308A Welding and Soldering for Sheet Metal 4 — Questions and Answers
Question 1: What is the AWS classification for a common MIG welding wire used on mild steel sheet metal?
- E7018
- ER70S-6 (Correct answer)
- E6010
- ER308L
Correct answer: ER70S-6
ER70S-6 is the most common MIG wire for mild steel. The 'ER' indicates electrode/rod, '70' indicates 70,000 PSI tensile strength, 'S' indicates solid wire, and '6' indicates the chemical composition with added deoxidizers.
Question 2: What is 'sweat soldering' and when is it used in sheet metal work?
- Soldering in hot weather conditions
- Pre-tinning both surfaces with solder, assembling them, then reheating to fuse the solder layers together (Correct answer)
- A type of brazing
- Soldering underwater
Correct answer: Pre-tinning both surfaces with solder, assembling them, then reheating to fuse the solder layers together
Sweat soldering involves pre-tinning (applying a thin coat of solder to) both mating surfaces, placing them together, then reheating until the solder reflows and bonds the surfaces. It is used for flat seams and patches.
Question 3: What polarity setting is typically used for MIG welding sheet metal?
- AC (alternating current)
- DCEP (Direct Current Electrode Positive / Reverse Polarity) (Correct answer)
- DCEN (Direct Current Electrode Negative / Straight Polarity)
- No polarity setting is needed
Correct answer: DCEP (Direct Current Electrode Positive / Reverse Polarity)
MIG welding uses DCEP (reverse polarity) where the electrode wire is positive. This provides deeper penetration, stable arc transfer, and proper cleaning action. DCEN would cause an unstable arc with MIG.
Question 4: What is the purpose of a 'heat sink' when welding thin sheet metal?
- To preheat the metal
- To absorb excess heat and prevent distortion or burn-through by drawing heat away from the weld zone (Correct answer)
- To store welding rods
- To hold the workpiece in position
Correct answer: To absorb excess heat and prevent distortion or burn-through by drawing heat away from the weld zone
Heat sinks (copper bars, aluminum blocks, or wet rags) placed near or behind the weld joint absorb excess heat, reducing the risk of burn-through and minimizing warping on thin sheet metal.
Question 5: What is the difference between 'acid core' and 'rosin core' solder?
- They are the same product
- Acid core has an aggressive flux for sheet metal and steel; rosin core has a mild flux for electrical work and must not be used on sheet metal (Correct answer)
- Rosin core is stronger
- Acid core is lead-free
Correct answer: Acid core has an aggressive flux for sheet metal and steel; rosin core has a mild flux for electrical work and must not be used on sheet metal
Acid core solder contains an aggressive acid flux suitable for sheet metal work on steel and galvanized surfaces. Rosin core solder has a mild, non-corrosive flux designed for electrical connections and is not effective on sheet metal.
Question 6: What safety precautions are required when brazing with silver-bearing alloys?
- No special precautions are needed
- Adequate ventilation to avoid inhaling flux fumes and cadmium vapors, proper eye protection, and fire safety measures (Correct answer)
- Only a hard hat is required
- Brazing is completely safe without precautions
Correct answer: Adequate ventilation to avoid inhaling flux fumes and cadmium vapors, proper eye protection, and fire safety measures
Brazing requires ventilation for flux fumes and possible cadmium vapors (in some older alloys), proper eye protection (shade 3-5 lens), fire safety due to open flames, and skin protection from flux chemicals.
308A Welding and Soldering for Sheet Metal 4
What is the AWS classification for a common MIG welding wire used on mild steel sheet metal?