DISTORTION IN WELDING

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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 

  1. The thickness of the joint is lower
  2. Process used has high heat input and  sequence followed was not balanced  against neutral axis 
  3. Fixture was not used or was not  designed based to counter expected  distortion 
  4. 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