Spot Welding

Questions and Answers

Bare steel and galvanized steel of the same thickness run at very similar weld schedule settings. The electrodes are the same. They vary in the amount of current or time applied due to the coating of galvanize (zinc) on the surface. This coating reduces the contact resistance and makes the overall resistance in the joint lower. More current or time must be applied to compensate for the reduced total joint resistance to reach the necessary temperature to form a nugget. The heat is generated by Joules Law:

Joules Law

The galvanized coating (Zinc) coats and alloys with the copper electrodes. This can result in:
• Increased the rate of electrode mushrooming
• Electrode face sticking to the part
• More frequent dressing
• Increased electrode wear
• Reduced electrode life
• Increased expulsion at weld initiation

Electrodes welding bare steel can make many thousands of welds. Welding galvanized steel they may only last a few hundred up to a two thousand before dressing is required.

Most operations work out schedules that allow for dressing or electrode changes at breaks or shift changes. In these operations they use current steppers to increase the current a few amperes each weld to match the electrode face growth until the next dressing or change. Then they reset the control counter and begin again.

Some operations use robotic dressers that move in every so many welds between part changes to slightly kiss the electrode face and renew the face continually. Then the current remains constant.

The basic resistance welding equipment can remain the same for bare or coated steel. The settings must change to compensate for the zinc coating. All resistance welders have the ability to do this. Due to the galvanized coating, dressing frequency enters into the process and must be addressed. Manual, pneumatic or automated systems can be used. 

Dresser

                     HAND DRESSER

 

Many modern controls have the stepper functions to compensate for electrode face growth. This is used to increase the time between dressing.

A related article in this blog is:

WHAT IS THE BEST METHOD TO SPOT WELD 16 GAGE ELECTROGALVANIZED SHEET METAL?

 

References: AWS Standard C1.1 Recommended Practices for Resistance Welding
                    RWMA - Resistance Welding Manual 4th Edition
                    CMW Inc. Welding Product Catalog

Weld is the second of the three main resistance welding functions. They are squeeze, weld and hold. During Squeeze the proper pressure builds up prior to current initiation. Weld is the period when the current flows and the weld nugget is formed. The Hold period marks the end of weld current and the beginning of the weld cooling and solidification under pressure.

 Preheat Sequence Timing Diagram

The amplitude and time of current flow is critical to forming the proper size nugget. The weld control controls the weld function and is able to accurately repeat exactly the same process each weld cycle for each part.

Reference: RWMA – Resistance Welding Manual 4th Edition

The number and spacing of spot welds will depend upon many factors. The material being welded for example – steel or aluminum. What type of part is being welded – a hair dryer or an automobile? Is the weld a structural weld? Is it a considered a safety weld? Shunting current must be avoided between welds that are too close together.

Reference Article: IS THERE A MINIMUM SPACING BETWEEN SPOT WELDS?

These and many other factors are considered by design engineers when they specify the number and location of spot welds. A full discussion of this topic is beyond the scope of this blog.

Reference: RWMA Resistance Welding Manual 4th Edition

Hastelloy X is a high temperature Nickel bearing alloy frequently used in the aerospace industry. It is readily weldable by Gas Tungsten Arc Welding (GTAW), Gas Metal Arc Welding (GMAW), Shielded Metal Arc Welding ( SMAW), and Resistance Welding (RW).

 Hastelloy X

      HASTELLOY X - AIRCRAFT APPLICATION

The recommended material preparation for welding HASTELLOY x is the same as for all resistance welding. The weld area and adjacent areas should be cleaned of all oil, grease, cutting oils, debris, scale, paint, markers and all foreign matter. Any appropriate solvent may be employed. There is no published standard for cleaning this material.

Reference: Haynes International – Hastelloy X
                   RWMA – Resistance Welding Manual 4th Edition

Machine qualification, building and design are out of the scope of this blog.

For information on machine qualification consult a machine builder or distributor.

Reference: RWMA – Resistance Welding Manual 4th edition
                   AWS - J1.2, Guide to the Installation and Maintenance of Resistance Welding Machines

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