The “Automotive Foundry Club” organized by the Foundry Branch of the Chinese Mechanical Engineering Society launched a series of special activities on the “Dialogue Large Integrated Die Casting” online seminar. On April 28, the first online live broadcast focused on high-end mold manufacturing. The event attracted more than 5,000 viewers to watch online, and the interactive atmosphere was warm, bringing a wonderful content presentation to the industry.
Expert Perspective
This issue of “Expert Perspectives” brings you Dr. Ye’s observations and thoughts on integrated die-casting and mold manufacturing from the Pan-Asian Automotive Technology Center.
Dr. Ye You, Senior Technical Manager of Metal Materials and Fasteners, Pan Asia Automotive Technology Center
1. Please share your observations and thoughts on large-scale integrated die casting.
A: I want to start from the source – the product design of the OEM. After all, the ideas and decisions of the OEM are very important to the development of this industry. For large-scale integrated castings, there are now many professional and non-professional introductions. Most of them talk about advantages, such as light weight, integration, streamlined industrial chain, and efficient development. I don’t want to talk about it here. Mainly talk about a few questions and my thinking.
The first point is the early acquisition of mechanical properties of large castings. Now the time from product design to mass production is getting shorter and shorter, and the early design review is more and more dependent on CAE simulation. We all know that the accuracy of material parameters is very important to the accuracy of various simulation analysis. We currently use the vast majority of body structural parts Most metal materials, such as steel plates, aluminum plates, profiles, and forgings, have various performance data measured in advance through test-plate-level experiments, which are used for CAE simulation of parts and vehicles with high confidence. However, this approach is not feasible for large die castings. The current possible approach is to enlarge the design redundancy and increase the safety factor. Is there any better workaround? In my opinion, computational materials science has established a quantitative relationship of composition-process-structure-property, and can calculate part of the mechanical properties of castings under ideal conditions. Then combined with our current casting simulation analysis software, whether it is possible to simulate the structure and defects after solidification and cooling, such as grain size, dendrite, composition distribution, inclusion, segregation, pores, shrinkage, etc. What about mechanical properties? CAE simulation analysis to support our product design. Of course, this work also requires the early intervention of materials, molds, and die-casting processes, but the core still requires software that can develop such computing functions with sufficient accuracy.
The second point is the cost of large castings. As far as the cost of a single piece is concerned, the current large-scale integrated castings do not have a cost advantage. Taking the rear floor of a medium-sized car as an example, the large casting weighs 70KG. Calculated at a price of 55 yuan per kilogram, the cost of a single piece is about 3850 yuan. The same part is designed with traditional steel plate stamping and welding, weighing about 100KG, plus various welding connections. The cost is about 2,000 yuan for a single assembly. If you consider the difference in mold life and pass rate, the price difference per piece will be larger. But I still have confidence in the reduction of the price of large castings. On the one hand, the price of die-casting structural parts has dropped significantly in recent years. With the entry of a large amount of investment, competition has intensified, and there should be room for cost reduction in the future. . On the other hand, from the perspective of part function, many parts of large castings do not need a thickness of 2-3mm. It is expected that with the improvement of material casting performance, die casting equipment, and technological level, the wall thickness can be made thinner, which reduces costs. more imagination.
The third point is the after-sales maintenance of large castings. Some time ago, it was reported by the media that the model Y damaged the large casting in a reverse collision, and the replacement cost exceeded 200,000 yuan, which aroused the public’s concern about the after-sales of the large casting. I think Tesla chose the rear floor as the entry point for large castings. After-sales maintenance should be considered a consideration. After all, the probability of rear-end maintenance, especially overhaul, is relatively much smaller. But if the local damage needs to be repaired, is there a better solution than the overall replacement? At present, the aluminum alloy 3D printing technology is becoming more and more mature, and the strength elongation of many products has reached or even exceeded the current die-casting products. Combined with suitable welding methods, such as CMT cold metal transfer welding technology, the welding heat input is small and can be better. It may be a relatively economical and reliable solution to avoid the influence on the structure and properties of the original casting, and it is worthy of further study.
