Titanium bolts, titanium screws manufacturing process and the development direction of automobile bolts

Bolts and screws are the most basic mechanical components and are used in various industries such as automobiles, industrial machinery, and construction. Unlike welding and riveting, the biggest feature of bolts is that they can be easily removed even after tightening, and can be reused by retightening.

     1 Production process The material for bolts is generally coiled wire. After secondary processing such as pickling, lubrication, annealing and wire drawing, the screw head and thread of the bolt are formed, heat-treated and surface-treated by cold forging. Depending on the type of product, some require heat treatment before thread forming, while others do not require heat treatment or surface treatment. Secondary processing is generally handled by professional handlers, but sometimes it is done in bolt making plants.

     1.1 Forming of the screw head Generally, the process of cold forging and forming the bolt head by using a forging machine such as a top heading machine or a jacking machine is called "top heading processing". The so-called cold forging is processing at room temperature, which is the opposite of warm processing and hot processing after the billet is heated. The forging process is to cut the coiled steel into an appropriate length, and use several forging die devices to forge the material. Forging includes four processes including upsetting, deep drawing, reverse extrusion and trimming. Upsetting is the process of squeezing the material from one end to make it expand to a larger diameter than the original; deep drawing is the opposite, that is, the process of squeezing the material from one end to shrink it to a smaller diameter than the original; reverse extrusion is Extrusion from the end surface of the material into a forging die smaller than the diameter of the material, so that the material is extruded outward while piercing; trimming is the process of removing excess thickness with a forging die. Depending on the shape of the processed product, these processing methods can be used separately or in combination for forming. The more complicated the shape of the product, the more processing steps are required to slowly carry out the forming process, but ordinary bolts can be processed and formed in only 2-5 processing steps.

     1.2 Forming of the thread part In the thread forming process of the bolt, a roll forming machine is used to cold forge the thread, and the forming method is the same as that of the bolt head. To process the thread shape, the blank is clamped between two forging dies in a set, and one of the two forging dies is rotated while the blank is rotated to form a thread shape through plastic processing. Forging dies used for thread forming are called "roll forming dies". There are three types of roll forming machines: flat forging die roll forming machine, round forging die roll forming machine and planetary roll forming machine. The flat forging die roll forming machine is equipped with two common roll forming forging dies, one of which is fixed and the other is moved back and forth to roll form the billet. The circular forging die roll forming machine rotates two cylindrical roll forming forging dies installed in parallel in the same direction, and rolls the blank sandwiched between the two forging dies. The processing method of the planetary rolling forming machine is to sandwich the blank between the cylindrical forging die and the fan-shaped forging die, and roll forming by rotating the cylindrical forging die.

     1.3 Heat treatment Bolts formed by cold forging are generally made of materials with hardness suitable for plastic processing. Materials with high carbon content or materials with added alloying elements are difficult to process due to their own hardness, so some need to be annealed to soften the material. Most materials cannot meet the required strength in the case of cold forging. Heat treatment is the treatment after cold forging, which can make the bolt have the required strength and mechanical properties, and is the most important process in the bolt manufacturing process. Bolts can be divided into various strength grades according to the location and purpose of use. In order to make the formed bolts have the required mechanical properties, heat treatment is required.

     1.4 Surface treatment Bolts used in automobile engines often adhere to lubricating oil, so even without surface treatment, they will not rust. However, this kind of bolt is only a small part, and most of the bolts are used in a corrosive environment, so if there is no surface treatment, they will rust soon. If the rusty bolts are left as they are, they will not be able to be unscrewed after being corroded. In severe cases, the bolts will break and cause a major accident. Therefore, bolts used in corrosive environments require surface treatments such as electroplating. The surface treatment of bolts can be roughly divided into two types: electroplating and coating. The most widely used method is electroplating, which has the advantages of low cost and good corrosion resistance. In the case where the corrosion resistance requirement is higher than that of electroplating, alloy electroplating such as galvanized iron and galvanized nickel can be used, or zinc-aluminum composite film can be coated.

     2Development trends of bolts for automobiles In recent years, automobile manufacturers have demanded lower parts costs, lighter weight, and higher strength. At the same time, in order to cope with environmental protection issues, CO2 emissions must be reduced, and in order to reduce fuel consumption, the weight of the vehicle body must be reduced as much as possible. In the production cost of bolts, the cost of blanks accounts for the main part, so the most effective way is to reduce the cost of the material itself. In order to reduce the supply cost of materials, various Japanese automobile manufacturers are studying the use of cheap materials from overseas. In order to meet the needs of users, bolt manufacturers have also conducted various researches on reducing the weight of bolts. As measures to reduce the weight of bolts, some use light metals such as aluminum or titanium, and some reduce the bolt size. Reducing the bolt size can reduce the weight, but if the bolt size of the same strength grade is reduced, the tightening force of the bolt will decrease due to the reduction of the cross-sectional area of the bolt.

     Therefore, in order to ensure the same degree of robustness while reducing the bolt size, the strength of the bolt must be increased. In the current public standards such as JIS and the own standards of automobile manufacturers, only bolts with a strength grade of 12.9 or less are specified. When the strength grade of ordinary quenched and tempered bolts exceeds 12.9, the delayed fracture characteristics will suddenly deteriorate. In some places where high-strength grade bolts of 10.9 or 12.9 have been used, if you want to reduce the size to achieve lightweight, you must use bolts with a strength grade exceeding 12.9 and use some method to improve the delayed fracture characteristics. In order to achieve high strength of quenched and tempered bolts, some alloy elements are added to improve delayed fracture characteristics. Although the delayed fracture characteristics can be improved to some extent by adding alloying elements, an increase in cost is inevitable due to the high price of alloying elements.

     Another measure is to use non-quenched and tempered bolts. Non-quenched and tempered bolts do not need to be quenched and tempered (heat treated) after forming, and the strength is mainly ensured by work hardening of the material. The metal structure of non-quenched and tempered bolts is completely different from that of quenched and tempered bolts, and it has strong resistance to delayed fracture. Non-quenched and tempered bolts use materials with a higher carbon content than quenched and tempered bolts. Through controlled cooling and heat treatment in the blank stage, a high area reduction rate can be obtained. Work hardening ensures the strength of the bolt. Ordinary bolts are not quenched and tempered after forming, but they must be blued in order to eliminate strain inside the bolts by cold forging. The disadvantage is that the hardness of the material is higher than that of the quenched and tempered type, and it is very difficult to form. At present, the strength of the non-quenched and tempered bolts used for the engine is 1600MPa, and the strength of the non-quenched and tempered bolts used for the body has reached 1400MPa. It can be expected that the demand for such high-strength bolts will further expand in the future.
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