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Manufacturing Processes > Joining processes > Manual Metal Arc Welding(MMA)

 

Manual Metal Arc Welding(MMA)

 

Process description

An electric arc is created between a consumable electrode and the workpiece at the joint line. The parent metal is melted and the weld created with the manual feed of the electrode along the weld and downwards as the electrode is being consumed. Simultaneously, a flux on the outside of the electrode melts covering the weld pool and generates a gas shielding it from the atmosphere and preventing oxidation.

Materials

Carbon, low alloy and stainless steels; nickel alloys and cast iron typically. Welding of non-ferrous metals is not recommended, but occasionally performed. Dissimilar metals are difficult to weld.

MMA

Process variations

  • Manual d.c. and a.c. sets. Only a few fluxes give stable operation with a.c.
  • Large selection of electrode materials with a variety of flux types for the welding of different metals and properties required. Core sizes are between Ø1.6 and Ø9.5mm and the electrode length is usually 460 mm.
  • Stud Arc Welding (SW): for welding pins and stud bolts to structures for subsequent fastening operations. Uses the pin or stud as a consumable electrode to join to the workpiece at one end. Portable semi-automatic or static automated equipment available.

Economic considerations

  • Weld rates up to 0.2 m/min.
  • Most flexible of all welding processes.
  • Manually performed typically, although some automation possible.
  • Can weld a variety of metals by simply changing the electrode.
  • More power required for a.c. welding than d.c. welding.
  • Suitable for site work. Welding can be performed up to 20m away from power supply.
  • Non-continuous process. Frequent changes of electrode are required.
  • Economical for low production runs. Can be used for one-offs.
  • Tooling costs low. Need for jigs and fixtures not as important as other methods and less accuracy required in setting up.
  • Equipment costs low.
  • Direct labor costs high. Skill level required is higher than MIG.
  • Finishing costs high relative to other welding processes. Slag produced at the weld area, which must be removed during runs and some grinding back of the weld, may be required. Weld spatter often covers the surface which may need cleaning.

Typical applications

  • Pressure vessels
  • Structural steelwork
  • Shipbuilding
  • Pipework
  • Machine frame fabrication
  • Maintenance

Design aspects

  • All levels of complexity possible.
  • Typical joint designs possible using MMA: butt, lap, fillet and edge in heavier sections (see Appendix B – Weld Joint Configurations).
  • Suitable for all welding positions.
  • Design joints using minimum amount of weld, i.e. intermittent runs and simple or straight contours wherever possible.
  • Balance the welds around the fabrication’s neutral axis.
  • Distortion can be reduced by designing symmetry in parts to be welded along weld lines.
  • Design parts to give access to the joint area, for vision, electrodes, filler rods, cleaning, etc. MMA excellent for welds inaccessible by other methods.
  • Sufficient edge distances should be designed for. Avoid welds meeting at end of runs.
  • Provision for the escape of gases and vapors in the design important.
  • The fabrication sequence should be examined with respect to the above.
  • Minimum thickness =1.5mm (6mm for cast iron).
  • Maximum sheet thickness, commonly for carbon, low alloy and stainless steels, nickel alloys and cast iron =200 mm.
  • Multiple weld runs required on sheet thicknesses ≥10 mm.
  • Unequal thicknesses difficult.

Quality issues

  • Moderate to high quality welds with moderate, but acceptable levels of distortion can be produced.
  • Quality and consistency of weld related to skill of welder to maintain correct arc length and burn-off rate.
  • Access for weld inspection important, e.g. NDT.
  • Joint edge and surface preparation important. Contaminates must be removed from the weld area to avoid porosity and inclusions after each pass.
  • A heat affected zone always present. Some stress relieving may be required for restoration of materials original physical properties.
  • Need for jigs and fixtures to keep joints rigid during welding and subsequent cooling to reduce distortion on large fabrications.
  • Backing strips can be used for avoiding excess penetration, but at added cost and increased setup times.
  • Can alter composition of weld by addition of alloying elements in the electrode. Addition of deoxidants in the flux minimizes carbon loss, which reduces weld strength.
  • Electrodes must be dry and free from oil and grease to prevent weld contamination.
  • Low hydrogen electrodes should be used when welding high carbon steels to reduce chance of hydrogen cracking.
  • The protective slag can help the weld to keep its shape during positional welding.
  • Weld ideally left to cool to room temperature before the slag removed.
  • When the electrode’s length reduced to approximately 50mm it should be replaced.
  • Welding current should be maintained during welding with a stable power supply.
  • Arc deflection can sometimes occur with d.c. supplies, especially in magnetized metals. The workpiece may need demagnetizing or the return cable repositioned.
  • Pre-heating of workpiece can reduce porosity and hydrogen cracking.
  • ‘Weldability’ of the material important and combines many of the basic properties that govern the ease with which a material can be welded and the quality of the finished weld, i.e. porosity and cracking. Material composition (alloying elements, grain structure and impurities) and physical properties (thermal conductivity, specific heat and thermal expansion) are some important attributes which determine weldability.
  • Surface finish of weld fair to good. Weld spatter often covers the surface.
  • Fabrication tolerances typically ±1 mm.

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