▶What is the difference between MIG and TIG welding, and when do I use each?
TIG (tungsten inert gas) uses a non-consumable tungsten electrode and a separate filler rod, allowing precise control but requiring two hands (one on torch, one feeding rod) and higher skill. TIG is ideal for thin materials, critical joints, and materials prone to cracking (stainless, aluminum, nickel alloys). MIG (metal inert gas) uses a wire feeder that continuously feeds filler metal, allowing one-handed operation and higher speed. MIG is faster and better for production work on mild steel and thicker sections. TIG produces higher-quality welds on thin or critical work; MIG is faster for volume.
▶What are tungsten electrode types and how do I choose?
Tungsten electrodes are classified by their alloy content: pure tungsten (green, low amperage, AC only), thoriated (red, most common, good heat resistance, slightly radioactive), lanthanated (gold, newer, less radioactive), and ceriated (orange, good starts). The type you choose depends on the base material, AC or DC power, and current range. For mild steel and stainless (DC power), thoriated or lanthanated are standard. For aluminum (AC power), pure tungsten is common. Higher amperage requires larger-diameter tungsten. Tungsten is expensive (use tungsten of 1/16" to 1/8" diameter) and should be re-sharpened in a tungsten sharpener or ground with a diamond wheel, never standard grinding wheel.
▶What is a back-purge and why is it necessary for critical welds?
A back-purge is inert gas applied to the back side of a weld to prevent oxidation on the underside. For critical applications (aerospace, pressure vessels, thick stainless), the underside of the weld can oxidize (turn dark blue or black) if not shielded, which weakens the joint. A back-purge kit connects a gas source to a dam or seal on the back, flowing inert gas (argon) during welding. For very critical welds, a final visual and metallurgical inspection confirms proper back-purge coverage. Without back-purge on critical work, the weld can fail inspection.
▶How do I TIG weld aluminum without cracking?
Aluminum is prone to hot cracking due to its thermal expansion and the difficulty of removing contaminants (aluminum oxide). Tips: (1) Use AC power (alternates polarity to break up the oxide film), not DC. (2) Preheat the metal to 200-300°F to reduce thermal stress. (3) Use a stainless steel brush (not carbon, which contaminates) to clean the surface immediately before welding. (4) Choose the right filler alloy: 5356 for structural (most aluminum alloys), 4043 for older or non-heat-treatable alloys. (5) Cool slowly; do not quench. (6) For thicker sections, use a larger electrode and lower travel speed to keep heat in. Aluminum welding is slower and trickier than steel but produces lighter, stronger joints.
▶What is porosity in TIG welds and how do I prevent it?
Porosity is gas bubbles trapped in the weld, caused by shielding gas contamination or moisture. Sources: (1) Dirty base metal or filler rod (oil, rust, or other contaminants), (2) Insufficient gas flow or wind blowing away shielding, (3) Moisture in the shielding gas (wet regulators or cylinders), (4) Using argon contaminated with oxygen or nitrogen. Prevention: Clean metal and filler wire with a stainless steel brush before welding. Monitor gas flow (typically 15-25 CFH for TIG). Dry out regulators and cylinders if moisture is suspected. Store filler rod in a dry location. Check shielding gas purity (99.99% argon minimum for critical work).
▶What is the difference between short circuit and spray transfer in MIG welding?
Short circuit (also called short arc or dip transfer) uses lower voltage and amperage, causing the wire to frequently touch the molten puddle and short-circuit, creating a distinctive clicking sound. It is slower but allows vertical and overhead welding with thin materials. Spray transfer uses higher voltage, and the wire doesn't touch the puddle; instead, the arc sprays tiny droplets of metal across the arc at high speed, resulting in a smooth, strong, fast weld. Spray requires flatwork or near-flat and causes more heat, which can distort thin materials. Choose short circuit for versatility and thin materials; spray for speed and thick sections.
▶How do I read and pass a TIG or MIG coupon test?
A TIG or MIG test coupon is a steel or aluminum plate that you weld in a specified position and joint type. The test is then sectioned (cut crosswise), polished, and examined for defects (porosity, undercut, lack of fusion, cracks). Mechanical testing (bend, tensile, hardness) may also be done. To pass: (1) Master the position (flat is easiest, overhead hardest). (2) Maintain consistent travel speed and arc length. (3) Keep the torch at the right angle (perpendicular to the joint). (4) Avoid excessive heat input (burn-through) or low heat (lack of fusion). (5) Practice extensively before testing; a failed coupon test is a setback.
