The welding of aluminium and its alloys

Cleaning and degreasing

Components for welding may be flat, preformed, sheared, sawn or milled to give the desired shape or to provide the weld preparation. Lubricants used during these processes must be removed if weld quality is to be main­tained. Degreasing may be accomplished by wiping, brushing, spraying or vapour degreasing with commercially available solvents. This is best done before any mechanical cleaning takes place. Mechanically cut edges may carry burrs along the cut edge that will trap dirt and grease. These burrs should therefore be removed from weld preparations by scraping with a draw tool - do not wire brush only as this may fail to remove them com­pletely. Scraping is also an excellent method for removing the oxide film. Stainless steel wire brushes, stainless steel wire wool or files may also be used to remove the oxide. As mentioned above in Section 4.6, under no circumstances should carbon steel, brass or copper brushes be used. Make sure that any cleaning tools are segregated and are used only on aluminium, otherwise cross-contamination can occur.

In certain cases, particularly when striving to achieve freedom from porosity, chemical cleaning or pickling may be required. A pickling shop is illustrated in Fig. 4.9 and a schedule of chemical cleaning treatments is given in Table 4.4.

Table 4.4 Chemical treatments for cleaning and oxide removal

Solution

Concentration

Temp (°C)

Procedure

Container material

Purpose

Nitric acid

50% water

18-24

Immerse 15min

Stainless steel

Removal of thin oxide film

50% HNO3

Rinse in cold water

for fusion welding

(technical

Rinse in hot water

grade)

Dry

Sodium hydroxide

5% NaOH in

70

Immerse 10 to 60 s

Mild steel

Removal of thick oxide film

followed by nitric

water

Rinse in cold water

Stainless steel

for all welding and

acid

brazing operations

Concentrated

18-24

Immerse 30 s

HNO3

Rinse in cold water

Rinse in hot water

Dry

Sulphuric - chromic

5 litres H2SO4

70-80

Dip for 2 to 3min

Antimonial lead lined

Removal of films and stains

acid

1.4 kg CrO3

Rinse in cold water

steel tank

from heat treating and

40 litres water

Rinse in hot water

oxide coatings

Dry

Phosphoric - chromic

1.98 litres of

95

Dip for 5-10min

Stainless steel

Removal of anodic coatings

acid

75% H3PO3

Rinse in cold water

0.65 kg of CrO3

Rinse in hot water

45 litres of water

Dry

Once degreased and cleaned the parent material should be welded within a short period of time; typically four hours would be regarded as reason­able. The component must be maintained in a clean condition during this time and this may require the item to be covered with polythene sheets or brown paper. If the item is left standing overnight the joints may require an additional cleaning operation so it is advisable to clean only those parts that can be welded within a four or five hour production period.

There are a couple of points concerning cleanliness that are worth men­tioning. If the chemical cleaning has been extremely good then it is possi­ble to leave the components for a longer period of time, perhaps overnight if the storage conditions are clean and dry. It has also been noticed that when the items are ‘super clean’ the TIG or MIG welder can experience arc starting and stability problems. The reason for this is not clear but is probably associated with the complete absence of any oxide. It is thought that a small amount of oxide assists in the formation of an active anode spot, resulting in a more stable arc.

The aluminium fabrication area ideally should be separated physically from other fabrication areas. For example, dust from activities such as grinding, settling on the surface will cause problems, particularly if this is from the grinding of steel items in adjacent bays. Aluminium and steel should never be welded in the same welding booth. It cannot be empha­sised too strongly how important attention to cleanliness is if sound, defect - free welds are to be made consistently.

The welding of aluminium and its alloys

Alloy designations: wrought products

Table A.4 BS EN BS EN Old BS/DTD Temperature (°C) numerical chemical number designation designation Liquidus Solidus IVIdUng range Al 99.99 1 660 660 0 AW-1080A Al 99.8 1A AW-1070A …

Principal alloy designations: cast products

Table A.3 BS EN numerical designation BS EN chemical designation Old BS number ANSI designation Temperature (°C) Liquidus Solidus Melting range Al 99.5 LM0 640 658 18 AC-46100 Al Si10Cu2Fe …

Physical, mechanical and chemical properties at 20°C

Table A.2 Property Aluminium Iron Nickel Copper Titanium Crystal structure FCC BCC FCC FCC HCP Density (gm/cm3) 2.7 7.85 8.9 8.93 4.5 Melting point (°C) 660 1536 1455 1083 1670 …

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