The strong affinity of the aluminum and oxygen in the air combine easily with oxygen to generate a dense solid AL2O3 film thickness of approximately 0.1μm, the high melting point of 2050 ° C, far exceeds the melting point of aluminum and aluminum alloy, and the density about 1.4 times the aluminum. In the welding process, an alumina film will hinder a good combination between the metal, and easy to cause slag. The oxide film can also adsorb moisture will lead to weld welding generate stomata. These defects would reduce the performance of the welded joints. In order to ensure the quality of welding, welding must strictly clean oxides on the surface of the weldment and to prevent the re-oxidation in the welding process, on the molten metal and at a high temperature of the metal to be effectively protected, which is the welding of aluminum and aluminum alloy is an important characteristics. Specific protection measures are: 1, before welding, mechanical or chemical methods to clear the workpiece groove around the part and the wire surface oxide;
2, the welding process to be adopted in conformity of the protective gas to be protected;
3, in the welding when using flux, constantly with wire prick the bath surface of the oxide film in the welding process.
Second, the aluminum thermal conductivity and large specific heat, thermal
Although aluminum and Aluminium Alloy Welding Wire of low melting point than steel, but the thermal conductivity of aluminum and aluminum than the heat capacity are large, more than steel and twice as large, a lot of heat during the welding process is quickly transmitted to the base metal internal, in order to obtain quality welded joints, the energy must be concentrated heat source of power sometimes requires the use of a preheating process measures, in order to achieve the welding process.
Third, the large linear expansion coefficient
The coefficient of linear expansion of aluminum and aluminum alloys are about 2 times that of steel solidification volume shrinkage rate of 6.5% -6.6%, and therefore easy to produce welding deformation. Effective measures to prevent deformation in addition to reasonable parameters and welding sequence using appropriate welding equipment is also very important, especially when welding sheet. Further, some of the aluminum and aluminum alloy welding, is formed in the weld metal solidification cracking tendency and the liquation crack in the heat-affected zone formed propensity are larger, often due to the large internal stress in the brittle temperature range thermal cracking. This is an aluminum alloy, in particular, is one of the most common serious defect in the welding of high-strength aluminum alloy. Measures to prevent such cracks in the actual welding site improved joint design, select a reasonable welding parameters and welding sequence, welding filler material to adapt to the characteristics of the parent material.
Fourth, easy formation of pores
The stomata is easy to produce in the welding of aluminum and aluminum welded joints defect, especially aluminum and rust-proof aluminum welding. Hydrogen is the main reason for the welding of aluminum and aluminum blowholes, and this has been proven by practice. The source of hydrogen, the water adsorbed by the moisture in the atmosphere of the arc column, the welding material and the base material, wherein the surface oxide film of the wire and the base material of adsorption of water, in order to weld the generation of holes, often a prominent position.
Easily absorb the gas of the liquid bath of aluminum and aluminum alloy, a large amount of gas dissolved at a high temperature, the solidification from the liquid state, solubility sharply precipitates too late, in the cooling and solidification process after welding, which gather in the weld formation of pores . In order to prevent the generation of holes, in order to obtain a good welded joint, the source of the hydrogen to be strictly controlled, before welding must be strictly limited to using the welding material (including wires, rods, flux, protective gas), moisture content, prior to use drying process. Best clean up after the base metal and wire welding is completed within 2-3 hours, up to more than 24 hours. TIG welding, a large selection of welding current with high welding speed. MIG welding is selected when a large welding current slow welding speed, and to improve the time of the presence of the molten pool. Al-Li alloy welded strengthening positive backside protection, with the groove scraper Clear Overview oxide film, the pores can be effectively prevented.
Welded joints is easy to soften
Some of the deterioration of the mechanical properties of weld heat treatment can be a reinforced Aluminum Electrode will soften due to the influence of the welding heat, the heat affected zone of welded joints, i.e. the strength is lowered, so that the matrix metal near the weld zone area. The same is true for cold hardening alloy, the joint performance is weakening, and the greater the energy of the welding line, the program is also more severe performance degradation. For such problems, the measures taken to develop the technology to meet specific material welding, to such restrictions welding conditions, to take the appropriate welding sequence to control the temperature between the preheat temperature and layer post-weld heat treatment. Soften the aluminum alloy can not be restored after welding, preferably by annealing or welding, soldering, and then subjected to a heat treatment in the solid solution state, if the welding is not allowed after heat treatment, should be used the concentrated energy welding method and a small line energy welding, in order to reduce the joint strength decreases.
