I. Waste Removal
After welding the workpiece, if gas welding or flux-cored welding rods are used, it is necessary to immediately clean residual flux and slag from the welding and surrounding area before performing visual inspection and non-destructive testing.
This step prevents corrosion of the weld and its surface by slag and residual flux, avoiding undesirable consequences. Common methods for post-weld cleaning are as follows:
(1) Scrub in hot water between 60°C and 80°C;
(2) Immersion in potassium dichromate (K2Cr2O2) or 2% to 3% chromic anhydride (Cr2O2);
(3) Additional rinse in hot water between 60°C and 80°C;
(4) Kiln or air drying.
To test the effectiveness of removing residual flux, distilled water can be placed on the weld of the workpiece. The distilled water is then collected and placed in a test tube containing a 5% nitric acid solution. If a white precipitate appears, it indicates that the residual flux has not been completely removed.
II. Welding Surface Treatment
Through proper welding process and correct operation techniques, the surface of aluminum and aluminum alloy welds after welding has a uniform, ripple-free and smooth appearance.
Anodizing, especially when combined with polishing and staining techniques, results in high-quality decorative surfaces. The zone affected by welding heat can be minimized, reducing color changes induced by anodization to a minimum. Fast welding processes can significantly reduce the heat affected zone. Consequently, the quality of anodizing treatment on flash welded seams is good.
Especially for alloy welding parts that cannot be heat treated for strengthening in the annealed state, the color contrast between the base metal and the heat-affected zone is minimal after anodizing. Furnace and immersion brazing, which does not involve localized heating, produces a very uniform metallic color appearance.
Alloys that can be heat treated for strengthening, often used in structural parts of buildings, are often anodized after welding. In these alloys, the heat from welding forms precipitates in the alloy elements.
After anodizing, differences will appear between the heat-affected zone and the weld seam. These halo zones close to the welding area can be minimized by rapid welding or by the use of cooling blocks and clamping plates. These halo zones, before anodizing and after welding, can be eliminated through precipitation hardening treatment.
When chemically treating welded parts, significant color differences can occur between the weld metal and the base metal. This requires careful selection of the filler metal composition, especially when it contains silicon, which can affect color matching.
If necessary, the weld can be mechanically polished. Common mechanical polishing methods include polishing, grinding, abrasive blasting, and sandblasting. Mechanical polishing improves the surface of aluminum parts through physical methods such as grinding, deburring, buffing, polishing or sandblasting. The objective is to achieve the desired surface quality in as few steps as possible.
However, aluminum and its alloys are soft metals with high coefficients of friction. Overheating during the grinding process can potentially cause deformation of welded parts or even lead to grain boundary fractures of the base metal. Therefore, sufficient lubrication is required during the polishing process and pressure on the metal surface must be minimized.
III. Post-weld heat treatment
The purpose of post-weld heat treatment is to improve the structure and performance of the welded joint or eliminate residual stresses. Heat-treatable aluminum alloys can undergo post-weld heat treatment, restoring the strength of the heat-affected zone of the base metal close to its original strength.
Generally, the point of joint failure occurs within the weld fusion zone. After post-welding heat treatment, the strength acquired by the weld metal depends mainly on the dispersed filler metal.
1. Features of welding aluminum and aluminum alloys
(1) Aluminum oxidizes easily in air and during welding, forming aluminum oxide (Al2O3) which has a high melting point and is extremely stable, making it difficult to remove. This prevents the original material from melting and fusing. The large specific gravity of the oxide film prevents it from floating on the surface, resulting in defects such as slag inclusions, lack of fusion and incomplete penetration.
The surface oxide film of aluminum and its absorbed moisture can cause porosity in the weld seam. Before welding, the surface must be thoroughly cleaned using chemical or mechanical methods to remove the oxide film. Protection must be reinforced during welding to avoid oxidation.
