Materials in welding: mild steel stainless and aluminium

A weld is never just a matter of method and setting — it is just as much a matter of what material is on the workbench. Mild steel, stainless steel and aluminum behave very differently when heated, and a technique that produces a nice bead on one material can produce a defective bead or even a weak structure on another.

§Mild steel: the robust base material

Unalloyed and low-alloyed steel—often called 'black steel' in everyday speech—is the most common material in construction work. It is relatively forgiving to weld because it does not form a protective oxide film that must be broken through and because most common welding methods are developed precisely for this material. With thicker goods or higher alloy content there can be a risk of hardening and cracking in the heat-affected zone and here preheating and a controlled cooling rate become important tools to keep the material tough and avoid brittle cracked joints.

§Stainless steel: the chromium oxide film must be protected, not destroyed

Stainless steel's corrosion resistance rests on an invisible, self-repairing surface film of chromium oxide. The film forms automatically when chromium in the steel reacts with the oxygen in the air, and it is the reason the material does not rust like ordinary steel. The problem arises when welding heat comes into play: if the steel is heated close to the melting point, chromium burns off the surface, and the protective film weakens locally.

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§Aluminum: the persistent oxide layer

Aluminum presents its own challenge: the metal instantly forms an oxide layer in air, and that layer has a melting point of about 2,060 °C – almost three times higher than the aluminum metal itself melts at about 660 °C. Without removing the oxide layer, the welding wire or electrode cannot melt together with the base material, and the result becomes a bonding error.

With TIG welding of aluminium this is solved by using alternating current (AC) instead of direct current: the current's changing direction cleans away the oxide layer during welding so a clean melt can form. Aluminium also conducts heat much better than steel which means more power must be supplied to keep the melt liquid — but in turn the heat also spreads quickly to the rest of the workpiece which increases the risk of burning and deformation on thinner material.

• 01Mild steel: be careful of hardening and cracking with thicker material or high alloying – preheat if necessary
• 02Stainless steel: limit heat input, use low-carbon or niobium-stabilized filler and keep groove surfaces free from iron contamination
• 03Aluminum: remove the oxide layer mechanically or with AC-TIG, and compensate for the material's high thermal conductivity
• 04Never mix abrasive discs or brushes between material types — it transfers contamination from one material to another

§The material determines the choice not the reverse

The skilled welder always starts by identifying the material and its properties before choosing method, filler material and parameters. Knowing the weak points of the three base materials — hardening in mild steel, sensitisation in stainless steel and the oxide layer in aluminium — is the difference between a seam that looks nice today and a seam that lasts for decades.