The characteristics and advantages of aluminum alloy make it one of the preferred materials for automotive lightweighting today.
The forming characteristics of aluminum alloy sheet and the technical applications in various processes such as stamping, welding, and painting in automotive body applications were studied and analyzed for the current application status of automotive aluminum sheet metal in China.
At the same time, some solutions and suggestions were proposed for the process problems that have arisen in China’s automotive industry during the use of aluminum alloy.
With the development of modern automotive industry and increasingly demanding requirements in energy, environmental protection, and safety in China, improving the lightweighting of vehicles can effectively save fuel consumption and reduce emissions.
Therefore, the application of aluminum alloy sheet metal in the automotive manufacturing industry is becoming increasingly widespread.
Aluminum has the advantages of small density, light weight, good formability, recyclability, energy saving, and environmental protection, as well as the ability to improve vehicle driving and safety performance, reduce fuel consumption and emissions, and reduce environmental pollution.
Therefore, aluminum alloy is recognized as one of the ideal materials for future automobiles.
The application of aluminum alloy in automotive bodies
Currently, more than 12% to 15% of the world’s aluminum consumption is used in the automotive industry, with some developed countries using more than 25%.
In 2002, the entire European automotive industry consumed more than 1.5 million tons of aluminum alloy in a year, of which approximately 250,000 tons were used for body manufacturing, 800,000 tons were used for manufacturing of automotive transmissions, and an additional 428,000 tons were used for the manufacture of automotive driving and suspension systems.
It can be seen that the automotive manufacturing industry has become the largest consumer of aluminum materials. The application of stamping aluminum sheet in automotive bodies is shown in Figure 1.
| Engine hood: Citroen C4, C5, Peugeot 307 | Engine hood: Volvo S60 | Engine hood, rear hatch, doors: Audi A6, Q5 | Audi A8, A2 – All-aluminum body Mercedes-Benz S/E-Class – Engine hood, fender Mercedes-Benz ML250 – Engine hood BMW 7 Series – All exterior panels except for the side panels Volvo S70 – Rear hatch Volvo XC60 – Engine hood |
| Engine hood: Toyota Mark X (Reiz), Prius, Subaru Legacy | – | Engine hood: Toyota Crown, Honda Mileage | Engine hood: Nissan Infiniti M56, EX35, FX, EV, Mazda RX-7, RX-8 Doors: Nissan Infiniti FX, EV |
| – | – | – | Engine hood: Lincoln, Jaguar, Land Rover |
| Engine hood: Shanghai GM Buick LaCrosse, Regal | Engine hood: Shanghai GM Buick GL8 | Engine hood, doors, and rear hatch: FAW Audi A6 | BMW 5 Series – Engine hood inner and outer panels, front door inner and outer panels. |
| Engine hood: SAIC Roewe 750 | Engine hood: SAIC Roewe 950 |
Stamping process requirements for aluminum alloy sheet metal
Forming and mold process requirements for aluminum alloy sheet metal.
Through research, the forming process for aluminum alloy is consistent with that of general cold-rolled sheet metal. By adding additional processes, the amount of waste tooling can be reduced, and aluminum scraps can be reduced.
However, the mold requirements for aluminum alloy sheet metal differ from those for general cold-rolled sheet metal. Table 1 shows the special requirements of stamping molds for aluminum sheet metal.
Table 1: Mold requirements for aluminum sheet metal
| Process steps | Special requirements |
| Drawing mold | The working surface of the drawing mold should be polished and hard chrome-plated. The hardness requirements for the mold: external plate HRC70, internal plate HRC55. There are many additional processes for the drawing ribs, which are made of round bars. The R angle of the drawing ribs is larger than that of the steel parts. The slip line of the aluminum parts is qualified when on the rounded corner, and should not exceed 18mm. The coloring of the retaining ring should be better than that of the steel parts. The sandpaper used for polishing the aluminum parts must reach a smoothness level equivalent to a 2000-grit sandpaper. |
| Trimming mold | The cutting edge of the aluminum scrap knife is designed with a disconnected structure to minimize debris. The upper die trimming blade is set at an angle of -2°, with a trimming depth of approximately 5mm and a base depth of 8mm. A stepped punch is used to reduce debris. The use of scrap knives should be minimized during trimming, while considering the sharpness of the blade. The gap for trimming aluminum parts should be slightly larger than that for steel parts, with a material thickness of 10%-12%. The width of the pressing plate should be 10-15mm. |
| Flanging and forming mold | The width of the flange pressing plate should be 20-25mm. The gap for flanging should be 95% of the material thickness. For aluminum flanging, the requirements for the insert block are high. The surface of the insert block must not have sand holes or air holes, and the surface finish must be good without obvious scratches, otherwise, it is easy to produce debris (especially at the corner radius). |
Precautions for long-term storage of aluminum alloy sheets
The yield strength of the aluminum sheet increases after failure hardening, and the machinability of the edge processing decreases. When making molds, consider using upper limit materials that meet the specifications, and conduct feasibility confirmation before production.
