Abstract: This paper studies and solves two key technologies in the localization of brake pads for Audi car disc brakes. First, combining regional resource advantages, carbon fiber asbestos-free composite friction material with performance superior to foreign friction materials was successfully developed. In addition, the fine stamping process of brake pad backing plate was developed. The results of this study lay a solid foundation for the localization of brake pads for Audi car disc brakes. Keywords: disc brake, brake pad, carbon fiber composite friction material, backing plate, fine stamping 1 Introduction Disc brakes are important actuators in the automotive braking system and are now widely used in cars. The braking element in the friction pair of this type of brake is the brake pad composed of friction pads (brake pads) and metal backing plate, and the friction pad is made of friction material. For more than half a century, asbestos-based friction materials have been used for automotive brakes. Asbestos friction materials have a short service life and are prone to "thermal fade" [1], especially asbestos dust pollutes the environment and harms human health. Therefore, in the past 20 years, countries around the world have restricted the production and use of asbestos friction materials and have begun to research and use a new generation of composite fiber reinforced friction materials to manufacture friction blocks for automobile brakes. In view of this development trend, and leveraging the relatively abundant carbon fiber resources in Jilin Province, we have conducted research and development on key technologies for the localization of brake blocks for Audi disc brakes. 2. Development of Carbon Fiber Composite Material Automobile Brake Friction Blocks The developed carbon fiber composite friction material is a multi-component system composed of reinforcing fibers, binders, and friction performance modifiers (fillers). Using orthogonal optimization design methods, the optimal formulation of the material components was determined, giving the friction material good comprehensive performance. After mixing, hot pressing, and heat treatment, it was pressed into friction blocks for disc brakes. 2.1 Determination of material formulation 2.1.1 Selection of materials (1) Selection of reinforcing fiber Due to the excellent comprehensive properties of carbon fiber: light weight, high specific strength and specific modulus (5 times and 7 times that of metal, respectively), good thermal conductivity, good wear resistance, and good high temperature resistance (the performance is very stable at less than 400℃)[2]. Therefore, carbon fiber, which is rich in resources in Jilin Province, was selected as the reinforcing fiber of the friction material to be developed. (2) Selection of binder The role of binder is to combine multiple components into a whole to transfer load and balance the load. Phenolic resin is usually used as binder because phenolic resin has certain advantages in terms of heat resistance, molding processability and economy. However, unmodified phenolic resin has high hardness and brittleness. Its extreme heat resistance temperature is about 250℃. When it exceeds 300℃, it decomposes quite severely, which reduces tensile strength and impact resistance, making the product prone to braking noise and slippage. At the same time, due to its high elastic modulus, the product does not fit well with the friction pair, which easily causes serious local overheating and thermal cracking. Therefore, modified phenolic resin is used in practical applications. Based on our years of experience in studying asbestos and semi-metallic friction materials, we selected phenolic resin modified with nitrile rubber (FH-908) as the binder. This binder has the advantages of high temperature resistance, good fatigue resistance, resistance to damp heat aging, resistance to atmospheric aging, and good resistance to media. (3) Friction performance modifiers The role of adding various friction performance modifiers (fillers) is to solve the problem of stable friction coefficient and wear resistance of friction materials under various conditions, and to improve thermal decay. The following principles should be followed when selecting fillers: the hardness of the filler should not be greater than the hardness of the mating surface; the melting point should be lower than the fusion temperature of the mating materials. The actual filler composition used is as follows: 1) Metal powder: It is mainly used to improve thermal conductivity, reduce wear and maintain the stability of high temperature friction coefficient. The metal powders we selected are copper powder (particle size 30-80 mesh) and HT200 cast iron powder (particle size 40-60 mesh). 2) Solid lubricant: It can improve the stability of friction coefficient and reduce noise. In this study, graphite and carbon black were used as solid lubricants. 3) Friction agent: Although solid lubricant can reduce material wear and make friction material work stably, it reduces the friction coefficient. Therefore, friction agent needs to be added in order to improve the friction coefficient. In this study, Al2O3, SiO2 and chromite powder were used as friction agents [3]. 2.1.2 Determination of friction material formulation In the research and development process, the determination of friction material formulation is carried out in two steps: the first step is to propose a preliminary formulation; the second step is to optimize the design. Finally, a reasonable formulation is determined. (1) Preliminary formulation By consulting relevant domestic and foreign materials and combining our experience in developing semi-metallic brake pads [3], the original formulation of carbon fiber composite friction material was proposed. The L9 (34) orthogonal table, that is, the four-factor three-level orthogonal table, was used for screening and evaluation. After two rounds of adjustment and testing, it was used as the preliminary formulation for the study. The preliminary formulation is as follows (wt%): Carbon fiber 16-25, Adhesive 11-17, Friction agent (SiO2+Al2O3) 4-10, Metal powder (cast iron powder) 19-28, Solid lubricant (graphite+carbon black) 10-19, Fixed component 10. (2) Final formulation determination In order to obtain the optimized formulation with fewer experiments and to understand the main and secondary factors affecting friction and wear performance, we