Aluminum welding technologies
Relevance
Al has a low melting point (657°C) with a high coefficient of thermal expansion and significant thermal conductivity - three times higher than that of mild steel. Hot aluminum is very brittle, but the main difficulty in welding it remains its mild oxidation. The oxide (Al203) is strong and refractory. It melts at 2050 °C and is poorly amenable to flux because of its chemical neutrality.
Welding
Gas and arc welding as well as special electrical contact welding machines are used. The most popular gas welding of aluminum with an acetylene-oxygen mixture. Preliminary metal edges are thoroughly cleaned: with sandblasters, steel brushes, scraping or washing in gasoline or in an aqueous solution of caustic soda. After caustic soda the parts are thoroughly washed with running water to prevent corrosion. When welding aluminium castings it is recommended to preheat them to 300 °C.
Flux
When welding thick-walled aluminium castings, flux may sometimes be unnecessary. But then the aluminum oxide must be constantly removed from the surface of the weld pool with a steel wire scraper, and the end of the filler rod is immersed in the weld pool to prevent oxidation. In the usual situation, special fluxes vigorously dissolve aluminum oxide even at low temperatures and greatly facilitate welding. Before the invention of good fluxes welding aluminum because of the complexity almost never practiced. Particularly strong dissolve Al203 halide compounds of lithium, so the fluxes are introduced lithium chloride or fluoride.
Until now the search for new, more perfect fluxes is underway. To check the quality of new fluxes, a small tray is melted with a gas torch on an aluminum plate covered with an oxide film with a grayish-matte dull surface. After a pinch of good flux into the bath, its surface is cleaned almost instantly.
Compositions of some of the fluxes used
| Flux content | Composition | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
|---|---|---|---|---|---|---|---|---|
| Chlorides | sodium NaCl | 30 | 45 | 28 | 35 | 33 | 30 | 19 |
| Chlorides | KCl potassium chlorides | 45 | 30 | 50 | 48 | 45 | 45 | 29 |
| Chlorides | Li CL | 15 | 10 | 14 | 9 | 15 | 15 | - |
| Chlorides | barium BaCl2 | - | - | - | - | - | - | 48 |
| Fluorides | sodium NaF | - | - | 8 | 8 | - | 10 | - |
| Fluorides | potassium KF | 7 | 15 | - | - | 7 | - | |
| Fluorides | calcium CaF2 | - | - | - | - | - | - | 4 |
| NaHS04 | 3 | - | - | - | - | - | - | |
Flux fabrication and storage
Fluxes and coatings are prepared from chemically pure substances. Fluxes are either thoroughly mixed with simultaneous grinding of the components, e.g. in a mill with a porcelain case and balls, or the components are pre-melted and then thoroughly milled. Flux fusion usually gives better results and less hygroscopicity. Note that aluminum fluxes change their properties when exposed to atmospheric moisture. Therefore, they are stored in tightly corked jars, from which portions are taken only for one shift.
Butt Contact Welding
With this method, aluminum is melted continuously on electric welding machines. At a current value of 15,000 A per 1 cm2 of welded cross-section the melting amount is 5-12 mm and the draft is 1.5-5 mm, depending on the cross-section of the weld. Continuous melting time is 30 to 70 AC periods and settlement time is 2 to 5 current periods. Current is switched off at the beginning of upsetting.
Spot welding
It is significantly hampered by high electrical conductivity of aluminum and its rapid melting during welding (for 0.002-0.005 sec.), so to maintain the value of pressure and contact with the metal you must quickly move the electrode of the machine. Spot welding with accumulated energy is also possible. Usually capacitor spot welding is used. Electrodes for such welding are taken from copper alloys with high hardness and electrical conductivity. Satisfactory results are given by the alloy
Alloys
Aluminum alloys have a higher strength than pure aluminum. Aluminum alloys can be divided into two groups: heat-strengthened and heat-strengthened alloys. An example of the first group is the alloy AMz, alloyed with 1.3% manganese, with a tensile strength of 13 to 20 kg/mm2 depending on the hardening. Such alloys are insensitive to heat treatment, relatively easy to weld, and the weld strength approaches that of the base metal in an annealed state.
Duralumin
Refers to thermally hardenable alloys. It has a number of grades with a tensile strength of 38 to 46 kg! mm2. Welding duralumin is still a challenge. Duralumin is an alloy of aluminum with copper and magnesium forming intermetallic compounds. Their solubility in aluminum depends on temperature. When aluminum is heated above the critical temperature, the compounds fully dissolve in the metal and remain in this form upon rapid cooling,
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