Defects in aluminium welded joints

When MIG welding of aluminium and aluminium alloys, the common defects in welded joints are poorly formed welds, cracks, porosity, burn-through, failure to weld through, failure to fuse and slag.
I. Poor weld seam formation
Poor weld formation is mainly manifested in the weld ripple is not beautiful, and not bright; weld bending is not straight, the width is not the same, too many joints; weld centre protrusion, both sides flat or depressed; weld overflow, etc..

1. Causes
a. Improper choice of joint specification.

b. Incorrect angle of the welding gun.

c. Unskilled welders.

d. Conductive nozzle aperture is too large.

e. welding arc is not strictly aligned with the bevel centre.

f. Moisture in the welding wire, welding parts and shielding gas.

2. Preventive measures
a. Repeated debugging to select the appropriate welding specification.

b. Maintain a suitable inclination of the welding torch.

c. Strengthen the skills training of welders.

d. Select the appropriate conductive nozzle diameter.

e. Strive to make the welding arc and the bevel strictly centered.

f. Carefully clean the welding wire and welding parts before welding; ensure the purity of the shielding gas.

II. Cracking

Cracks in aluminium and aluminium alloy welds are produced during the crystallization of the weld metal and are called thermal cracks, also known as crystallization cracks. It takes the form of longitudinal cracks, transverse cracks (often extending to the base metal), as well as root cracks, arc pit cracks and so on. Cracking will reduce the strength of the structure or even cause sudden damage to the whole structure and is therefore totally unacceptable.

1. Causes
a. Excessive depth to width ratio of the weld gap.

b. Rapid cooling of the arc pit at the end of the weld.

c. Mismatch between the composition of the welding wire and the base material.

d. Incorrect operating technique.

2. Preventive measures
a. Properly increase the arc voltage or reduce the welding current to widen the weld channel and reduce the depth of melt.

b. Properly fill the arc pit and use attenuation measures to reduce the cooling rate.

c. Ensure that the welding wire is reasonably matched to the base material.

d. Select the appropriate welding parameters, welding sequence, increase the welding speed and take preheating measures if preheating is required.

C. Porosity

In aluminium and aluminium alloy MIG welding, porosity is one of the most common defects. To completely remove the pores in the weld is very difficult to do, only to minimize its content. The main types of porosity in aluminium welds are surface porosity, diffuse porosity, locally dense porosity, single large porosity, root chain porosity and columnar porosity. The pores will not only reduce the denseness of the weld, reduce the bearing area of the joint, and make the strength of the joint, plasticity, especially the cold bending angle and impact toughness is reduced more, must be prevented.

1. Causes
a. Poor gas protection, impure shielding gas.

b. contamination of welding wire, welding parts.

c. The absolute humidity in the atmosphere is too large.

d. Unstable arc, arc too long.

e. Excessive extension of the wire, excessive distance between the nozzle and the welded part.

f. Improper selection of wire diameter and bevel form.

g. Repeated arc in the same part, the number of joints is too much.

2. Preventive measures
a. ensure the quality of the gas, appropriately increase the flow of shielding gas to exclude all the air in the welding area, eliminate the spatter at the gas nozzle, make the shielding air flow evenly, the welding area should have measures to prevent air flow, prevent the air from invading the welding area, the flow of shielding gas is too large, the flow should be appropriately and properly reduced.

b. careful cleaning of the wire, weld surface oil, dirt, rust, scale and oxide film before welding, the use of high deoxidizer containing wire.

c. Rational choice of welding site.

d. when reducing the arc length.

e. Maintain a reasonable range of distance between the nozzle and the welded part.

f. try to choose a thicker wire and at the same time increase the thickness of the blunt edge of the workpiece bevel, which on the one hand allows allowing the use of high currents and also makes the proportion of wire in the weld metal decrease, which is effective in reducing the hole rate.

g. Try not to repeat the arc in the same part, repeat the arc to be polished or scraped to clean up the arc; a weld once the arc is started to try to weld longer, do not arbitrarily break the arc to reduce the amount of joints, in the joints need to have a certain amount of weld overlap area.

Four, burn through

1. Causes
a. Excessive heat input.

b. improper bevel processing, welding assembly gap is too large.

c. Point solid welding when the distance between the weld joints is too large, the welding process produces a large amount of deformation; incorrect operating posture.

2. Preventive measures
a. Properly reduce the welding current, arc voltage and increase the welding speed.

b. Increase the size of the blunt edge and reduce the root gap.

c. Properly reduce the spacing of welding joints when spot welding.

d. Welding process, hand holding the torch posture should be correct, the operation should be skilled.

V. Not welded through

1. Causes
a. welding speed is too fast, the arc is too long.

b. improper bevel processing, assembly gap is too small.

c. welding technology is low, improper control of operating posture.

d. welding specification is too small.

e. Unstable welding current.

2. Preventive measures
a. When slowing down the welding speed, depressing the arc.

b. Properly reduce the blunt edge or increase the gap between the parts.

c. Make the angle of the welding torch to ensure that the maximum depth of melt is obtained when welding, the arc is always maintained at the front of the welding pool, to have the correct posture.

d. Increase the welding current and arc voltage to ensure that the base material has sufficient heat input to obtain the amount.

e. Increase the voltage stabilisation power supply device or avoid opening the electricity peak.

Six, not fused

1. Causes
a. welding parts oxidation film or rust is not removed clean.

b. Insufficient heat input.

c. Improper welding operation techniques.

2. Preventive measures
a. careful cleaning of the surface to be welded before welding.

b. Improve the welding increase current, arc voltage, deceleration of small welding speed.

c. Welding to slightly adopt the way to transport the bar, there is a momentary pause in the bevel surface, the wire in the front of the molten pool, improve the welders technology.

VII. Slagging

1. Causes
a. Incomplete cleaning before welding.

b. welding current is too high, resulting in local melting of the electric nozzle mixed into the molten pool and the formation of slag.

c. Welding speed is too high.

2. preventive measures
a. Strengthen the cleaning work before welding, multi-channel welding, each weld after a similar weld cleaning.

b. In the case of ensuring penetration, properly reduce the welding current, high current welding, the conductive nozzle is not pressed too low.

c. Reduce the speed appropriately, use a wire containing a higher deoxidiser and increase the arc voltage.
Source:Pressure vessel people
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