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Abstract: The structure and performance characteristics of 473QB for automotive engine crankshaft main bearing caps were analyzed, and powder metallurgy materials were used to replace cast iron or ductile iron, reducing production costs. We have developed a reasonable development plan, determined the material formula, production process flow, and designed the mold. By conducting metallographic analysis of the product through trial production, the powder metallurgy process and mold structure have been improved, and a powder metallurgy crankshaft main bearing cover that meets the requirements and can withstand complex loads has been successfully developed.
Keywords: powder metallurgy; Main bearing cover; Materials; workmanship mould
With the application of new materials, technologies, and processes, powder metallurgy technology is constantly expanding into high-level new fields. Especially in the field of iron-based products, with the improvement of production equipment and processes, products have further developed towards high-density, high-strength, and complex shaped structural components, which has led to the rapid development and application of powder metallurgy technology in the manufacturing of automotive parts [1-3]. Figure 1 shows a 473QB crankshaft main bearing cap produced by a certain automotive company using imported technology. It serves to fix the crankshaft and withstand impact loads, with high accuracy and performance requirements. It is an important irregular powder metallurgy structural component. Its accuracy and performance directly affect the performance and lifespan of the host. On the basis of analyzing the accuracy and structural performance of the parts, a reasonable development plan has been formulated based on the situation of China's raw material resources and process equipment. After multiple experiments, the powder metallurgy main bearing cap was finally localized, with a material utilization rate of up to 98%, which has significant economic and social benefits.
Figure 1 473QB crankshaft main bearing cover
1. Product analysis and material technology
When the engine is running, the crankshaft converts the reciprocating motion of the piston into rotational motion, thereby driving the wheels to rotate. The main bearing cap should not only keep the crankshaft in place safely, but also be able to rotate freely, while also bearing the combustion stress and alternating load of the engine. Traditionally, this part was produced by cutting cast iron or nodular cast iron. However, due to the small cutting volume, material saving, and energy saving of powder metallurgy, production costs have been greatly reduced. Therefore, powder metallurgy has been used to replace cast iron or nodular cast iron.
As shown in Figure 1, this product is different from ordinary bearing covers in terms of appearance and structure. Its appearance and structure are more complex, with the smallest size being only 0.3mm, and there are circular corners with R=0.1mm on both sides; High geometric and positional accuracy, with some dimensions having an accuracy of ± 0.1mm. Therefore, the difficulty of mold manufacturing is relatively high. After analysis and research, it has been preliminarily determined to adopt a combination mold structure of one up and two down to avoid damage to weak points during pressing.
According to usage requirements, the main bearing cap should have good comprehensive mechanical properties and high accuracy. The physical and mechanical performance requirements of this product are as follows.
1) Density ≥ 6.5g/cm3.
2) Hardness: 45-90HRB.
3) Magnetic testing, no cracks allowed, demagnetization after testing.
4) The surface of the parts shall not have any bumps, burrs, pores, looseness or other defects.
The Fe-C-Cu powder metallurgy material was selected based on the Japanese material standards JMPA1-1970 and GB/T14667.1-1993. In order to ensure product quality, water atomized 300W iron powder with good compressibility and formability was selected, and its properties and chemical composition are shown in Tables 1 and 2.
Adding alloy element carbon can significantly increase the mechanical strength of products; The addition of copper further strengthens the physical and mechanical properties of the product, and can effectively suppress the sintering shrinkage of the product; Adding manganese sulfide can improve the processing performance of products. The performance of auxiliary raw materials is shown in Table 3, and the material ratio is shown in Table 4.
According to different ratios of materials, three experimental formulas were selected to press various test bars according to GB2865-86 and GB2866-86 under the same process conditions, sintered, and then subjected to strength testing, hardness testing, and metallographic analysis. Simultaneously pressing product samples, sintering under the same conditions, testing size change patterns, shape and position accuracy, and hardness. Select materials with good performance and small size changes from them, and prepare new samples and samples for testing. After installation testing, determine the product material and production process.
2. Production process flow
The production process of the main bearing cover is as follows: mixing powder → pressing → sintering → deburring → chamfering → polishing → cleaning → rust prevention treatment → inspection → packaging.
1) Mixing powder: Weigh the main and auxiliary materials according to the formula, add them in a certain order into a 1000kg double cone mixer, and mix for 1.5 hours to make a mixed powder.
2) Pressing: Press on a 4000kN mechanical fully automatic powder product press, with a pressing rate of 10 pieces/min. The volumetric method is used for powder loading, and the female mold, core rod, and lower inner punch are floating and pressed, using a pull-down demolding method. The density of the compact is 6.7-6.8g/cm3.
