After nearly two decades of market cultivation and development, electrification of passenger cars has formed a trend and is sweeping global automobile manufacturing industry. According to 2022 annual report of China Passenger Car Association, domestic new energy vehicle market penetration rate has reached 25.6% (global 13.5%). At the same time, body-in-white, as the largest assembly of passenger cars, has seen coexistence and competition of steel bodies and steel-aluminum hybrid bodies. Relevant passenger car electrification technologies developed around body-in-white, such as body safety technology, comfort technology, lightweight technology, and intelligent manufacturing technology, are constantly emerging, engineering development connotation and process of complete vehicle and body-in-white assembly have changed. Due to current market's continuous pursuit of mileage, driving safety, manufacturing efficiency, single vehicle cost, brand effect, etc., as well as a more open vehicle and body development process, body-in-white structure of domestic electric passenger cars reflects better performance as a whole, which has an impact on traditional body manufacturing process with a large number of body parts, complex splicing relationships, and long supply chains. In terms of steel thin-plate car bodies, continuous introduction of high-strength steel forming and warm/hot forming technologies, pipe high-pressure forming technology, complex thin-plate laser welding processes and advanced connection technologies has led to trend of integrated development in design of car body structural parts at part level. In terms of vehicle body forming of lightweight materials such as engineering plastics, carbon fiber reinforced materials, and aluminum/magnesium alloys, thin plate stamping, injection molding, profile rolling, extrusion molding, high-pressure die-casting, steel-aluminum composite material assembly and connection technologies are increasingly widely used. Development of body structural parts has broken through limitations of original steel body, and degree of integration of parts has been extended to body level. Among them, high-strength steel warm-formed parts, internal high-pressure formed parts, large-scale aluminum/magnesium die-casting parts have become focus of innovation in structural design and manufacturing process technologies in integrated design of vehicle body structures due to their advantages such as lightweight, high integration, high rigidity, and high precision. Typical joint areas in body structure include aluminum die-cast shock absorber mounts, aluminum die-cast rear floor panels, aluminum die-cast front floor panels, extruded door sill longitudinal beams, internal high-pressure formed A-pillars, ultra-high-strength steel hot-formed B-pillars, high-strength steel one-piece thermoformed shock absorber mounts, etc., are gradually being widely used. At the same time, passenger electric vehicles adopt chassis, body and vehicle integrated structure wrapped with power batteries, such as battery pack (celltopack, CTP), battery body integration (celltobody, CTB), battery chassis integration or battery vehicle integration ( celltochassisorcelltocar, CTC) and skateboard chassis structures. Above-mentioned trend of large-scale application of new processes, new materials, and new structures not only increases importance of body in vehicle development process, but also puts forward higher requirements for technical integration and engineering complexity of body development. Whether at product level or project level, development team is required to pay more systematic consideration in product design and verification, development cost and cycle.
In terms of definition of body integration, according to existing research progress, body integration in a narrow sense refers to integration of product structure and its functions, that is, body integration. It mainly refers to using the latest achievements in body materials and related processes to carry out integrated research and application of body-in-white and its components to obtain design and development with excellent comprehensive indicators such as lightweight, high precision, structural mechanics, and batch manufacturability. Its main features: using first principles, using as few materials and unified processes as possible to realize design and manufacturing of complex product functions. Broad concept of integration includes three levels of content, namely high degree of integration of materials and processes, products and equipment, products and commodities, involving all aspects of the entire development process. Through first principles, JK Automotive attempts to use as many real materials as possible to establish a relationship database between material genes and various physical properties, which can provide a basis for rapid material selection for mass production applications. NIO uses return-type and snake-type testing methods to develop heat-treatment-free materials to ensure that performance of aluminum alloy materials meets standards, promotes mass production application of integrated die-casting process on its second-generation models by conducting test verification at sheet level, body-in-white level, and vehicle level. Iron and Steel Research Institute carries out research on application of hot forming technology for high-strength and high-plasticity steel. Medium-manganese steel forming technology it developed has been applied in manufacturing of integrated side parts, has promoted development of low-carbon technology for part integration. Large die-casting equipment units such as die-casting machines and die-casting molds are prerequisite for realization of integrated molding technology. JiKrypton Automobile has developed a three-stage vacuum structure to ensure that vacuum degree is less than 50Mbar. Structural parts have requirements for mechanical properties such as tensile strength, yield strength, and elongation. They are sensitive to die-casting defects such as slag inclusions, shrinkage cavities, and pores, so they have higher requirements for mold vacuum. At present, there are few reports on process optimization. Based on an electric passenger car project, this paper analyzes from aspects of vehicle (body) development process and quality control of aluminum alloy die-casting integrated parts, put forward solutions to die-casting quality problems and optimization suggestions for development process, develop a process control-oriented rapid development method for body integration based on maturity evaluation.

