As a material, Aluminum requires extra attention when welding. Defects such as porosity, cracking, and voids can occur more frequently compared to other metals. Lynn Welding is especially good at giving aluminum the attention it deserves; whether it’s 2-ply, 3-ply, or even welding through sealant.
Considerations during the resistance welding process for aerospace grade aluminum
During the seam and spot-welding processes, it is vital that the electrodes are cleaned and visually inspected throughout the operation. As you weld, the electrodes may become contaminated by material transfer. Aluminum will begin to deposit on the surface of the electrodes which can occur in as little as five spot welds, creating irregular intervals that require constant inspection. If the electrodes are contaminated, the probability of a defect occurring within the weld increases significantly.
Aluminum contaminated electrodes
Metallurgical sandpaper is often used to remove this material from the electrodes. Over time this will grind the electrode down into the cooling water jacket of the electrode. The thinning of the electrode surface will often cause it to deform, or even rupture in rare cases. Once the electrode is damaged or deformed, it no longer has direct contact with the surface of the material and will begin to create irregularly shaped spot welds, such as a donut shape consisting of one larger oval and one smaller oval.
When welding aluminum, it is important to clean and preserve the surface from all contamination. The primary contaminants are greases, oils, residues, and oxides. If the surface contaminants are left on the stock, the weld will contain defects, experience expulsion, or the stock may not even be welded through the contamination. To clean the surface from the first three contaminants, a simple wipe down with acetone or alcohol is required. The oxide layer must be either acid cleaned or stainless-steel wire-brushed off the surface of the stock. For acid cleaning, the parts must be welded within 24 hours if the part is extremely critical, or up to 144 hours depending on the safety classification of the weld. This cleaning process when combined with proper electrode maintenance provides a perfect resistance weld with no external or interior defects.
To inspect the cleanliness of the surface, a surface resistance analyzer must be used. The machine simply checks the resistance value between the two surfaces measured in ohms. Generally, the value must below 100µΩ to create an acceptable weld. The images below show the surface resistance value of an uncleaned test coupon, where the resistance is off the charts! The high resistance value is due to the aluminum oxide layer which begins to form directly after cleaning. A very high resistance value will not allow current to flow freely between the electrodes, creating defects or even a defective weld. After cleaning, the resistance is much lower, and the aluminum must be welded within a certain time interval as the oxide layer begins to reform as soon as the clean aluminum is exposed to oxygen.
Uncleaned vs Cleaned test coupons
Another issue arises when the material is not wiped down prior to welding. Surface contaminants such as oil transfer from touching the stock increase the resistivity of surfaces as seen below. The clean surface had been tested, and then retested after a finger had smudged and transferred oils and grease to the surface. That simple act had increased the surface resistance by a substantial amount! The impurities on the surface will cause deformities in the weld itself and may even cause expulsion of the weld material.
Surface Resistance Analyzer
How Surface Contamination Affects Resistivity
At the end of the day, aluminum requires more attention and preparation to create perfect welds. Here at Lynn Welding, we go through the precautions listed above no matter what the job, where others would cut corners.
Pietrasz, Matthew. "Special Considerations when Resistance Spot Welding Aerospace-Grade Aluminum" ,Lynn Welding Co,. Inc., 13 February 2020.
Stryjewski, Arkadiusz. "Special Considerations when Resistance Spot Welding Aerospace-Grade Aluminum" ,Lynn Welding Co,. Inc., 13 February 2020.