Clamp down - welding to avoid distortion

There's no 'silver bullet’ to solve all types of distortion, but these are a some steps that could help prevent costly mistakes.

It’s a universal truth in fabrication: parts are manufactured to strict tolerances and any discrepancies slow down production, eventually costing the manufacturer money.

When welding, metal is heated and this causes the metal to distort. There is no avoiding this. “When welding, distortion in metal is problematic for a variety of reasons, one of the most crucial is the fact that the weld might not be structurally sound,” said Bruce Clark, Director, Marketing and Export Sales for Lincoln Electric Canada.

“Welding distortion is caused by non-uniform heating and cooling in and near the weld, which produces residual stresses in the metal,” said Chris Conrardy, CTO & VP Technology and Innovation at EWI.

“The form and magnitude of the distortion depends on a number of factors, such as the component geometry, material type, weld design, welding process heat input, welding sequence, tooling, etc.”

Welding heats a very specific point of the metal surface, usually the edge, and this causes the material to expand unevenly. “The metal is restricted in the amount it can expand by the cooler metal further away from the weld. This causes the expanding metal to compress, which upon cooling makes the now compressed metal shrink pulling the weld into tension. This leaves the weld area distorted,” said Kodi Welch, welding engineer at Miller Industrial Systems Group.

Different metals will move in different ways when heated.“Each metal has a different coefficient of expansion and contraction based on heat input,” said Tom Wermert, Senior Brand Manager for Victor Technologies.

“What you will find is that aluminium transfers heat very quickly through the material where mild steel would have a medium transfer of heat and stainless steel the heat becomes very localized to the specific area depending on the amount of nickel and chrome in the product. Materials react and distort differently based on the amount of heat that is put in them.”

All material will distort in some way, even thick material.

“The type and magnitude of distortion depends on many factors, including thickness,” said Conrardy. “Very thin materials, such as sheet metal, suffer most from a form of distortion known as ‘buckling’, which is characterized by metastable ‘oil-canning’ and ‘waviness’. Angular distortion is more common for greater thicknesses, and is often most pronounced in materials of thickness from 5 to 20 mm. Thicker materials tend to have less distortion due to their greater inherent ability to resist the residual stresses that cause distortion.”

Simple avoidance:

The simplest way to avoid distortion in a weld is not to over-weld the product. “There is no single ‘silver bullet’ that solves all types of distortion,” said Conrardy. “The optimum solution for a particular application may involve combining multiple distortion control methods. In general, one of the simplest ways to minimize welding distortion is to minimize the total welding heat input. This is done by optimizing weld joint designs and procedures to avoiding over welding.

"For example, using an 8-mm fillet weld where a 5-mm weld would be sufficient more than doubles the weld volume and can dramatically impact distortion.”

Spending less time over the weld joint can also minimize distortion in the metal part. “Increase your speed and deposition of filler metal on the weld. Heat input and travel speed on a weld are inversely proportionate. If you were to doubleyour travel speed, you would actually only put in a fourth of the amount of heat into the product,” said Wermert.

Clamp it down: 

Proper clamping of the workpiece is crucial to creating a proper distortion free weld. “Tooling can have a significant effect on distortion,” said Conrardy. “Properly restraining a component during welding and cooling can resist and reduce certain types of distortion such as angular distortion and bowing.”

Proper clamping technique must be observed to eliminate the potential of distortion. Ensuring that the workpiece is evenly clamped is essential.

“You have to make sure that all your clamping is balanced so that you don’t have one piece of material that is able to move versus another piece that is held more tightly,” said Wermert. “So clamping across from each other or a balanced clamping really helps and will eliminate some distortion.”

Clamping a workpiece at the ends and not the middle can lead to distortion in the middle of the workpiece. For some crucial applications, water-cooled jigs can be deployed.

“When welding sheet-metal, a water-cooled jig will carry heat away from the welded components,” said Clark.

Improper clamping can have the opposite effect of increasing or exacerbating the distortion.

“Improper work holding methods can have negative consequences, by shifting the distortion to other portions of the structure which are not restrained, or increasing the likelihood of cracking in highly restrained areas,” said Conrardy.

Justin Durik, a welding engineer at Miller Industrial Systems Group said “clamping a part then welding on that part will put added stress into the material due to the shrinkage from welding,” while Lex Palmer, a welding engineer at Miller Industrial Systems Group says, “depending on the material, it can restrict and strain the materials microstructure which can cause cracking.”

Predicting distortion:

Knowing how the part will distort will help the welder avoid — or at least minimize — distortion.

“There are software packages that offer distortion predictions,” says Palmer. “It is often difficult to predict without previous testing and measuring. A welding procedure should be put into practice to ensure welding variables stay within pre-tested ranges.”

Since heat is the cause of the distortion knowing how much heat is going into the workpiece is important to know. “A simple calculation for heat input is (V*A*60/travel speed[ipm]). Reducing heat input into the weld joint will help reduce distortion,” said Durkin. “For simple component geometries (e.g., a single stiffener filet welded on a square plate), empirically derived equations can give estimates of expected distortion for different material types, thicknesses, and weld sizes,” said Conrardy.

“For more complex ‘real-world’ applications, computer simulations are often required to predict distortion. Computer simulation technology is rapidly evolving and accuracy is improving, however some physical testing is often required to validate and fine-tune the predictions.”

Welding distortion is something all welders have to contend with and in a large manufacturing environment it can be a significant cost when parts are not made to required tolerances and there is considerable amount of reject parts.

“Heat input must be controlled. Use welding processes and joint designs that allow for faster travel speeds and smaller weld sizes. Use the smallest bead size allowable that will still give the required strength. When able, use pulse equipment to help lower the average amperage at given travel speed as compared to conventional CV spray. Preheat if allowable. Use rigid fixturing.” said Welch.

“In some instances you cannot avoid distortion, so what you can do is pre-bend the parts so when you weld, the distortion will distort in the position where you want it to be,” said Wermert.

Another technique is to preset the parts in such a way that the metals will distort in such a way as to pull the completed part into the correct tolerances.

“Prebending, presetting or prespringing the parts to be welded, is a simple example of the use of opposing mechanical forces to counteract distortion due to welding,” according to Lincoln Electric.

“Another common practice for balancing shrinkage forces is to position identical weldments back-to-back, clamping them tightly together. The welds are completed on both assemblies and allowed to cool before the clamps are released. Prebending can be combined with this method by inserting wedges at suitable positions between the parts before clamping.

“By the use of proper tooling or fixturing, distortion can be greatly reduced. Also, reducing the overall heat input into the part will help reduce distortion. This could be anything from travelling faster to switching from a CV weld to a pulsed arc weld. Design of the part will come into play, because a bend in the material will resist weld distortion,” said Durkin.

“To minimize distortion, manufacturers should take a holistic view of the entire production sequence to assess potential distortion contributors,” said Conrardy. “The assessment should include the component and weld design details, construction sequence, the materials preparation methods (e.g., handling, cutting, weld joint preparation), fitting practice, tooling methods, and welding procedures.

"An approach which focuses on controlling distortion at each step in the production sequence will usually minimize the resultant welding distortion.”