Alloying elements evaporation and burning
Certain aluminum alloys containing a low boiling point of the alloy elements, these elements easily evaporated at high temperature burning, thereby changing the chemical composition of the weld metal, reduces the performance of the welded joints. In order to compensate for these burning while adjusting the process, often higher than the base metal wire or other welding materials containing these boiling element content.
VII, low aluminum in the high temperature strength and ductility
Aluminum strength of only 370 ℃ 10Mpa, welding live because they can not support liquid metal leaving the weld forming bad, and even the formation of collapse or burn-through, in order to solve this problem, often used when welding aluminum and aluminum alloy pad.
Eight, the corrosion resistance of welded joints below the base metal
Intensive heat treatment of aluminum alloys (e.g., duralumin) obviously decrease the corrosion resistance of the fittings, the joint structure is more uneven, the corrosion resistance is reduced. Purity or denseness of the weld metal also affects the joint corrosion resistance. Impurities, coarse grains, as well as brittle precipitates, corrosion resistance will be decreased, not only from the local surface corrosion and often appear to intergranular corrosion, corrosion resistance is an important addition to the presence of aluminum alloy welding stress also affect factor.
In order to improve the corrosion resistance of welded joints, take the following measures:
1, to improve the non-uniformity of the joint tissue components. That the weld by welding material alloys, grain refinement and prevent defects; adjust the welding process at the same time to reduce the heat-affected zone, and prevent it from overheating, and post-weld heat treatment.
2, eliminate welding stress, such as local surface tensile stress can be a way to eliminate local hammering.
3, to take protective measures, such as take anodized or coating.
Nine, no color change, brought difficulties to the welding operation
The change from a solid to liquid aluminum and aluminum alloy welding, no significant change in color, so bring a lot of difficulties to the operator in the welding process. Therefore, the requirements of welders heating temperature when welding, maximize the use of flat welding arc cited (put out) board primers (quenching) arc.
1) chemical cleaning chemical cleaning efficiency, stable quality, suitable for cleaning up little wire and size, mass-produced artifacts. Available immersion method and scrub and French. Available acetone, gasoline, kerosene and other organic solvent surface oil, with 40 ℃ ~ 70 ℃ 5% ~ 10% NaOH solution alkaline cleaning 3min ~ 7min (pure aluminum little longer, but not more than 20min), mobile water rinse, followed by room temperature to 60 ° C, 30% HNO3 solution pickling 1min ~ 3min, mobile wash, air dry or low-temperature drying.
2) Mechanical cleaning: the large size of the workpiece, the production cycle is longer, the multilayer welding or chemical cleaning stains and then, often using mechanical cleaning. First wipe the surface with organic solvents such as acetone, gasoline degreasing, followed directly with a diameter of 0.15mm ~ 0.2mm brass wire brush or stainless steel wire brush, brush the exposed metallic luster date. Generally should not use the grinding wheel or sandpaper, sand so as not to stay on the metal surface, into the molten pool generated welding slag and other defects. Also available scraper, rasp clean up the surface to be welded.
Clean up after such long time storage (if more than 24 hours) should be re-processing.
2, the pad: aluminum alloy at high temperature strength is very low, the flow properties of the liquid aluminum, the weld metal during welding is easy to produce discover phenomenon. Welding is often used in order to ensure penetration while avoiding collapse, pad to lift up the molten pool and nearby metal. The pad may be graphite plates, stainless steel plates, carbon steel, copper, or copper rod. The surface of the pad to open a circular arc-shaped grooves, to ensure that the seam opposite molding. Also can not add the pad side welding molding proficient or advanced technology measures taken strictly for arc welding energy feedback control, but requires
3, preheating before welding: thin, small pieces of aluminum generally do not preheat thickness 10mm ~ 15mm can be preheated before, according to the different types of aluminum Welding Alloys preheating temperature of 100 ° C. to 200 ° C. Available oxygen acetylene flame, electric stove or blowtorch heat. Preheat the weldment can reduce distortion to reduce porosity and other defects.