Hints for the Welder


Aluminium

Aluminium and it’s alloys, because of their lightness, corrosion resistance and strength, are finding increasing use in chemical plant structural work. They are made in wrought and cast iron forms. The alloys may be:

  1. Heat-treatable (containing small amounts of silicon, copper, magnesium, chromium and zinc) and obtain their strength by quenching and age-hardening
  2. Non-heat-treatable (containing mainly manganese and magnesium) and depend on cold-working for extra strength)

Welding of Aluminum

Aluminium is very different to steel in its properties and weldability because:

  1. It has a melting point of 660ºC (800ºC lower than steel) but requires as much heat per pound to melt it
  2. It has a thermal conductivity five times that of steel, hence heat loss is rapid, making preheating necessary
  3. It expands twice as much as steel for a given temperature increase, with greater danger of distortion
  4. It forms a tough, adherent oxide film on its surface which prevents globules of molten weld from “wetting” the plate
  5. It absorbs hydrogen readily when molten, but rejects it on solidification, creating a danger of porosity

Welding tips:

  1. When welding all but very thin sections, use a preheat to ensure proper fusion of weld with the base metal, and use copper backing if necessary.
  2. Allow for high rate of expansion when setting up jobs. If possible, use jigs to prevent distortion and employ frequent tack welds.
  3. Design joints so that the weld has the least possible restraint placed on it to avoid hot-cracking. Butt welds are generally stronger than fillet welds, because of more uniform stress distribution. They are also better than fillet welds in chemical plants because they are easier to clean and less likely to trap c orrosive slag.
  4. Keep all aluminium type electrodes in a warm, dry place, and dry at 150ºC for half an hour before use.
  5. Clean the surface of the joints with stainless steel wire brush just before welding.
  6. After welding, the joints must be thoroughly cleaned with a brush and hot water to remove slag.

Copper

Copper and its alloys, the bronzes and brasses, are in most cases weldable with arc.

  1. Copper: may be "deoxidised" or "tough pitch" copper. "Deoxidised" copper is welding quality. "Tough pitch" copper is not welded satisfactorily with the arc, due to gross porosity forming in the weld junction.
  2. Bronzes: plain bronzes are alloys of copper and tin. Aluminium bronzes contain up to 11% aluminium, which gives high tensile strengths and excellent corrosion resistance.
  3. Brasses: alloys of copper and zinc, with other alloys added in special cases.

Welding of Copper and Alloy

The most important factor is the high rate of conductivity of copper, making a preheat of heavy sections necessary to give proper fusion of weld and parent metal. It also has a high coefficient of expansion; about 35% greater than mild steel, for which allowance must be made in setting up.

Hints for the Welder

  1. Preheat to give good fusion
  2. Insulate to prevent loss of heat
  3. When building up parts such as bronze bearings, cleanse first with petrol to remove oil, dry and heat to drive oil from cracks

Recomendations:

  • Pacweld 451: Copper - Bronzes - Brasses
  • Pacweld 321: Aluminium

Causes of Wear and How to Over Come Them

When selecting filler materials for hardfacing, it is important to consider not only the alloys but the complete interrelationships of the wear problem. These interrelationships are illustrated most clearly with the aid of tribology (the study of friction and wear).

Wear System

  1. Basic-body: component which is to be hard-faced usually metal or alloy
  2. Counter-body:
    • solid: metal, alloy, mineral
    • liquid substances
    • gases and vapours (there are no intermediate materials)
  3. Intermediate materials:
    • solid: mineral, e.g. dirt in a bearing, plastics between worm and cylinder
    • liquid: lubricating oils
    • gaseous: vapours, dust, gases (counter-body may be missing)
  4. Movement: flowing - sliding - rolling - striking - jolting, alternatively fast and slow
  5. Load (pressure): Low - medium - high, constant - intermittent - rising - falling - in bursts

An optimum solution to the problem is impossible to obtain without the maximum possible information about all the facts and influences; it must be borne in mind here that in the actual practical situation wear problems are more complex.

When we remember for instance, that a piece of rock never has an identical composition or hardness and that, as well as the known factors, increased, constant or varying temperatures may occur, it will be clear that there are numerous ways in which PACWELD welding filler materials can be used to combat wear in industrial and engineering applications.