Introduction
Incomplete
Penetration
Lack of
Longitudinal
Undercutting
Fusion
Porosity
Cracking

 

 

 

Variations-
Metal
Transfer
Equipment
Power
Supply
Shielding
Gases
Wire
Electrodes
Safety
Welding
Techniques
Welding
Conditions
Economics
Weld
Defects
Mig Spot
Welding
Tables

 

9 LONGITUDINAL CRACKING Longitudinal or centerline cracking, of the weld bead is not often encountered in mig welding. However, that which does occur can be one of two types: hot cracks and cold cracks. Typical hot cracks are shown in Figure 10-5. Hot cracks are those that occur while the weld bead is between the liquidus (melting) and solidus (solidifying) temperatures. In this temperature range the weld bead is ”mushy”. Hot cracks usually result from the use of an incorrect wire electrode (particularly in aluminum and stainless steel alloys). The chemistry of the base plate can also promote this defect (an example would be any high carbon stainless steel casting). Any combination of the joint design, welding conditions and welding techniques that results in a weld bead with an excessively concave surface can promote cracking. One form of this defect which may often be encountered, particularly with any 5000 series aluminum, is called a crater crack. These are small cracks which appear at the end of the weld where the arc has been broken. Although small, these cracks are troublesome since they can propagate into the weld bead. A crater crack is shown in Figure 10-6. The major reason for this defect is the incorrect technique for ending the weld. To properly end a weld, the crater should be filled. This is done by reversing the arc travel direction before breaking the arc. This technique is depicted in Figure 10-7. In addition, if the welding control is designed to supply gas for a short time after the arc is broken, the crater should be shielded until it is completely solidified.
Figure 10-5 - Example of Longitudinal
Cracking
Figure 10-6 - Example of Crater Cracking