3) Sintering: Use a push rod or mesh belt sintering furnace for sintering. The furnace temperatures of the pre firing section, high temperature section, and insulation section are 8001140850 ℃, respectively. The residence time of the high temperature section is 1 hour, and the boat is pushed every 6.5 minutes. The cooling rate is about 260 ℃/h. The protective atmosphere is decomposition ammonia, with a dew point of -40 ℃.
4) Rust prevention treatment: Treat with special rust prevention liquid and dry.
3. Mold design and manufacturing
The preliminary design of the pressing mold structure is shown in Figure 2. Based on the product accuracy and performance requirements, as well as the characteristics of the press, the mold design should be reliable in action, convenient in use, and have a long service life. Therefore, the preliminary design of this mold is a combination structure of one upper and two lower parts, mainly composed of upper punch, female mold, lower punch (2 pieces), core rod (2 pieces), and some pads and covers. The mold material, except for the cushion seat, is made of Cr12MoV and the product is pressed on a 4000kN mechanical press. In addition, considering the large size of this part and the corresponding large size of the mold, strict dimensional and positional tolerances are required. Therefore, precision wire cutting machines, electric discharge machining machines, lathes, grinders, and other equipment are used to process the mold, and detailed processing techniques are developed to ensure the quality of the mold. During design, the strength of the main components should also be checked.
4. Trial production of products
4.1 Metallographic analysis of samples
According to the metallographic analysis of the sample, the microstructure of the main bearing cover is pearlite+ferrite+free copper+graphite. Adding copper to form liquid phase sintering, which plays a solid solution strengthening role; Copper can strengthen ferrite. It can be seen that iron based sintered materials are mainly made of carbon, and adding copper on the basis of improving strength and hardness can further improve their physical and mechanical properties. During the development process, a small batch of tests were conducted, and after bench testing and vehicle testing, all performance indicators met the design requirements and were recognized by users. Afterwards, mass production was carried out, and the product quality was stable and reliable.
Figure 2 Pressing Mold Structure (One Up, Two Down)
4.2 Sample reflects mold design issues and improvement measures
1) Cracks appeared at the straight edge of the part during pressing, which were analyzed to be caused by the sharp straight edge of the mold cavity during demolding. Therefore, the straight edge of the mold cavity was trimmed (2 ° to 3 °) × 15mm conical surface, cracks eliminated during re pressing. 2) Due to the fact that the part has three bosses with different cross-sectional thicknesses, using a combination of one up and two down method for pressing makes it inconvenient to adjust the mold during pressing, and the density of the three steps is different. The thick part of the steps has a lower density, and there is a density boundary zone on the transition surface of the part from thick to thin, making it obvious that there is a strength boundary zone in the strength of the part. When parts are working, the density boundary zone will first crack due to fatigue, leading to part damage. Therefore, it was decided to use a mold structure of one up and three down for pressing to ensure uniform and consistent component density, meeting the quality requirements of the product. The improved pressing mold structure is shown in Figure 3.
Figure 3 Improved Pressing Mold Structure (One Up, Three Down)
The working process of the improved one up and three down pressing mold is as follows: first, the female mold is filled and moves upwards, and the core rod seat moves upwards with the female mold on the fixture. Lower plate one (the template where lower punch one is located) and lower plate two (the template where lower punch two is located) move upwards at the same time, moving to each self set position. The feeding box feeds the material, and the feeding is completed. At this time, the upper slider moves downwards and enters the mold cavity. After entering the mold cavity for a certain distance, the mold cavity begins to float, and the down punch 1 and down punch 2 also begin to move downward under the pressure of the upper slider. When the upper slider reaches 155 °, the slider of the lower plate 1 is pressed onto the forming block. At this point, the powder no longer undergoes lateral movement and only moves downwards to prepare for formation. When the upper slider moves to 165 °, the air pressure resets, the downward air pressure opens, and the upper slider continues to move to 180 ° to form. Subsequently, the demolding action of the machine tool begins. After the female mold detaches the parts, it continues to demould downwards. At this time, the demolding top rod under the female template pushes against the inclined wedge of the lower plate 1. The inclined wedge opens the slider of the lower plate 1, and the lower plate begins to demould. At the same time, the demolding top rod of the lower plate 2 on the negative template also repeats the demolding action of the lower plate 1, pulling down the lower plate 2 to separate its formed part from the punch. When the female mold is demolded to 270 °, the upper slider returns, the core rod is demolded, and the demolding is completed.
5. Conclusion
The 473QB main bearing cap product developed through trial production has been inspected by Nanjing Dongmu Powder Metallurgy Co., Ltd. and the host supporting factory. The geometric dimensions, shape and position accuracy, appearance, and various physical and mechanical performance indicators have met the design requirements and have been recognized by users. It can be seen that the mold design of the main bearing cover is reasonable, the production process is reliable, the product quality is stable, the production efficiency is high, and the cost is low, providing a good reference for the future development of similar products.
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2023-08-28