Aluminum Alloys MIG Welding Procedure Instructions

1. Base-metal preparation:

To weld aluminum alloys, operators must take care to clean the base material and remove any aluminum oxide and hydrocarbon contamination from oils or cutting solvents or NC machines coolants. Aluminum oxide on the surface of the material melts at about 2035^o C while the base-material aluminum underneath will melt at 650^o C. Therefore, leaving any oxide on the surface of the base material will inhibit penetration of the filler metal into the work piece. To remove aluminum oxides, use a stainless-steel bristle wire brush (and/or grinding) or solvents and etching solutions. If grinding is used for cleaning/preparing weld areas it should be paid attention to the brand used since some grinding wheels use an organic binder that leaves a residue behind, that will possibly cause weld porosity. Thus, it is recommended to test the particular brand of grinding wheels before actual use, in a welding test work piece

When using a stainless-steel brush, brush only in one direction. Take care to not brush too roughly: rough brushing can further embed the oxides in the work piece. Also, use the brush only on aluminum work-don’t clean aluminum with a brush that’s been used on stainless or carbon steel. When using chemical etching solutions, make sure to remove them from the work before welding. To minimize the risk of hydrocarbons from oils or cutting solvents entering the weld, remove them with a degreaser. Check that the degreaser does not contain any hydrocarbons. Suitable and effective degreasers are acetone or toluene. These are effective also for removal of machining coolant lubricants. Alternatively, machining coolant lubricants can be removed by washing the part in a mild alkaline cleaner. Regarding moisture effect the surfaces must be completely dried before welding. The welding must be done in sheltered locations from rain, snow or drafts.

2. Preheating:

Preheating the aluminum workpiece, can help us to avoid weld cracking. Preheating temperature should not exceed 110^oC (required the use of a suitable temperature indicator to check temperature and prevent overheating). In addition, placing tack welds at the beginning and end of the area to be welded will aid in the preheating effort. Welders should also preheat a thick piece of aluminum when welding it to a thin piece; if cold lapping occurs, should be used run-on and run-off tabs at welding ends.

 3. Tack Welding

Tack welds shall be deposited in such as manner as to facilitate their incorporation into the final weld without causing discontinuity in the deposit. The  placement of tack welds shall be made with care to minimize stresses in welds caused by their restricting the contraction of parts being welded.

 4. Welding Sequence

An approved Welding sequence plan is necessary for efficient control of residual stresses. The logic of welding sequence plans is to minimise – as far as practical – the residual welding stresses by defining the various steps to be followed during the welding process so that external restraint shall not resist the shrinking incident to the welding area and add stresses to the structure. The welders should follow with a stringent way the approved -for the specific weld areas- Welding Sequence plan. Deviations from this plan can be approved in cases that is quite impractical or ineffective the complete application of the plan in specific steps of this. On this case it should be tested and assured the equivalent result of final welds, that means, clean and sound welds free of defects and with minimized residual stresses.

5. Welding Technique

With aluminum, pushing the gun away from the weld puddle rather than pulling it will result in better cleaning action, reduced weld contamination, and improved shielding-gas coverage.

 6. Travel speed

Aluminum welding needs to be performed “hot and fast.” Unlike steel, the high thermal conductivity of aluminum dictates use of hotter amperage and voltage settings and higher weld-travel speeds. If travel speed is too slow, the welder risks excessive burnthrough, particularly on thin-gauge aluminum sheet

 7. Shielding Gas

Argon, due to its good cleaning action and penetration profile, is the most common shielding gas used when welding aluminum. Welding 5XXX-series aluminum alloys, a shielding-gas mixture combining argon with helium – 75 percent helium maximum – will minimize the formation of magnesium oxide.

 8. Welding filler metal (wire)

Select an aluminum filler wire that has a melting temperature similar to the base material. The more the operator can narrow-down the melting range of the metal, the easier it will be to weld the alloy. Obtain wire that is 1,2 or 1,6 mm diameter. The larger the wire diameter, the easier it feeds. To weld very thin-gauge material, an 1,0mm diameter wire combined with a pulsed-welding procedure at a low wire-feed speed – 3 to 8 m/min – works well.

 The most commonly used wire is ER5356 (AlMg5) because of its good strength and good feed-ability when used as a MIG electrode wire. It is designed to weld 5xxx series structural alloys but is also quite effective for 6xxx series alloys. For castings it is preferred the ER4043 wire (AlSi5) since castings are high in silicon. The 5356 on the other hand is recommended over 4043 for components that will be anodized after welding. The 4043 has lower melting point and more fluidity than 5xxx series filler alloys and is less sensitive to weld cracking with the 6xxx series base alloys. Also makes brighter looking welds with less smut (because it doesn’t contain magnesium).  The ER 5356 are not suitable for service temperatures over 65^o C due to their susceptibility to stress corrosion cracking.

 9. Convex-shaped welds crater

In aluminum welding, crater cracking causes most failures. Cracking results from the high rate of thermal expansion of aluminum and the considerable contractions that occur as welds cool. The risk of cracking is greatest with concave craters, since the surface of the crater contracts and tears as it cools. Therefore, welders should build-up craters to form a convex or mound shape. As the weld cools, the convex shape of the crater will compensate for contraction forces.

 10. Fillet welds

For abutting plates up to 10 mm the fillet weld throat size should not exceed the nominal throat by 1-1,5 mm. Slight convex form is desirable. Excessive convexity – more than 2mm – should be corrected by grinding or chipping. For plates >10 mm thickness the accepted excessive convexity from nominal throat is 2-3 mm. For more than 3 mm is required grinding or chipping. The throat depth and leg length must be always greater equal of 90% of the design values (as specified in the drawings) but the values limits are also dependent on the applicable standards of acceptance for the specified welding class.

Where intermittent welds (chain or staggered) are applicable the size/pitch (l/d) of welds shall be in accordance with the drawings; the same for the end connections double continuous welds extent. The intermittent weld ends shall be smooth with light convex-shaped finish.

 11. Butt Welds, Weld reinforcement

For plates more than 12 mm the reinforcement of butt welds shall not be more than 2,5-3 mm. For plates <12 mm the reinforcement shall not exceed 25% of material thickness.

 12. Welding finish

Welds received for final approval from the production shall be free from all slag, flux, weld spatter and other residues and foreign materials

Aluminium jointed with steel plate, with the use of bitellalic bar

Grinding area for the removal of aluminum oxide