The drawing oil/anti-rust oil used in production is easy to volatilize. The sheet should be used immediately after opening the packaging, or cleaned and oiled before stamping.
The surface of the sheet is vulnerable to oxidation and must be specially managed (packaged) and cannot be left out in the open.
Requirements for welding aluminum stampings:
The main processes for welding aluminum alloy car bodies include resistance welding, CMT cold transition welding, tungsten argon arc welding, riveting, punching and polishing.
No-rivet connection process for aluminum alloy sheets
No-rivet connection of aluminum sheets is accomplished by using pressure equipment and dedicated molds to cold-extrude 2 or more layers of metal sheets into a certain shape, and then form an internally embedded connection point with a certain tensile and shear strength.
The thickness of the connecting sheets can be the same or different, and can have adhesive layers or other intermediate layers, and the material can be the same or different.
The connection effect is good, and auxiliary connection components are not needed.
Punch-riveting connection process for aluminum alloy sheet
The punch-riveting connection of aluminum sheets, as shown in Figure 2, uses punch-riveting equipment to press punch-rivet pins into the connecting material, forming a stable, reliable, and beautiful connection point.
Characteristics of the punching and riveting process
- High strength: The shear, tensile, and dynamic fatigue strengths of the riveted joint are higher than or equal to those of the spot welded joint, and its impact resistance is also stronger.
- Visual inspection of the quality of the joint: The rivet is completely inserted into the workpiece, and at the same time, a protruding contact point is formed on the other side of the joint, which is considered to be a qualified connection.
- The material to be joined does not need to be drilled in advance, and the process is simple.
- The joint is sealed and will not be affected by gas or liquid, which may cause material corrosion.
- Various types of metal and non-metallic materials with different thicknesses, strengths, and properties can be joined together.
Resistance Welding
Currently, aluminum alloy resistance welding generally uses medium-frequency or high-frequency resistance welding processes. The welding process melts the base metal within a very short time frame by generating a molten pool within the range of the diameter of the welding tongs/electrodes.
The welding point cools rapidly to form a connection, resulting in a minimal likelihood of generating aluminum-magnesium dust. The welding fumes produced are mostly oxide particles of the metal surface and surface impurities.
Local exhaust is provided at the workstations during the welding process, which can promptly and effectively remove these particles from the atmosphere through the duct, thereby producing practically no deposition of aluminum-magnesium dust.
CMT Cold Transition Welding and Argon Arc Welding
Due to the protection of non-reactive gases, these two welding processes do not allow sufficient contact between the molten metal and oxygen in high-temperature conditions.
Under the effect of the arc jet, they can produce smaller aluminum-magnesium metal particles that splash onto the work environment, causing the deposition of aluminum-magnesium dust.
The risk of aluminum-magnesium dust explosion exists, and dust explosion prevention and treatment work should be done.
Process requirements for aluminum alloy stamped parts on the roller
The difference between aluminum alloy roller bonding and cold-rolled sheet roller bonding is significant since aluminum is less ductile than steel.
Therefore, the bonding force during the rolling process should not be too great, and the rolling speed is relatively slow, generally 200-250mm/s. The bonding angle of each step cannot exceed 30 degrees, and V-shaped bonding is not allowed.
Temperature requirement for aluminum alloy bonding: it should be performed at room temperature of 20℃. Parts taken directly from the freezer should not be bonded immediately through roller bonding.
Aluminum alloy stamped sheet metal roller edge types and characteristics
Roller edge types for aluminum alloy stamped sheet metal
Ordinary bonding is divided into three steps: initial pre-bonding, secondary pre-bonding, and final bonding. It is usually used when there is no special strength requirement, and the flange angle of the outer plate is normal.
