Casting is a production method in which molten metal is poured into the cavity of a mold and cooled and solidified to obtain a product. In the automobile manufacturing process, there are many parts made of cast iron blanks, which account for about 10% of the total vehicle weight, such as cylinder block, transmission case, steering gear housing, rear axle housing, brake drum, various brackets, etc. . Sand molds are usually used to manufacture iron castings. The raw material of the sand mold is mainly sand, mixed with a binder, water, etc. The sand material must have a certain adhesive strength so that it can be molded into the required shape and can withstand the erosion of high-temperature molten iron without collapsing. To mold a cavity in the sand mold that matches the shape of the casting, wood must be used to make a model, which is called a wooden mold. The volume of the hot molten iron will shrink after cooling. Therefore, the size of the wooden mold needs to be increased according to the shrinkage rate based on the original size of the casting, and the surface that needs to be cut is thickened accordingly. Hollow castings need to be made into sand cores and corresponding core wooden molds (core boxes). With wooden molds, cavity sand molds can be turned (casting is also called "sand foundry"). When making sand molds, it is necessary to consider how to separate the upper and lower flasks to take out the wooden mold, and also consider where the molten iron flows in and how to fill the cavity to obtain high-quality castings. After the sand mold is made, it can be poured, that is, the molten iron is poured into the cavity of the sand mold. When pouring, the temperature of molten iron is 1250-1350 degrees, and the temperature is higher during smelting.
In the automobile manufacturing process, forging is widely used. Forging is divided into free forging and model forging. Free forging is a processing method in which a metal blank is placed on an anvil and subjected to impact or pressure (known as "ironing"). The blanks of automobile gears and shafts are processed by free forging. Model forging is a processing method in which metal blanks are placed in the cavity of a forging die and subjected to impact or pressure. Model forging is a bit like the process of pressing dough into a biscuit shape in a mold. Compared with free forging, the shape of the workpiece produced by die forging is more complicated and the size is more accurate. Typical examples of automobile die forgings are: engine connecting rods and crankshafts, automobile front axles, steering knuckles, etc.
Three, cold stamping
Cold stamping or sheet metal stamping is a processing method in which sheet metal is cut away or formed under pressure in a die. Daily necessities, female aluminum pots, lunch boxes, washbasins, etc. are made by cold stamping processing methods. For example, to make a lunch box, you first need to cut out a rectangular blank with 4 rounded corners (known as "blanking" by experts), and then use a punch to press the blank into a concave mold to form (known as "drawing" by experts) ). In the deep drawing process, the flat sheet material becomes a box shape, and its four sides are vertically bent upwards, and the material at the four corners is piled up and wrinkles can be seen. The auto parts processed by cold stamping include the engine oil pan, brake bottom plate, car frame, and most body parts. These parts are generally formed through processes such as blanking, punching, deep drawing, bending, flanging, and trimming. To manufacture cold stamped parts, a die must be prepared. The die is usually divided into 2 pieces, one of which is installed above the press and can slide up and down, and the other is installed under the press and fixed. During production, the blank is placed between two punching dies. When the upper and lower dies are closed, the stamping process is completed. The productivity of stamping processing is very high, and parts with complex shapes and high precision can be manufactured.
Welding is a processing method in which two pieces of metal are locally heated or heated and pressurized at the same time to join them together. We often use the welding method in which workers hold the mask in one hand and the welding tongs and the welding rod connected to the wire in the other hand is called manual arc welding, which uses the high temperature generated by arc discharge to melt the welding rod and the welding parts to join them. Manual arc welding is not widely used in automobile manufacturing. Spot welding is the most widely used in automobile body manufacturing. Spot welding is suitable for welding thin steel plates. During operation, two electrodes apply pressure to the two steel plates to bond them together, and at the same time make the bonding point (circular with a diameter of 5-6 barrels) be heated and melted to be firmly bonded. When two body parts are welded, the edges are welded at a point every 50-100 to make the two parts form a discontinuous multi-point connection. To weld the entire car body, usually thousands of solder joints are required. The strength requirements of the solder joints are very high. Each solder joint can withstand a tensile force of 5kN, even if the steel plate is torn, the solder joints cannot be separated. The common gas welding in the repair shop is to burn acetylene and use oxygen to support combustion to generate a high-temperature flame to melt and join the welding rod and the weldment. This kind of high-temperature flame can also be used to cut the metal, which is called gas cutting. The application of gas welding and gas cutting is more flexible, but the heat-affected zone of gas welding is larger, which causes deformation and metallographic structure changes of weldments, and performance declines. Therefore, gas welding is rarely used in automobile manufacturing.
