and thus calculating the weldment initial resistance

Equipment for RSW of aluminum

• AL Can be spot welded by all conventional RSW systems and power supplies, if they meet the following :

(1) High electrode force (4-8 KN)

(2) High welding current (20-45 KA)

(3) Minimal deflection in gun arms (<1 mm)

(4) Efficient water cooling of electrodes ,gun arms ,& all current-carrying components

(5) Low-friction force actuator, to maintain weld force as electrodes move during weld formation (i.e. good follow-

up).

Electrodes for (RSW) aluminum • (Because of Lower Resistance of Aluminum Than

Steel, More Current is Needed to Make the Weld)• The Main properties of ideal electrodes:

(1) Max electrical and thermal conductivity

(2) Max hardness or resistance to deformation

(3) Tip design which reduce wear

(4) Minimum tendency to alloy with the material being welded

Alloys used as a material in AL spot welding electrodes

• The copper alloy RWMA Class I (a copper-cadmium alloy) has found wide acceptance as electrode material for aluminum spot, roll-spot and seam welding.

• The RWMA Class II copper alloy, which is heat-treatable and available in cast form, has also been used by many fabricators for spot, roll-spot and seam welding. Because of superior hardness, this is especially true for the latter two welding processes which results in greater electrode life.

Properties of Some Copper-Alloy Electrodes for Aluminum

Arc-Cleaning Method of Spot Welding Al

• High frequency arc: Arc cleaning is a relatively new approach to the resistance welding aluminum alloys. This approach is the result of the sputtering action of positively charged argon ions which displaces the oxide from the negatively charged sheet surface, as shown in figure ,Although this technique has shown considerable success, additional equipment and power sources are required

Defects in spot welds• Unconformities which can appear in spot welds

cause the spot welds can have less strength and can lead into total destruction of manufacturing parts of cars bodies.

• The typical unconformities of spot welds are : (1) Cold weld (2) Small-diameter nugget

(3) bad shape of welding nugget

(4) cracks inside/around welding nugget

(5) deep indentation of welding electrodes in sheets.

Typical flows in spot welding joints

Conclusion

• the RSW process of AL and its alloys is finding a wide range of applications in joining technology. Its offered new perspectives for innovative working and opportunities for cost saving and high productivity particularly in vehicle manufacturing.

References • http://www.fabricator.com• http://link.springer.com• http://www.twi-global.com/• http://aws.org• http://www.welding-advisers.com