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Perform tack welding

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&#;Tack welding involves welding two or more metal pieces together by merely applying pressure and heat to the area to be welded. Tack welding joins the two pieces of metal by using electrodes to send electrical current through the work pieces. The parts are locally heated.

What Is Tack Welding?

&#;After items to be welded together have been positioned as required, generally by clamping them on suitable fixtures, tack welds are used as a temporary means to hold the components in the proper location, alignment, and distance apart, until final welding can be completed.

&#;In general, tack welding is performed by the same process that is used for the final weld. For example, aluminum-alloy assemblies to be joined by friction stir welding are tack-welded by the same process using a small tool developed for this purpose. Or electron beam tack welds, created with reduced power, are used to supplement or replace fixturing and to maintain the correct shape and dimensions during final electron beam welding.

Why are Tack Welds Important?

The temporary nature of tack welds may give the false impression that the quality of these auxiliary joining aids is not as important as that of final weld and that this operation doesn't have to be properly programmed, performed, and inspected. This is not true.

Tack welding is real welding, even if the welds are deposited in separate short beads. It performs the following functions:

&#;Holds the assembled components in place and establishes their mutual location

&#;Ensures their alignment

&#;Complements the function of a fixture, or permits its removal, if necessary

&#;Controls and contrasts movement and distortion during welding

&#;Sets and maintains the joint gap

&#;Temporarily ensures the assembly's mechanical strength against its own weight if hoisted, moved, manipulated, or overturned

Defective Tack Welding Risks

&#;When hoisted, improperly tack welded assemblies can rupture, and portions or subassemblies can fall and endanger people or damage property.

&#;Tack welding must not interfere with or degrade the quality of final welding. It must not introduce weld defects, such as arc strikes, craters, cracks, hard spots, and slag left in place.

&#;Many steels used in fabricating pipes and vessels are sensitive to rapid cooling or quenching, especially following short tack welds, because of the limited heat input required to tack weld. 

Note: Higher heat input slows the cooling rate, which minimizes the occurrence of hard and brittle microstructures.

Check gap alignment

&#;To ensure the satisfactory performance of a welded structure, the quality of the welds must be determined by adequate testing procedures. Therefore, they are proof tested under conditions that are the same or more severe than those encountered by the welded structures in the field.

&#;These tests reveal weak or defective sections that can be corrected before the materiel is released for use in the field. The tests also determine the proper welding design for ordnance equipment and forestall injury and inconvenience to personnel.

In most welds, quality is tested based on the function for which it is intended. If you are fixing a part on a machine, if the machine functions properly, then the weld is often considered correct. There are a few ways to tell if a weld is correct:

&#;Distribution: Weld material is distributed equally between the two materials that were joined.

&#;Waste: The weld is free of waste materials such as slag. The slag after cooling should peel away from the project. It should be removed easily. In Mig welding, any residue from the shielding gas should also be removed with little problem. TIG , being the cleanest process, should also be waste free. In Tig, if you see waste, it usually means that the material being welded was not cleaned thoroughly.

Porosity: The weld surface should not have any irregularities or any porous holes (called porosity). Holes contribute to weakness. If you see holes it usually indicates that the base metal was dirty or had an oxide coating. If you are using 

 or Tig, porosity indicates that more shielding gas is needed when welding.  Porosity in aluminum welds is a key indicator of not using enough gas.

&#;Tightness: If the joint is not tight, this indicates a weld problem. In oxyacetylene welding, if using autogenous welding, where there is no filler material, the weld must be tight. Same for Tig autogenous welding. The gap is not as critical in other types of welds since any gap is filled in by the filler material. That said, gaps, in general, indicate a potential quality problem.

&#;Leak-Proof: If you are repairing an item that contains liquid, a leak is a sure-fire way (and obvious way) to see that there is a problem. Same for something that will contain a gas. One testing method is to use soap bubbles to check for problems (can be easily applied with a squirt bottle.

&#;Strength: Most welds need to demonstrate the required strength. One way to ensure proper strength is to start with a filler metal and electrode rating that is higher than your strength requirement.

Common Weld Faults

Incomplete Penetration

This term is used to describe the failure of the filler and base metal to fuse together at the root of the joint. Bridging occurs in groove welds when the deposited metal and base metal are not fused at the root of the joint. The frequent cause of incomplete penetration is a joint design which is not suitable for the welding process or the conditions of construction. When the groove is welded from one side only, incomplete penetration is likely to result under the following conditions.

&#;The root face dimension is too big even though the root opening is adequate.

&#;The root opening is too small.

&#;The included angle of a V-groove is too small.

&#;The electrode is too large.

&#;The rate of travel is too high.

&#;The welding current is too low.

Lack of Fusion

Lack of fusion is the failure of a welding process to fuse together layers of weld metal or weld metal and base metal. The weld metal just rolls over the plate surfaces. This is generally referred to as overlap. Lack of fusion is caused by the following conditions:

&#;Failure to raise to the melting point the temperature of the base metal or the previously deposited weld metal.

&#;Improper fluxing, which fails to dissolve the oxide and other foreign material from the surfaces to which the deposited metal must fuse.

&#;Dirty plate surfaces.

&#;Improper electrode size or type.

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&#;Wrong current adjustment.

Undercutting

&#;Undercutting is the burning away of the base metal at the toe of the weld. Undercutting may be caused by the following conditions:

&#;Current adjustment that is too high.

&#;Arc gap that is too long.

&#;Failure to fill up the crater completely with weld metal.

Slag Inclusions

Slag inclusions are elongated or globular pockets of metallic oxides and other solids compounds. They produce porosity in the weld metal. In arc welding, slag inclusions are generally made up of electrode coating materials or fluxes. In multilayer welding operations, failure to remove the slag between the layers causes slag inclusions. Most slag inclusion can be prevented by:

&#;Preparing the groove and weld properly before each bead is deposited.

&#;Removing all slag.

&#;Making sure that the slag rises to the surface of the weld pool.

&#;Taking care to avoid leaving any contours which will be difficult to penetrate fully with the arc.

Porosity

Porosity is the presence of pockets that do not contain any solid material. They differ from slag inclusions in that the pockets contain gas rather than a solid.

&#; The gases forming the voids are derived form:

&#;Gas released by cooling weld because of its reduced solubility temperature drops.

&#;Gases formed by the chemical reactions in the weld.

&#;Porosity is best prevented by avoiding:

&#;Overheating and undercutting of the weld metal.

&#;Too high a current setting.

&#;Too long an arc.

Visual Inspection before Welding

&#;Check drawings

&#;Look at the weld position and how it corresponds to the specification. Watch the vertical direction of travel

&#;Check fillet welding symbols

&#;Does the procedure align with local codes and the weld specification

Weld Material Inspection

&#;Do the materials purchased match the specification for base metal size and type? Check electrode size, gas selection and grade.

&#;Check materials for defects. Look for contaminants such as rust, scale, mill, lamination etc.

&#;Are materials prepared for correct angles

Equipment Inspection

&#;Check for damage (cables, ground clamps, electrode holder).

&#;Check arc voltage

&#;Check amperage meter for range against specification

Visual Inspection during Welding

&#;Check electrodes for size, type and storage (low hydrogen electrodes are kept in a stabilizing oven)

&#;Watch root pass for susceptibility to cracking

&#;Inspect each weld pass. Look for undercut and required contour. Ensure the weld is cleaned properly between each pass.

&#;Check for craters that need to be filled

&#;Check weld sequence and size. Gauges are used to check size.

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