For welding non-ferrous metals, the preferable electric supply is DC. There are several reasons why DC is preferred over AC for welding non-ferrous metals. Let's discuss them in detail:

1. Polarity Control:
DC welding offers the advantage of polarity control, which allows the welder to choose between electrode positive (EP) and electrode negative (EN) polarity. EP polarity is commonly used for non-ferrous metals as it provides deeper penetration, higher welding speed, and better control of the heat input. The ability to control the polarity is crucial for achieving high-quality welds in non-ferrous metals.

2. Heat Distribution:
DC welding provides better heat distribution compared to AC welding. AC current constantly changes direction, leading to rapid heat fluctuations during the welding process. This can result in uneven heat distribution and inconsistent weld quality. On the other hand, DC current maintains a constant direction, resulting in more stable and controlled heat distribution. This is particularly important for non-ferrous metals, which have different thermal conductivity compared to ferrous metals.

3. Oxide Removal:
Non-ferrous metals tend to form oxides on their surface, which can hinder the welding process and negatively affect the weld quality. DC welding, specifically with EP polarity, helps in removing these oxides more effectively. The EP polarity generates a higher concentration of heat at the electrode, which helps in cleaning the oxide layer and promoting better fusion between the base metal and the filler material.

4. Arc Stability:
DC welding offers better arc stability compared to AC welding. The stable arc is crucial for achieving consistent and high-quality welds in non-ferrous metals. AC welding, due to its constantly changing direction, can lead to arc instability, resulting in spattering, uneven penetration, and difficulty in controlling the weld pool. DC welding provides a more stable arc, allowing for better control and improved weld quality.

In conclusion, DC is the preferable electric supply for welding non-ferrous metals due to its ability to control polarity, better heat distribution, effective oxide removal, and improved arc stability. These factors contribute to achieving high-quality welds in non-ferrous metals, ensuring strength, durability, and overall weld integrity.