5. Metal cutting
Metal cutting is to use a tool to cut the metal blank layer by layer; the processing method to obtain the required shape, size, and surface roughness of the workpiece. Metal cutting processing includes two methods-fitter and mechanical processing-fitter is a processing method in which workers use hand tools to cut. It is flexible and convenient to operate and is widely used in assembly and repair. Machining is done using machine tools, including turning, planing, milling, drilling, and grinding methods.
1) Turning: Turning is the process of machining workpieces on a lathe with a turning tool. The lathe is suitable for cutting various rotating surfaces, such as inner and outer cylindrical or conical surfaces, as well as turning end faces. Many shaft parts and gear blanks of automobiles are processed on lathes.
2) Planning: Planning is the process of processing workpieces with a planer on a planer. The planer is suitable for processing horizontal planes, vertical planes, inclined planes, and grooves. The rhyme surface of the cylinder block and cylinder head on the automobile, the mating plane of the transmission case and the cover, etc. are all processed with a planer.
3) Milling: Milling is the process of machining workpieces with milling cutters on a milling machine. Milling machines can process bevels, grooves, and even gears and curved surfaces, and other old milling are widely used in processing various auto parts. The molds for cold stamping of car bodies are all processed by milling. The computer-operated CNC milling machine can process workpieces with complex shapes and is the main machine tool for modern machining.
4) Drilling and boring: Drilling and boring are the main cutting methods for machining holes.
5) Grinding: Grinding is the process of processing workpieces with a grinding wheel on a grinder. Grinding is a finishing method that can obtain workpieces with high precision and roughness and can grind workpieces with high hardness. Some car parts mould after heat treatment are all finished with a grinder.
Six, heat treatment
Heat treatment is a method of reheating, heat preservation, or cooling of solid steel to change its structure to meet the requirements of the use of parts or process requirements. The level of heating temperature, the length of holding time, and the speed of cooling can cause different structural changes in steel. The blacksmith immerses the heated steel parts in water for rapid cooling (experts call it quenching), which can increase the hardness of the steel parts. This is an example of heat treatment. The heat treatment process includes annealing, normalizing, quenching, and tempering. Annealing is to heat the steel parts for a certain period and then slowly cool them together with the furnace to obtain a finer and uniform structure and reduce the hardness to facilitate cutting. Normalizing is to heat the steel parts, take them out of the furnace after heat preservation, and then cool them in the air, which is suitable for refining low-carbon steel. Quenching is to heat the steel parts and quickly cool them in water or oil after heat preservation to increase the hardness. Tempering is usually the subsequent process of quenching. The quenched steel parts are reheated, kept warm, and cooled to stabilize the structure and eliminate brittleness. There are many auto parts, to retain the toughness of the core, but also to change the surface structure to improve the hardness, it is necessary to use surface high-frequency quenching or carburizing cyanidation, and other heat treatment processes.
Assembling is to connect each component according to certain requirements through connecting pieces (bolts, nuts, pins or buckles, etc.) to connect and combine them into parts, and then connect and combine the components to form a complete vehicle. Regardless of whether the parts are combined into parts or parts are combined into a complete vehicle, they must meet the cooperation relationship specified in the design drawings to make the parts or the complete vehicle achieve the predetermined performance. For example, when assembling the transmission to the clutch housing, the centerline of the transmission input shaft must be aligned with the centerline of the engine crankshaft. This centering method is not adjusted by the assembler (assembler) during the assembly process but is guaranteed by the design and manufacturing. If you visit a car factory, the most attractive thing is the assembly line. On this assembly line, we drive a car every few minutes. Take the FAW Jiefang truck assembly line as an example. This assembly line is a 165m long conveyor chain. The car moves to each station along with the conveyor chain and is gradually assembled. There is a conveying chain around, which can continuously convey the engine assembly, cab assembly, wheel assembly, etc. from each workshop to the corresponding station on the assembly line. At the beginning of the conveyor chain, first place the frame (bottom-up), and then install the rear axle assembly (including leaf springs and hubs) and the front axle assembly (including leaf springs, steering knuckles, and hubs) on the frame, Then turn over the frame to install the steering gear, air tank, and brake pipeline, fuel tank and tubing, wires and wheels, and finally install the engine assembly (including the clutch transmission and the central brake), connect the drive shaft, and then install the cab and front Panel. At this point, the car can drive off the assembly line.