During tungsten inert gas (TIG) welding, an alternating current power source must be used to remove the oxide film through “cathodic cleaning”. When gas welding, a flux must be used that removes the oxide film. When welding thick plates, the welding heat may be increased. For example, helium arc has high heat, so use helium or argon-helium mixed gas shielding, or large diameter gas metal arc welding (GMAW) with positive direct current, which eliminates the need for “cleaning” cathodic”.
(2) The thermal conductivity and specific heat of aluminum and its alloys are more than twice those of carbon steel and low-alloy steel. The thermal conductivity of aluminum is tens of times greater than that of austenitic stainless steel.
During the welding process, a large amount of heat can be quickly transferred to the base metal, therefore, when welding aluminum and its alloys, in addition to the energy consumed in melting the metal pool, more heat is wasted on other parts of the metal. This energy waste is more significant than in steel welding. To obtain high-quality welded joints, high-power, concentrated energy sources should be used as much as possible. Preheating and other process measures can also be used.
(3) The coefficient of linear expansion of aluminum and its alloys is approximately twice that of carbon steel and low-alloy steel. Aluminum exhibits significant volumetric shrinkage during solidification, causing substantial deformation and stress upon welding. Therefore, measures to prevent welding deformation must be employed. Solidification of an aluminum welding puddle tends to result in shrinkage cavities, porosity, hot cracking and high internal stress.
To prevent the occurrence of hot cracks, production can adjust the composition of the welding wire and the welding process. In corrosion-resistant situations, aluminum-silicon alloy welding wire can be used to weld aluminum alloys, excluding aluminum-magnesium alloys.
In aluminum-silicon alloys, the tendency to hot cracking is considerable when the silicon content is 0.5%. As the silicon content increases, the alloy's crystallization temperature range decreases, its fluidity improves significantly and its shrinkage rate decreases, consequently reducing the tendency to hot cracking.
According to production experience, when the silicon content is 5% to 6%, hot cracking does not occur, therefore, the use of SAlSi wire (with silicon content of 4.5% to 6%) provides better crack resistance.
(4) Aluminum has strong reflectivity for light and heat. During the transition from solid to liquid state, there is no noticeable change in color, making it difficult to assess during the welding process. High temperature aluminum has very low strength, making it difficult to support the weld pool and prone to burns.
(5) Aluminum and its alloys can dissolve a significant amount of hydrogen in the liquid state, but hardly any in the solid state. During the solidification and rapid cooling of the welding pool, hydrogen does not have time to escape, leading to the formation of hydrogen porosity.
Moisture in the arc atmosphere, welding materials, and moisture adsorbed on the surface oxide film of the parent material are significant sources of hydrogen in the weld seam. Therefore, strict control over hydrogen sources is necessary to prevent porosity formation.
(6) Alloying elements tend to evaporate and burn, reducing the performance of the weld seam.
(7) If the base metal of the parent material is strain hardened or solution heat treated, the heat of welding may reduce the strength of the heat-affected zone.
(8) Aluminum, having a face-centered cubic lattice and no allotropic shapes, does not undergo phase transition during heating and cooling. This leads to coarse grains in the weld, which cannot be refined through phase transitions.
2. Welding methods
Almost all welding methods can be used to weld aluminum and its alloys. However, the adaptability of aluminum and its alloys to different welding methods varies, and each method has its own applications.
Gas welding and shielded metal arc welding are simple and convenient. Gas welding can be used to repair seams on aluminum sheets and castings where quality requirements are not high. Shielded metal arc welding can be used for repair of aluminum alloy castings.
Inert gas shielded welding (TIG or MIG) is the most widely used method for welding aluminum and its alloys. Aluminum sheets and aluminum alloys can be welded using alternating current tungsten inert gas welding or pulsed tungsten inert gas welding.
Aluminum and aluminum alloy plates can be welded using helium tungsten arc welding, tungsten inert gas welding with argon-helium mixture, metal inert gas welding and pulsed metal inert gas welding. The application of metal inert gas welding and pulsed metal inert gas welding is becoming more and more widespread (argon or argon/helium mixture).
