Common defects of aluminum and aluminum alloy MIG welding joints include poor weld seam formation, cracks, porosity, burn-through, incomplete fusion, lack of penetration, slag inclusion, etc.
I. Poor weld seam formation
Poor weld seam formation is mainly manifested as uneven ripples, non-bright surfaces, bent and uneven width, too many joints, center protrusion with flat or concave sides, overflowing, etc.
1. Causes:
Improper selection of welding specifications;
Incorrect angle of the welding gun;
Lack of proficiency in operation;
Too large diameter of the conductive nozzle;
The welding arc is not strictly aligned with the center of the groove;
Moisture content in the welding wire, workpiece, and shielding gas.
2. Prevention measures:
Repeatedly adjust and select appropriate welding specifications;
Maintain a suitable angle of the welding gun;
Strengthen welding skills training;
Select an appropriate diameter of the conductive nozzle;
Strive to align the welding arc strictly with the center of the groove;
Carefully clean the welding wire and workpiece before welding and ensure the purity of the shielding gas.
II. Cracks
Cracks in aluminum and aluminum alloy welds are generated during the crystallization process of weld metal, which is called hot crack or solidification crack. Its forms include longitudinal cracks, transverse cracks (often extending to the base metal), root cracks, arc crater cracks, etc.
Cracks can reduce the structural strength and even cause sudden failure of the entire structure, which is completely unacceptable.
1. Causes:
The depth-to-width ratio of the weld gap is too large;
The arc crater at the end of the weld cools too quickly;
The composition of the welding wire does not match the base material;
Incorrect operation techniques.
2. Prevention measures:
Properly increase the arc voltage or reduce the welding current to widen the weld and reduce the penetration;
Properly fill the arc crater and use attenuation measures to reduce the cooling rate;
Ensure that the welding wire matches the base material properly;
Select appropriate welding parameters and welding sequence, appropriately increase welding speed, and adopt preheating measures if necessary.
III. Porosity
Porosity is the most common defect in aluminum and aluminum alloy MIG welding joints. It is difficult to completely remove porosity from the weld, so the goal is to minimize its content.
According to its type, the main types of porosity in aluminum welds are surface porosity, dispersed porosity, locally concentrated porosity, single large porosity, root chain-like porosity, columnar porosity, etc.
Porosity not only reduces the compactness of the weld seam and the bearing area of the joint but also reduces the strength and ductility of the joint, especially the decrease in cold bending angle and impact toughness, which must be prevented.
1. Causes:
Poor gas protection, impure shielding gas;
Contaminated welding wire or workpiece;
The absolute humidity in the atmosphere is too high;
Unstable arc, long arc length;
Excessive extension length of the welding wire, too far distance between nozzle and workpiece;
Improper selection of welding wire diameter and groove form;
Too many arcs repeated at the same location.
2. Prevention measures:
Ensure the quality of the gas, appropriately increase the flow rate of the protective gas to eliminate all air in the welding area, eliminate splashes at the gas nozzle, make the protective gas flow evenly, and take measures to prevent the airflow in the welding area.
To prevent air from entering the welding area, if the flow rate of the protective gas is too high, it should be appropriately reduced;
Before welding, carefully clean the oil, dirt, rust, scale, and oxide film on the surface of the welding wire and workpiece and use welding wire with higher deoxidizer content;
Reasonably choose the welding site;
Appropriately reduce the arc length;
Maintain a reasonable distance range between the nozzle and the workpiece;
Choose a larger welding wire as much as possible while increasing the thickness of the blunt edge of the workpiece groove. On the one hand, it allows the use of higher welding currents and reduces the proportion of welding wire in the weld metal, which is effective in reducing porosity;
Try not to repeat the arc at the same location. When repeating the arc, the grinding or scraping should be carried out at the arc starting point.
Once a weld seam has been started, it should be made longer as much as possible, arc breaking should not be done randomly, and there should be a certain overlap area for the weld seam at the joint.
IV. Burn-through
1. Causes:
Excessive heat input;
Improper groove processing, too large assembly clearance of the weldment;
The spacing between spot-welded points is too large, and a large amount of deformation is generated during the welding process; incorrect operation posture.
2. Prevention measures:
Appropriately reduce the welding current and arc voltage and increase the welding speed;
Increase the size of the blunt edge and decrease the root gap;
Appropriately reduce the distance between spot-welded points;
During the welding process, hold the welding gun in the correct posture and be proficient in operation.
V. Incomplete fusion
1. Causes:
Welding speed is too fast, arc length is too long;
Improper groove processing, too small assembly clearance;
Low welding skills, improper control of operation posture;
Small welding specification;
Unstable welding current.
2. Prevention measures:
Appropriately slow down the welding speed and press down the arc;
Appropriately reduce the blunt edge or increase the assembly clearance of the root;
Ensure that the welding gun angle can obtain the maximum penetration depth during welding, keep the arc always at the forefront of the welding pool, and maintain the correct posture;
Increase the welding current and arc voltage to ensure sufficient heat input of the base material;
Increase the voltage stabilization device or avoid peak hours of electricity use.
VI. Lack of fusion
1. Causes:
Oxide film or rust on the welding site is not completely removed before welding;
Insufficient heat input;
Improper welding techniques.
2. Prevention measures:
Carefully clean the surface of the welding area before welding;
Increase the welding current and arc voltage, reduce the welding speed appropriately;
Use a weaving technique during welding, pause momentarily on the groove surface, and keep the welding wire at the forefront of the weld pool to improve the welding skills.
VII. Slag inclusion
1. Causes:
Incomplete cleaning before welding;
Excessive welding current, causing the partial melting of the gas nozzle to mix with the molten pool and form slag inclusion;
High welding speed.
2. Prevention measures:
Strengthen the cleaning work before welding. For multi-pass welding joints, the weld seam should be cleaned after each pass;
Reduce the welding current appropriately while ensuring penetration. Do not press the conducting nozzle too low when using high welding current;
Reduce the welding speed appropriately, use welding wire with higher deoxidizer content, and increase the arc voltage.