used an L16 (45) orthogonal array of five factors and four levels for screening and analysis. At the same time, wear tests were conducted using an MM-200 friction and wear tester, and the optimization level of the components and the order of influence of the components were analyzed according to the friction coefficient and volumetric wear amount. Thermal decay tests were also conducted on a D-MS tester. Based on the test results, the optimization level of the components and the order of influence of the components were analyzed according to the friction coefficient. The optimization level of the components and the order of influence of each factor on the wear rate were also analyzed according to the volumetric wear amount. Finally, based on a comprehensive analysis of the above experimental results, the optimal formulation range (wt%) for carbon fiber composite friction materials with good friction and wear performance is as follows: Friction agent (Z1) 6-8, Carbon fiber (Z2) 16-19, Binder (Z3) 13-15, Metal powder (Z4) 22-25, Solid lubricant (Z5) 10-13. To reduce costs and expand the application range, based on the above experiments, cheaper pre-oxidized carbon fiber (8mm in length) was selected for experimental research. Based on multiple rounds of experiments, an economical formulation for actual production was determined. Its composition range is: Z1: 8%-10%; Z2: 19%-22%; Z3: 15%-17%; Z4: 22%-25%; Z5: 13%-16%. 2.2 Friction and Wear Performance of Carbon Fiber Composite Friction Blocks 2.2.1 High Coefficient of Friction Due to the high specific strength and specific modulus of carbon fiber, and its self-lubricating properties. When the carbon fiber content is 19% to 22%, the friction material has a high coefficient of friction and a low volume wear rate. 2.2.2 Good resistance to thermal decay. Under high temperature friction, the carbon fiber composite friction material has increased resistance to ploughing, showing a large coefficient of friction at high temperature and a thin thermal decomposition layer, which can keep the friction and wear performance stable at very high temperatures and has good resistance to thermal decay. The carbon fiber composite friction block developed was tested by the National Automotive Quality Supervision and Inspection Center, and its technical performance indicators met the national standard GB5763-1986. Its service life is 2 to 3 times that of asbestos friction material[3]. 3 Fine stamping process of friction block backing plate The product picture of the friction block backing plate of Audi car disc brake is shown in Figure 1. According to the technical conditions of Lucas.Girling Company of the United Kingdom and the supply standard of Volkswagen (TL-VW-110), the characteristics and technical requirements of the backing plate are shown in Table 1. [align=center] Figure 1 Product drawing of friction block backplate for Audi car disc brake[/align] [align=center] Table 1 Characteristics and technical requirements of backplate parts[/align] Given that the backplate has high precision requirements and large usage, the fine stamping process has significant economic benefits. 3.1 Fine stamping process design and calculation According to the product drawing and technical requirements of the backplate, domestic steel 35 (GBT699-1999) material is used, and the steel plate thickness is t=5±0.2mm. The main mechanical properties of this material are: tensile strength σb= 530MPa, and the hardness in the annealed state at the time of delivery is 197HB. The blanking force is calculated by the following formula W =1.2 Lτ t (1) Where: W——blanking force, kN; L——circumference of the inner and outer contours of the blanked part, mm; τ——shear strength of the sheet, MPa; t——thickness of the sheet, mm. Substituting the relevant values ​​into formula (1), we get W=1250kN. The blank holder force is calculated by the following formula: P2=KL1＊2h＊σb. Where P2 is the blank holder force, kN; K is the coefficient, K=1.6; h is the height of the V-shaped tooth, h=0.8mm; σb is the tensile strength of the sheet, σb=530MPa. The blank holder force and counterforce of the strong tooth ring are calculated as follows: P2=200kN. Combining the above two items, the total tonnage of the blanking force is about 150t. The overlap value of the blanking is determined according to the reference [4], where the overlap value between workpieces is b=8mm and the outer overlap value is a=6mm. The blanking process is arranged as follows: hot-rolled steel plates of GB3275－1991 are used. After the shearing machine cuts the material, pickling is performed. The compound die is used for fine blanking on the Y26-630 domestic hydraulic fine blanking press. 3.2 Design and manufacturing points of fine blanking die To obtain a back plate stamping part with a flat surface and a flat outer contour, a fixed punch pattern compound die as shown in Figure 2 is adopted. The JD003-82 series fine blanking die set is selected for this die to improve the quality of the die and shorten the processing cycle [5]. Since the blanking plate is relatively thick and is a medium hardness steel plate, the main working parts of the die are made of Cr12MoV steel, and the quenching hardness is 60~64HRC. The blanking process clearance is Z=1.0t%, which can achieve a better cross-sectional quality. As shown in Figure 3, a single-sided V-shaped toothed ring structure is adopted [4], and the tooth inner angle α = 40°. [align=center] Figure 2 Back plate fine blanking compound die[/align] [align=center] Figure 3 V-shaped pressure edge toothed ring[/align] 4 Conclusion The brake pad of the Audi car disc brake is a key component with high technological content and long service life. According to Volkswagen's supply standard TL-VW-110, the friction pads are made of asbestos-free friction material, and the backing plate is a precision-stamped medium-hard steel part. Currently, the brake pads for domestically produced Audi disc brakes have been supplied with imported parts from Lucas Green GmbH in the UK. This research, addressing this background and fully utilizing the abundant carbon fiber resources in Jilin Province, independently researched and developed a precision-stamping process for carbon fiber composite asbestos-free friction pads and backing plates. This solved two key technical problems in the localization of this product, laying the foundation for the localization of brake pads for Audi disc brakes.