European-style roller bonding, as shown in Figure 3, is divided into four steps: initial pre-bonding, secondary pre-bonding, final bonding, and European-style bonding, which is usually used for long-edge bonding.
All internal parts are repeatedly flushed by the working solution in the groove. All grooves are equipped, such as the front cover and back cover. European-style bonding can also be used to reduce or eliminate surface defects.
Characteristics of Roller Edges for Aluminum Alloy Stamped Sheet Metal:
The bottom die and block of the aluminum press equipment should be periodically polished and maintained with 800-1200 grit sandpaper to ensure that there are no aluminum shavings on the surface.
Various Reasons for Defects Caused by Roller Edges on Aluminum Parts:
The various reasons for defects caused by roller edges on aluminum parts are shown in Table 2.
Table 2: Various Reasons for Defects Caused by Roller Edges on Aluminum Parts.
| Defects caused by crimping | Causes |
| Inner and outer panel surfaces are mismatched | The relative position of the inner and outer panels can cause surface defects. |
| Wear of crimping bottom die | If the bottom die is smaller than the workpiece or if it is dirty, it can cause surface wrinkles or dents. |
| Poor pre-crimping angle or final crimping angle | This can cause the flange edge after crimping to flash out or not fully cover the inner panel. |
| The outer panel flange is too long | Interference with the feed plate, resulting in warping of the outer panel flange edge after crimping, and unnecessary wear on the feed plate. |
| Defects in glue application or insufficient crimping leading to the formation of small and dense air bubbles. | After glue application, empty spaces or insufficient crimping can cause small and dense air bubbles. |
The Technological Requirements for Coating Aluminum Alloy Stamped Sheet Metal Parts
The Principles and Functions of Washing and Passivating Aluminum Alloy Stamped Sheet Metal Parts.
Water wash and passivation refer to the process of removing the naturally formed oxide film and oil stains from the surface of the aluminum parts, and then wrapping a dense oxide film on the surface of the parts through the chemical reaction between the aluminum alloy and the acidic solution.
The oxidation film, oil stains, and adhesive used in stamping will all have an impact on the surface of the aluminum parts.
In order to improve the adhesion of the adhesive and weld, chemical processes need to be employed to maintain the long-term adhesive connection and electrical stability of the surface, achieving better welding results.
Therefore, parts that require laser brazing, cold metal transfer welding (CMT), and other welding methods require water washing and passivation.
The Process Flow of Water Washing and Passivating Aluminum Alloy Stamped Sheet Metal Parts.
The equipment for water washing and passivation consists of degreasing area, industrial washing area, passivation area, clear water washing area, drying area, and exhaust system.
The aluminum parts to be processed are loaded into a cleaning basket and fixed, then lowered into the tank.
Usually, solvent is used in a tank for long-side pressing, and all the working solutions in the tank are repeatedly flushed for internal parts.
All tanks are equipped with circulating pumps and nozzles to ensure that the tank liquid can uniformly rinse all parts.
The process flow for water washing and passivation is: degrease 1 → degrease 2 → wash 2 → wash 3 → passivate → wash 4 → wash 5 → wash 6 → dry. Aluminum castings can omit wash 2.
The Drying Process for Water Washing and Passivating Aluminum Alloy Stamped Sheet Metal Parts.
The time required for the part temperature to rise from room temperature to 140℃ is about 7 minutes, and the minimum time for curing the adhesive is 20 minutes.
The aluminum parts are heated from room temperature to the holding temperature for about 10 minutes, and the aluminum is kept at this temperature for about 20 minutes.
The cooling time from the holding temperature to 100℃ of the parts is about 7 minutes. After the holding is finished, it is cooled to room temperature. Therefore, the overall drying process of the aluminum parts is 37 minutes.
Conclusion
Modern automobiles are developing towards lightweight, high-speed, safety, comfort, low cost, low emissions, and energy efficiency. The development of the automotive industry is closely related to energy, environmental protection, and safety.
With the strengthening of environmental awareness, aluminum alloy sheet metal has incomparable advantages in cost, manufacturing technology, mechanical performance, and sustainable development compared to other lightweight materials.
Therefore, aluminum alloy will become the preferred lightweight material in the automotive industry.
