Spot Welding
Questions and Answers
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 - 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
Aluminum is a soft low resistance material. Additionally, it readily forms nonconductive oxides on its surface very quickly. Due to its low resistance high currents are required 30-40,000 amperes. As compared to steel at 10,000 amperes. It melts at low temperatures and has a high coefficient of expansion. Care must be taken to maintain force as the nugget shrinks (fast follow-up) (secondary forge cycle) or voids will be present.
To address sticking, it melts at low temperatures. It requires high currents. High currents through the electrode mean heat and could lead to electrode face pickup and sticking. This must be minimized. Use the most conductive electrode alloy RWMA Class 1. Use a radius face to concentrate the current in a small area for affect. This helps to break through any remaining surface oxides not removed in the recommended aluminum surface cleaning. Additionally the weld should be hot but short. Just a few cycles if AC and a few millisecs if MFDC. The point is to minimize the time of the weld and keep the electrode face cool. The cooler it is the less pick up there will be. Internal water cooling of the electrode is always a requirement.
What I have described is outlined in AWS standard:
AWS C1.1 Recommended Practices for Resistance Welding
The need is to deliver the proper current but keep the contact resistance low RC
Reference: AWS C1.1 Recommended Practices for Resistance Welding
RWMA Resistance Welding Manual 4th Edition
It is known that titanium resistance welding is sensitive to heat and exposure to air. Titanium expulsion can be very exciting if the process is not controlled.
EXCESSIVE EXPULSION
AWS C1.1 “Recommended Practices for Resistance Welding” states that titanium can be resistance welded. The molten nugget is sealed away from air by the surrounding solid material. Shielding gases are not necessary.
AWS C1.1 offers some surface cleaning options and a table with weld schedule data for spot welding Titanium Ally 6%Al-4%V.
Reference: AWS C1.1, Recommended Practices for Resistance Welding
RWMA - Resistance Welding Manual 4th Edition
It is always best to spot weld to a clean surface. This means that preferably all oils and oxides would not be present in the weld interface. Any that are present will lead to interference with and an alteration of the contact resistance. In some instances, the dirt may be expelled as flash. Sometimes it merely absorbs energy and slows or alters nugget formation. Clean is good. Silicon Carbide clean is not desirable. It is a nonconductor. If it gets imbedded in the surface, surface resistance goes up and the heat distribution will shift and maybe make it difficult to conduct current consistently into or through the part. Three surfaces in the diagram can be affected. Two contact resistances Rc and the interface resistance Rx.
One wants to remove oxides and nonconductors not imbed new ones in the surface and leave them behind like silicon carbide.
A similar article on this subject is:
HOW CAN SANDBLASING AFFECT THE SURFACE RESISTANCE DURING RESISTANCE WELDING?
References: RWMA - Resistance Welding Manual 4th Edition
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