Steel body is essentially a manifestation of integrated development. After years of development, a manufacturing industry chain with a sound system and mature technology has been formed. Driven by current advanced use of steel (high-strength steel, ultra-high-strength steel, JiPa steel, etc.) and its forming technology, steel bodywork is still an important direction for integrated development. Relatively speaking, light alloy materials face problems such as little experience in fast-paced mass manufacturing processes, high equipment costs, complex process control, and environmental pressure from carbon emissions. This has become a challenge for existing development processes to cope with rapidly evolving complex systems, but it is also an opportunity for process updates and upgrades.
Current complete vehicle development process is invested as a core competitiveness by various OEMs, including the entire process from product planning, conceptual design, engineering development, trial production testing, production introduction, mass production to after-sales service stages, requires design, R&D, testing, manufacturing, sales, after-sales and other departments to be organized in an orderly manner and coordination. Development process of complex products is based on serial workflow. Changes in a certain link in serial system affect process output of the entire system, ultimately impacting development quality objectives. In particular, engineering design changes in the middle and later stages of development process will pose challenges to workload, development cycle, project investment, and management dimensions of development process, as shown in curve I in Figure 1. Statistics show that although product design and development costs only account for 10% to 15% of the total cost, they determine 70% to 80% of the total cost. In order to effectively control costs, shorten development cycle, and respond to market changes more quickly, in project practice, a more radical development process gradually moves key design content to early stage. Early stage of a development project has two distinctive characteristics: ① Incomplete development information (incomplete); ② Explore possibilities through multiple options. Incompleteness of information makes development work particularly dependent on available resources, including internal resources of OEM and the resources of external partners, requiring development process to have excellent ability to integrate resources; multi-solution explorability means that there are multiple feasible solutions to achieve goal. Development process is required to support evaluation and decision-making of multiple options and give full play to important role of design stage in the entire development process.

 

In view of problem of body integration development and incompleteness of early development information, a pre-development framework is proposed. As shown in Figure 2, during conceptual design stage of vehicle (body) development, OEM joins industry or supply chain system to form a body integration pre-research team. Pre-research team aims to develop high-maturity body integration solutions, summarizes and analyzes the latest achievements in integration (referring to aluminum alloy die-casting mold technology and systems) industry, builds a professional database, such as best practices, mass production experience and lessons , mass production case library, etc., explores multiple solutions by selecting key assemblies and combining component design and simulation analysis, develops component molds based on the best and trial-produces physical verification assemblies, forming a closed-loop development mechanism of design, simulation, verification, and improvement. All feasible solutions will form a database to guide subsequent project development after maturity evaluation. Integrated solution that meets project goals will be used as a pre-research deliverable and become an input engineering document for subsequent development process, providing support in subsequent physical development process, shortening time of design and product verification phases. Body integrated development process is shown in Figure 3. Body development goals are fully discussed and defined in the early stage, all aspects of project are implemented to avoid costly engineering adjustments in the middle and later stages. At the same time, structural design work can only be carried out in the middle of original development process can be more manufacturability research based on manufacturing process in pre-research stage. On the one hand, it reduces time required for mid-term modeling, and on the other hand, it also saves multi-department collaboration and CAE computing power requirements required for structural optimization.

