This article appeared in the October 1973 issue of Airsport. Written by Frank Rogers
CLEANING
Prior to welding, all surfaces within 1/2-3/4 inch of the area to be welded should be cleaned, firstly with a suitable solvent such as white spirits, M.E.K. or trichlorethylene to remove any surface oil or grease then surface cleaned with a fine wire stainless steel brush, preferably power driven.
Remove only contaminants in the cleaning process; do not reduce material thickness. Remove all burrs, swarf, marking, inks, paint etc. from the area to be welded. Do not handle cleaned surfaces with bare hands as this causes contamination. Wear clean cotton or latex gloves if necessary.
Welding should take place as soon as possible after cleaning, since with non stainless steel and aluminium, corrosion could begin within 24 hours on an unprotected surface.
EDGE PREPARATION
Prepared edges are not normally required for steels thinner than 16 SWG (.064″), but to ensure sufficient penetration and adequate strength with thicker steels, i.e. from 16 SWG to 10 SWG (.128″), the edges are prepared as shown on Fig. 1.
To preserve the strength of the joint, the gap between the mating parts should not exceed .005″ for steels of .028-.048″ and .010″ for thicker material.
When butt welding aluminium alloys up to 12 SWG (.104″) thick, a gap of approximately 1/4 the material thickness is left between the plates. Vee grooving is unnecessary for this thickness of material. For butt welding aluminium alloys from 12 SWG to 1/4″ thick, the edges are prepared as shown in Fig.1 except the angle should be 60-70 degrees and the gap between the plates should not exceed 1/3 the plate thickness. Sheet metal up to .040″ thick which is to be flange edge welded should have the edges prepared as shown in Fig. 2.
The preparation of the ends of tubes is normally based on the same considerations of material thickness as given above. Where tubes intersect, particular care is necessary in shaping the ends to ensure a good fit. The centre lines of tube clusters should normally intersect.
WELDING JIGS
To maintain correct alignment and gap, the parts may be jig located and/or held by mechanical means such as clamps or by tackwelds. If a jig is used it must be unaffected by heat, give free access to the area to be welded, and be fairly rigid, but not to such an extent that the parts may become stressed during cooling, thus clearance should be allowed for expansion and contraction.
WELDING
All welding must be performed by a person holding a current D.C.A. licence for the particular material being welded. A builder may be approved by D.C.A. to carry out his own welding by performing the tests required in ANO 104.9 satisfactorily.
Welding low alloy steel.
Carbon and low alloy steels used by amateur builders are normally oxyacetylene welded. A neutral or slightly reducing flame should be used and the inner cone should be held close to the material being welded, the torch and welding rod being held at angles of about 60 and 30 degrees respectively to the flame of the weld. Fluxes are not required for welding carbon steel as the oxides of the various elements unite to form a slag at a temperature below that of the molten steel; this slag tends to float to the surface of the weld. The welder should endeavour to float these oxides away from the joint. This action is assisted by using welding rods containing alloying elements which act as cleaners. Pre-heating and post weld heating should be performed as described in AIRSPORT December 1972.
Welding Aluminium alloys
Although oxyacetylene welding of aluminium alloy is still practiced, welding by means of the argon arc process is considered to be more advantageous.
In the latter process a shield of inert gas such as argon, surrounds the area to be welded i n order to reduce oxidation of the joint. Two methods of argon arc welding are used namely, TIG and MIG welding. In TIG the electric arc is drawn between the work and a tungsten electrode, a filler rod being used. TIG welding is used for materials up to 1/4″ thick.
In MIG welding an automatically fed filler wire is used to draw the arc to the work. As i n TIG welding no flux is necessary. Material from 1/4″ to 4″ can be welded by the MIG process.
A flux is used for oxyacelylene welding of aluminium alloys and should be of free flowing consistency and applied by a brush. A slightly reducing flame is used as an excess of oxygen causes a rapid formation of aluminium oxide. The torch should be held at about 30 degrees to the plane of the weld, the angle decreasing as the end of the weld is approached. When welding material of 16 SWG and thinner, tack weld at 1″ to 1-1/2″ intervals. For thicker materials the spacing is 2″-3″.
Removing flux.
Unless the flux is specifically approved as being non corrosive, it is essential that all traces are removed. Flux, slag and scale may be removed using a fine stainless steel wire brush. All weld spatter must be removed as it is a source of corrosion attack. Do not file or grind welded joints. Flux may be removed from aluminium alloy parts by cleaning in a neutralising solution consisting of:
| Nitric acid | 10% by volume |
| Water | Remainder |
| Temperature | 150°F |
| Time | 20 minutes |
or scrubbing with a non metallic bristle brush in boiling water. If using the weak nitric acid solution, rinse the part thoroughly in hot water afterwards. The efficiency of the final washing operation can be checked by adding about 2% silver nitrate test solution to a sample of the washing or rinse water. If a white precipitate appears, it indicates that flux residues are still present and further cleaning is necessary.