3. Welding materials
(1) Welding wire
In addition to considering the good performance of the welding process, the selection of aluminum and aluminum alloy welding wires must ensure that the tensile strength and plasticity (through bending tests) of butt joints meet the specified requirements of according to the vessel's requirements.
Impact resistance requirements must be met for aluminum-magnesium alloys with a magnesium content greater than 3%. For vessels with corrosion resistance requirements, the corrosion resistance of the welded joint must reach or be close to the level of the base material. Therefore, the selection of welding wire is mainly based on the properties of aluminum.
1) The purity of pure aluminum welding wire is generally not inferior to that of the original material;
2) The chemical composition of aluminum alloy welding wire is generally similar or close to that of the original material;
3) The content of corrosion-resistant elements (magnesium, manganese, silicon, etc.) in aluminum alloy welding wire is generally not lower than that of the original material;
4) When welding different types of aluminum materials, choose the welding wire based on the most corrosion-resistant and strongest original material;
5) High-strength aluminum alloys (aluminum alloys reinforced with heat treatment) that do not require corrosion resistance can use welding wires of different compositions, such as crack-resistant aluminum-silicon alloy welding wires such as SAlSi-1 ( Please note that the strength may be lower than the original material).
(2) Shielding Gas
The shielding gas is argon, helium or a mixture of these. For high-frequency alternating current TIG welding, use more than 99.9% pure argon. Positive polarity direct current welding is suitable for helium.
In MIG welding, it is recommended to use argon with the addition of 50% to 75% helium when the plate thickness is 75 mm. Argon must meet the requirements of GB/T 4842-1995 “Pure Argon”. The pressure of the argon cylinder is insufficient and cannot be used when it is below 0.5 MPa.
(3) Tungsten electrodes
Four types of tungsten electrodes are used in argon arc welding: pure tungsten, thoriated tungsten, cerium tungsten and zirconium tungsten. Pure tungsten electrodes have high melting and boiling points, making them less prone to melting, evaporation, electrode burnout, and tip contamination.
However, they have a lower electron emission capacity. Thoriated tungsten electrodes, made by adding 1% to 2% thorium oxide to pure tungsten, have greater electron emission capacity, allow for greater current density, and maintain a more stable arc. However, thorium is slightly radioactive, so appropriate protective measures must be taken during use.
Cerium tungsten electrodes are made by adding 1.8% to 2.2% cerium oxide (with impurities ≤0.1%) to pure tungsten. These electrodes have low electronic work function, high chemical stability, allow high current density and do not have radioactivity. They are the most used electrodes today.
Zirconium and tungsten electrodes can avoid contamination of the base metal electrode. Their tips are easy to maintain in a hemispherical shape, making them suitable for AC welding.
(4) Flow
The flux used in gas welding consists of chlorides and fluorides of elements such as potassium, sodium, lithium and calcium, which can remove the oxide film.
4. Preparation before welding
(1) Pre-welding cleaning
Before welding aluminum and its alloys, it is essential to carefully remove the oxide film and grease from the part's welding line and the surface of the welding wire.
The quality of this cleaning directly affects the welding process and the quality of the joint, such as the tendency to generate porosity in the weld and various mechanical properties. This cleaning is typically achieved using chemical or mechanical cleaning methods.
1) Chemical Cleaning
Chemical cleaning is highly efficient and of consistent quality, making it suitable for cleaning welding wires and small, mass-produced parts. Submersion and scrubbing are two common methods.
Organic solvents such as acetone, gasoline and kerosene are used for surface degreasing, followed by alkaline washing using a 5% to 10% NaOH solution at 40°C to 70°C for 3 to 7 minutes (longer for aluminum pure, but not exceeding 20 minutes), then rinse with running water.