 

 

A typical body-in-white is generally composed of 300 to 500 thin plate parts spliced together by spot welding. Development of industry has been restricted due to large number of parts, long manufacturing chain paths, and high pressure to reduce quality. Development of lightweight materials and their forming and joining processes has created solution conditions. However, high complexity and high investment threshold of aluminum/magnesium alloy die-casting processes and equipment have restricted large-scale application of integrated body parts.
Integrated structures mainly made of high-strength steel materials are limited by shape and forming size of parts, and cannot further improve integration level of integrated design. Rise of mid-to-high-end electric passenger car market represented by Tesla has promoted integrated manufacturing of large-scale parts and components. Scale has reduced shortcomings caused by die-casting process and equipment itself, allowing level of integrated structural design to be improved in practice.
Compared with typical steel body, integrated design of body structure has greatly simplified number of parts and body-in-white manufacturing process, reducing development and investment of related tooling equipment. However, from perspective of body product performance requirements and product quality assurance , complexity of manufacturing system, manufacturing cost control, etc., integrated design is still a highly complex system process and more challenging in many aspects, so evaluation of integrated design plan will also be based on the entire product process development comprehensive consideration of needs.
Based on analysis of steel body structure design evaluation content, through development of cases, impact of integrated die-casting process on integrated design was studied. In preliminary research of project, front cabin assembly area and rear floor assembly area were selected as integrated development objects. Parts are shown in Figure 4. Rear floor dimensions are: 1560mm*1570mm*480mm, and weight is 46.43kg; front cabin dimensions: 967mm×1604mm×771mm, mass 43.23kg. Preliminary analysis shows that rear floor assembly only needs to withstand rear-end collision impact (body safety test project), and wheel vibration during driving is borne by subframe, so structural design requirements are relatively simple; front cabin needs to withstand multiple impact tests in collisions, performance requirements for castings are higher than those for rear floor; in terms of dimensional effects, elongation of filling end during die-casting process may decrease significantly. Filling end of rear floor is located at beam, and simulation analysis is required to determine its acceptability. It was found in the case that front cabin filler ends at sill beam connection, which may affect product performance. Figure 5 shows production process flow of die-cast aluminum for front engine room, which mainly includes key processes such as production preparation, die-casting molding, and casting post-processing. Post-processing process involves changing factors such as part (casting) temperature, part pickup, and storage. Actual production will encounter problems including deformation of parts and loose internal quality. It is necessary to perform fault source diagnosis and analysis based on data status in pre-research and design stages and actual process status. Common production problems and countermeasures (part) are as shown in table 1 shown. Taking die-casting parts of front cabin as an example, front cabin assembly is assembled from two integrated die-casting parts on left and right (suspension and shock absorber tower support seats), front partition and lower connecting beam. Connection length exceeds 2m. According to its geometric specification requirements, there are many difficulties in dimensional control. Connection area between tower package and longitudinal beam requires a profile of 0.15mm. Because precision of matching parts is high, parallelism tolerance of connection area is specified. There are deviations between actual part status in local areas and design requirements. According to actual part measurement and correlation analysis, it was found that deviations are correlated in same area, so deviations may appear in three process steps of mold forming, aging treatment and orthopedics. By scanning shape and size of parts in each process step, aging treatment stage can be determined. During aging treatment of this part, a horizontal simple bracket is used for batch processing to supplement aging temperature + aging time. Far end of part is exactly area where deviation occurs. After comparing aging tests of different bracket parts, local deviations were improved. Handling of this problem also shows that aging treatment is not only an improvement of material structure, but also a key workstation for dimensional quality of parts, which needs to be paid attention to in the early design.

 

 

Table 1 Common production problems and countermeasures (part)

Product data related to integrated assembly in pre-research stage is still incomplete, determined goals are of a qualitative conceptual nature and still need to be estimated in CAE stage. Pre-research content prepares basic boundary conditions for formulating subsystem technical specifications to support smooth development of subsequent stages of development process. In order to further quantify "qualitative" nature of design plan and support exploration of multiple options in pre-research stage, design summary and optimization in subsequent stages (including detailed structural design and physical stage, etc.), maturity is used as an indicator to measure level of integrated design in the early stage, and a design scheme maturity evaluation method was developed. Maturity of design solution is evaluated according to four dimensions, namely design reliability, design performance indicators, design technical defects and design completion. Maturity is calculated as follows:

 

