Surendra M Vaidya
Advisor – Technical,
Godrej & Boyce Mfg. Co. Ltd.
Executive Vice President &
Business Head, Godrej Aerospace
Distortion in welding is a very common phenomenon and occurs due to expansion and contraction of weld seam during its gradual welding. Since in all welding process we weld drop by drop and area where we weld gets expanded due to heat of welding and rest of the area which is cooler or at ambient temp does not allow it to expand easily. Same happens when weld area is cooling down, it shrinks or contracts but surrounding area does not allow for free shrinkage of weld and contraction of HAZ.
As a result after welding we see preparations that will help to minimise distortion with least fixturing. Finally if you land up in distortion, do not worry, you may grind weld in opposite direction from where you
parts being joined and the assembly set up condition may deviate from its dimensions. These can be classified in following types.
- Longitudinal shrinkage
- Transverse shrinkage
- Angular distortion
- Bowing and dishing
- Buckling
- Twisting
Distortion depends upon type of material, thickness , process used in welding and welding sequence, fixture, residual stresses in the parts to be welded from earlier manufacturing processes.
The risk of distortion increases if
1. The coefficient of expansion is higher
- The thickness of the joint is lower
- Process used has high heat input and sequence followed was not balanced against neutral axis
- Fixture was not used or was not designed based to counter expected distortion
- Parts used for joining had undergone huge mechanical working ( Forming, bulk of material removed by machining) as a result had stresses in it
started and reweld as if we are doing repair of normalising of earlier weld. Do this immediately as a part of welding sequence. You may heat the weld area if metallurgically acceptable to around
Important point is while welding we are pouring metal in small drops and every drop when transferred from consumable to base material and heat of arc locally raises temp to and beyond melting point. Rest of the area is either yet to be welded or cooling down after welding. Also this is near the weld joint which may be just 10% of the overall assembly area.
Preheat job to 50-60deg C, good fixture that will allow expansion and contraction but will prevent
distortion shall be used, use high speeds of welding, use joint geometry and sequence that will weld around neutral axis and will balance distortion forces. This will require use of good manipulators, make use of latest IoT sensors like strain gauges, temp sensors and can decide sequence that will result the best control. Keep assembly on wheels, or jacks that will make easy movement, for thin sheets, use small welds like stich welding, use of endo thermic paste helps in absorption of unwanted or extra heat, or helps in cooling down the assembly faster.
Best way is to develop AI using data collected from similar joints and materials for future predication and decide on pre-set up joint or edge 600deg C on other side of distortion and cool the first side, this is called as heat correction and is a generally acceptable distortion control technique
