Cause: Weld Time Long

Definition
For a typical weld, weld time is the amount of time welding current flows through the metal. Weld time is long when it exceeds applicable specifications.

Description
Since electrical power arrives at the weld control as alternating current, at a 60-cycle per second rate (50 in some areas outside the U.S.), weld time is usually measured in cycles. This has become a convenient measuring standard for duration of weld heat. One cycle equals 1/60th of a second, for a 60-cycle per second supply.

For Direct Current (DC) welders, weld time is usually measured in cycles as a convenience. In some instances, however, with mid- and high-frequency DC welding, milliseconds are often used to measure weld time. However with DC welders, there is some difference in programmed weld time and actual weld time as the Fig.1 below shows.

In a typical single-pulse weld the metal between the electrodes is heated from room temperature to welding temperature and rapidly cooled. The growth and shape of the weld nugget is governed by the heat/cool cycles of the weld schedule (see Fig. 2).

When weld time is too long, high indentation, excessive expulsion, and electrode sticking can occur. Worst case produces "burn-through" where metal between the electrodes is completely melted, producing a hole in the parts to be welded. The electrodes penetrate through this molten metal and may contact each other.

Detection

• Excessive indentation
• Expulsion/burn through
• Poor Class A appearance
• Sticking/stuck tips

Significance
Quality, Workplace Issues, Cost, Downtime, Maintenance, Throughput (cycle time; PPH), are all potentially affected by this condition. Special considerations are noted below:

Downtime: Stuck tips, resulting from long weld time, is a maintenance issue that will also affect downtime.

Subordinate Causes

• Wrong weld schedule
• Weld control malfunction