2. Compared with traditional multi-piece metal parts spliced into large parts, what are the advantages and disadvantages of integrated die castings?
A: The first thing to point out is that we do not go to the myth of this product. Integrated casting is only one of the many design schemes for body parts, and it does not bring new functional improvements to customers. Its biggest advantage may lie in the shortening of product development cycle and industrial chain, and the halo of high technology. Another advantage is that the casting process can realize the design of almost any shape and feature, which also makes it possible to integrate parts and components, which is also not available in other existing manufacturing processes that can achieve economical mass production scale. Everyone pays attention to the advantages here, and I do not emphasize light weight, because from a professional point of view, the weight efficiency of cast aluminum is not high, and the aluminum plate splicing design will be lighter.
In terms of disadvantages, in addition to the cost, maintenance and other issues mentioned above, I would like to talk about it from the aspects of design and manufacture. From the product design point of view, the multi-piece splicing design has many materials to choose from, the strength ranges from 200-2000MPA, and the thickness ranges from 0.5mm-3mm. There is abundant design optimization space to achieve the best balance between weight, cost and performance. In this regard One-piece die-casting is obviously impossible. Another obvious problem is that compared with the design of multi-piece metal splicing, the design change cost of large castings will be very high. Although this will be alleviated with the improvement of CAE simulation capabilities, the design change cost is definitely higher.
From the perspective of product manufacturing, the die-casting process is also much more complicated than traditional stamping. This problem can be understood in this way. The main change in the stamping process is the shape. From the flat plate to the part, the performance is basically unchanged. In the die-casting process, not only the shape, but also the structure and properties have undergone qualitative changes, which mainly occur within a few seconds of solidification and cooling. The engineer in question feels very distressed. For example, we are dealing with this virus today. We can go to the sky and build the Three Gorges Dam, but small microbes give us a lot of headaches.
3. Please talk about your understanding of heat treatment-free materials.
A: The mature application of die-casting body structure has been more than 20 years, and it has been widely used in China for nearly ten years. Typical models such as Cadillac CT6, currently generally adopt vacuum die-casting + heat treatment process, the performance can basically reach 120MPA yield and 200MPA tensile strength , the elongation is more than 10% (casting body sampling). Due to the heat treatment deformation problem of large integrated castings, heat treatment-free materials may be a better solution at present.
First of all, we need to understand why die-casting structural parts have been using heat treatment in the past, and what benefits can the heat treatment bring us? In addition to optimizing material properties, the heat treatment process can also improve the uneven distribution of internal stress, structure and composition caused by the non-equilibrium solidification process of die casting. The former is understood by everyone, and the latter is often ignored. In fact, commercial heat-free materials appeared more than ten years ago, but they have not been well recognized for use. Considering the requirements of process performance such as fluidity, the mainstream heat-free materials are based on traditional Al-Si alloys by adding appropriate chemical modification elements, combined with the actual solidification conditions of die casting, to form fine grains and fully dispersed. The refined structure can obtain the mechanical properties that can only be obtained by traditional solution aging strengthening.
The reason why I say this here is mainly to remind everyone that since the heat treatment process is omitted, in order to obtain the ideal structure and properties in the as-cast state, very careful material composition design and die-casting process control are required. It is very sensitive, and it is also very sensitive to the fluctuation of the die-casting process, especially the elongation, which is directly related to the crash performance of the body and the connection process. Therefore, from the perspective of industrial mass production, there is still a lot of work to be done. It requires the close cooperation of related parties such as product design, materials, molds, casting processes, casting production, and die-casting machines, complementing each other, and making full use of the role of simulation software.
Dr. Ye You: I talk more about issues here. Many viewpoints are also put forward for the first time in the industry. While firmly optimistic about integrated large-scale castings, I hope to bring some calm thinking to everyone, so as to facilitate the rational and benign development of the industry.