Among them, CSD is maturity score of an integration solution; E(N) is maturity score corresponding to Nth maturity indicator; w(N) is weight coefficient of indicator, N≥1, N is factors included in indicators, each factor has a corresponding maturity value; p is number of indicators included in evaluation of maturity. At this stage, it mainly includes four indicators: structural design reliability, structural design performance, design technical defects, and design completion. Content of indicators and influencing factors included are shown in Table 2. Integrated structural design with high maturity has better performance in terms of material usage efficiency, manufacturing cost, manufacturing process stability, component service performance, etc.
Table 2 Integrated die casting design maturity evaluation (part)

Due to advantages of integrated body parts in terms of high precision, weight reduction, and process steps, lightweight technologies represented by aluminum alloy die-casting and high-strength steel warm forming have been widely used in car bodies, promoting technological progress in lightweight materials, body structure design, and product development processes. Different from relatively mature manufacturing environment of steel car bodies, implementation of an integrated design plan based on lightweight alloy die-casting technology involves consideration of the entire production process such as material development, manufacturing technology and equipment development. Now, from aspect of development process, we will sort out the early development of integrated body represented by die-cast aluminum, improve efficiency of integrated development through maturity evaluation method for multi-solution exploration, establish a database and process architecture suitable for integrated body development, so that vehicle development process has ability to quickly respond to market demand. At present, integrated development based on lightweight alloy die-casting technology is in ascendant. Integration of body structure continues to increase, and challenges will be unprecedented. In the future, we will continue to face bottleneck of industrial technology, and we hope to continue to work in following aspects.
(1) Research on related low-carbon short-process processes will be main process direction for lightweight vehicle bodies in the future. Integrated modular multi-material hybrid bodies will gradually enter mainstream vehicle solutions. Design and development method based on materials-process-structure will provide support for application of new materials.
(2) Traditional die-casting method mainly uses heat treatment to improve material strength, but large parts are difficult to heat treat, and super large parts cannot be heat treated. After heat treatment, they are easy to deform, difficult to correct, and have a low yield rate. In order to meet die-casting process requirements and effect of heat treatment-free, a certain proportion of silicon, magnesium, copper, iron, manganese, zinc and other elements are added in aluminum alloy material production process to develop heat-treatment-free aluminum alloy materials. Heat treatment-free aluminum alloy materials can be used to produce complex thin-walled parts without heat treatment, can be used to produce high-strength and heat-resistant parts after heat treatment. With continuous improvement of enterprises' requirements for high-strength and heat-resistant properties of parts, use of same series of materials for all vehicle aluminum alloy parts, which facilitates recycling of aluminum alloy materials and realizes pursuit of low-carbon and environmental protection, new heat-treatment-free integrated molded aluminum alloy materials with high strength, high plasticity and high fluidity will become future development trend. 
(3) In die-casting process of integrated parts, in order to solve typical technical difficulties such as purity of aluminum liquid, abnormal microstructure, and casting defects, new rheological die-casting molding technologies are developed to improve structural properties of aluminum alloy solidification process, ensure that product quality meets standards, which has become a new idea for large-scale thin-walled aluminum alloy die-casting.
(4) Maintenance cost of die-casting aluminum alloy molds is high. Because of influence of alternating temperature field, core in the area in contact with product is prone to fatigue damage. At the same time, structural parts generally have more deep cavities and deeper ribs, resulting in poor hardenability of most deep rib parts of mold during heat treatment. Therefore, partial inlaying is used to improve hardenability of mold parts and extend their service life, but it is still necessary to carry out research on mold part material selection, mold temperature control, mold structure optimization, renewable mold technology, etc., to further improve mold usage efficiency, extend service life, and reduce single-piece production costs.
(5) In the direction of development of steel car bodies, current mature steel body process technologies (stamping, welding, painting, etc.), advantages of low-cost manufacturing and green manufacturing still exist, which will inevitably focus on high-strength steel and its composite processes such as material selection, forming process and equipment development, and accurate simulation technology.
(6) In terms of digital development, standard application and research of geometric specifications will focus on expression of dimensional technical specifications of integrated parts and their manufacturing processes, development of efficient detection systems, and rapid process-based data analysis.