Subsequently, an acid wash is carried out with a 30% HNO3 solution, at room temperature up to 60°C, for 1 to 3 minutes, followed by another rinse with running water and drying with air or low heat.
2) Mechanical Cleaning
Mechanical cleaning is often used when the workpiece is large, the production cycle is long, multilayer welding is involved, or when the workpiece is recontaminated after chemical cleaning.
First, organic solvents such as acetone or gasoline are used to degrease the surface, followed by brushing with a 0.15mm to 0.2mm diameter copper or stainless steel brush until a metallic shine is revealed.
The use of grinding wheels or ordinary sandpaper is generally discouraged to prevent grains of sand remaining on the metal surface, which could enter the weld pool during welding and cause slag inclusions or other defects. Alternatively, scrapers or files can be used to clean the surface to be welded.
After cleaning the part and welding wire, an oxide film will reappear during storage, especially in humid environments or environments contaminated by acidic or basic vapors, where the oxide film grows even faster.
Therefore, the storage time from the end of cleaning to the start of welding must be minimized. In humid conditions, welding should generally be carried out within 4 hours of cleaning. If the storage time after cleaning is too long (e.g. exceeds 24 hours), the cleaning process must be repeated.
(2) Backing plate
Aluminum and its alloys exhibit low strength under high temperatures, and molten aluminum flows easily, leading to potential collapse of the weld metal during welding. To ensure complete penetration without collapse, backing plates are often used to support the weld pool and adjacent metal.
Backing plates can be made of graphite, stainless steel, carbon steel, copper plates or copper bars. A curved groove is made on the surface of the backing plate to ensure weld formation on the back.
Alternatively, one-sided welding with two-sided forming can be carried out without backing plate, but this requires skilled welding operations or the application of advanced process measures such as strict automatic feedback control of welding energy.
(3) Preheating before welding
Thin and small aluminum parts generally do not require preheating. Preheating can be carried out for thicknesses between 10 mm and 15 mm, with temperatures ranging from 100°C to 200°C depending on the type of aluminum alloy.
Techniques such as oxyacetylene flames, electric furnaces or torches can be used for heating. Preheating helps to minimize welding deformation and reduce defects such as porosity.
5. Post-welding Treatment
(1) Post-Weld Cleaning
Residual welding flux and slag left on and near the weld can damage the passivation film on the aluminum surface and can even corrode aluminum parts, thus requiring thorough cleaning. For simple-shaped parts with general requirements, simple cleaning methods such as hot water rinsing or steam brushing can be used.
For complex-shaped and high-demand aluminum parts, after brushing in hot water with a stiff brush, they should be soaked in an aqueous solution of chromic acid or potassium dichromate solution with a concentration of 2-3% at approximately 60° C-80°C for 5 -10 minutes, followed by scrubbing with a stiff brush.
They must then be rinsed in hot water and dried in an oven or with hot air. Natural drying is also acceptable.
(2) Post-welding heat treatment
Generally, heat treatment is not necessary after welding aluminum containers. However, if the aluminum material in use exhibits significant sensitivity to stress corrosion cracking under the conditions of the medium in contact with the container, it is necessary to undergo post-weld heat treatment to eliminate high welding stresses, thereby reducing the stress on the container. below the limit for stress corrosion cracking.
This requirement must be especially indicated in the container design documents before stress relieving heat treatment is carried out after welding. If post-weld annealing heat treatment is required, the recommended temperature for pure aluminum, 5052, 5086, 5154, 5454, 5A02, 5A03, 5A06, etc., is 345°C; for 2014, 2024, 3003, 3004, 5056, 5083, 5456, 6061, 6063, 2A12, 2A24, 3A21, etc., it is 415 ℃; for 2017, 2A11, 6A02, etc., it is 360 ℃.
Depending on the size and requirements of the workpiece, the annealing temperature can be adjusted up or down by 20°C-30°C, while the holding time can vary from 0.5 to 2 hours.
