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Farm workshop: Top tips for welding odd metals

In our latest workshop edition, we look at the process behind welding odd metals.

 

Alex Heath reports on how to repair and fabricate with stainless steel, aluminium and cast iron in the farm workshop...

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Farm workshop: Top tips for welding odd metals

The farm workshop often sees all manner of mishaps being brought in to be fixed.

 

Now, thanks to to the versatility of modern welders, it is possible to tackle more complex tasks and metal types.


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To gain expert insight into the complex world of welding, we teamed up with Telford Group, Shropshire which has run a welding certification school for the past 10 years and been an independent welder dealer for 40 years.

 

Steve Woodhouse, managing director, and Richard Lamb, training and testing engineer, gave us some tips on welding less common metals – stainless steel, cast iron and aluminium.

Preparing stainless steel and aluminium

Preparing stainless steel and aluminium
  • The single most important point when cleaning stainless steel and aluminium ready for welding is to use a non-ferrous wire brush. A Scotch Brite pad or medium grit sand paper can also be used if easier to hand.
  • If a normal mild steel wire brush were to be used, small bits of the brush would come off and become embedded in the work piece, leading to rusting on the metals that are supposed to be rust free.
  • Having dirty and oxidised materials will lead to inclusions and porosity in the weld, meaning weak and potentially dangerous repairs will be made.
  • Grinding the edges so they fit flush is also important, as it will make the welding easier and potentially stronger.

Welding aluminium

Welding aluminium
  • Options for welding aluminium are extensive, however, using metal inert gas (MIG) or tungsten inert gas (TIG) are the best. In both cases the gas used should be pure argon, and wire or rods changed to match the grade of aluminium. TIG welding is slower than MIG, but offers the benefits of tidier welds, less heat distortion and easier control of the molten pool.
  • TIG welding aluminium requires a zirconium-tungsten tip, with the welder set up to weld with alternating current. The frequent changes in polarity cause cyclical heating of the metal which fuses and cleans the workpiece.
  • There are several different grades of aluminium. However, on farm the majority of aluminium will likely be a magnesium or silicon alloy. A quick test run should show what grade it is. Excess hissing and splatter will occur if the wrong wire is being used. On new aluminium the grade should be shown on the test certificate.
  • Because of its soft nature, aluminium wire will not feed through the pipe to the torch easily, without kinking. To combat this a liner can be put into the feed pipe. However, the best option is to use a spool gun which has the wire feeding from a roll onto the gun itself.
  • The action of welding aluminium is much more important than mild steel. You have to push the molten pool forward, otherwise a build-up of soot will occur on the weld. Also, the sowing action does not need to be so pronounced.

Welding stainless steel

  • Welding stainless can be done in the three main ways; using manual metal arc (MMA Arc), MIG or TIG. The preferred method would be utilising an external source of inert gas, as is done with MIG or TIG methods.
  • Both methods require the use of a high Argon gas mix, above 98 per cent to avoid oxidisation of the chrome, present in the base material. Other elements of the gas mix include helium, which allows more heat to be transferred to the weld, carbon dioxide for a stable arc, and hydrogen which helps remove any oxygen from the weld area.
  • TIG welding stainless is ideal as, although slower than MIG welding, it transmits less heat
    into the metal, reducing the chance of warping while at the same time leaving a neat finish.
  • TIG welding stainless requires
    a thorium-tungsten electrode which should be ground to a point to produce a very concentrated area of heat. The welder for all steels should be configured so that it is running direct current with straight polarity.
  • Welding stainless up to 2.5mm thick may not require the use of a rod to add extra material to the weld pool. However, any thickness over that and for angled joints, will require the use of a filler rod.

Preparing and welding cast iron

  • Cast iron can be treated in much the same way as mild steel in its preparation, using wire brushes to remove any dirt and paint. Where cast then differs is the need for the cracked area to be gouged out using a grinder. Depending on the depth and severity of the crack, a V-profile of 3-5mm deep should be cut along the crack.
  • Welding cast iron will never return the item back to its original strength. However, it can be useful for cosmetic purposes, or to keep a machine running while a replacement part is found.
  • Repairing cast iron can be achieved through MIG brazing. Imperative to the success of the weld is to not subject a single area to extreme changes in temperature. Before welding commences, heating up the area next to the crack with a gas axe will help lower the stresses welding will put on the cast.
  • Welding should be in short bursts, to reduce the chances of heating up the cast too much. A large bead is required too, so several passes are required.
  • A wire with a flux core should be used, and regular farm standard gas is fine. Mild steel wire can also be used; the welding material should be more malleable than the cast to allow for a certain amount of flex in the weld.
  • The cooling down process is also vital. If the cast cools too quick, it will shrink and crack, so covering it in sand or a fireproof blanket to delay the cooling process is advised.
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