New solutions for press forming of body panels
The area of ​​the body covering products is large, and the technical requirements for each part are different. The original separation forming and the overall forming method have different defects, and the application of the laser tailored welding board technology provides a new idea for the overall forming.
There are two methods of forming auto body parts in traditional crafts: separate forming and integral forming. The separation forming method is to separate large parts into small single pieces and then weld them into parts. The advantage is that different materials and different thicknesses can be selected according to the requirements of various parts; the disadvantage is that more tools and equipment are needed for investment. The manufacturing cost is high, and the matching accuracy and overall vehicle quality of the welding assembly are also reduced. Integral forming method is to directly form large parts with the whole sheet material. The main advantage is that the investment of tooling molds and equipment is greatly reduced, the manufacturing cost is relatively low, and the product quality is improved; the disadvantage is that materials with the same material and the same thickness must be used for all parts of the part, and it is difficult to achieve a good structure optimization. .
Laser Welded Blanket Technology
Tailored laser blanking plate technology is a modern processing technology developed based on mature laser welding technology. It is a high-energy laser welding several pieces of steel with different materials, different thicknesses, and different coatings into an integral plate and stamping production. Satisfy the different performance requirements of different parts of the parts. The appearance of the tailor-welded plate process solves the technical problems that the ultra-wide plates and parts of different parts have different process performance requirements that cannot be satisfied by conventional single-thickness materials. FIG. 1 is a schematic diagram of the production of a side skirting panel for a sedan by separately forming, integrally forming, and laser fitting.
Fig. 1 Comparison of forming methods for side panels of cars
Tailored laser welding plate
The use of tailored blanks for lasers has tremendous advantages, which can bring significant economic benefits to the automotive industry, mainly reflected in: the number of vehicle parts is greatly reduced, the spot welding process is simplified, and the dimensional accuracy of the body is improved, reducing the quality problems. The variability of the thickness of the material ensures the reinforcement of important positions.
Fig. 2 shows an example of successful production of a laser tailored welded blank for the overall roof of the Dongfeng mid-size vehicle cab. Fig. 3 shows a schematic diagram of the welded top cover assembly after the Dongfeng heavy truck is separated and formed. The current status, process parameters and investment of the two parts are shown in the table. The table focuses on the characteristics of the overall forming scheme of the tailor-welded plate and the stamping scheme of the parts for the heavy-duty roof.
Figure 2 The overall roof of a mid-size car
Figure 3 Front and rear top assemblies of heavy-duty vehicles
From the following table, we can see that the overall stamping of the welded joints has resulted in significant economic benefits: the investment in the mold has been reduced from the original 4.9 million yuan to 3.6 million yuan; the area occupied by the equipment and the number of operators have been reduced; the weight of the parts has been lapped The reduction in surface area was reduced by 0.55 kg, material utilization reached a relatively high 76.8%, and material consumption was reduced by 5.33 kg/vehicle.
Table Comparison of Overall Stamping and Part Stamping
Stamping Formability of Laser Welded Blanks
The technical problems in the use of tailor-welded blanks are mainly due to the problem of tooling manufacturing caused by factors such as the decrease in forming performance caused by structural changes in the weld zone and weld seam movement.
1. Stamping performance of tailor welded blanks
The research on tailoring properties of tailor welded blanks shows that:
(1) The strength of welded joints after laser welding is increased to a certain extent than that of the base metal. The effect of the change in thickness ratio on the strength is not affected by the change in material grade ratio. The strength of welded joints of different grades depends mainly on the low strength grade. s material.
(2) The forming performance of the welded joints is reduced to a certain extent compared with that of the base metal. With the increase in the thickness difference and the strength difference of the tailored welds, the forming performance is reduced.
(3) For non-thickness tailored welded blanks, the plastic deformability of the tailor-welded blank is significantly reduced when the tensile direction is the same as the direction of the weld seam, and the smaller the proportion of thin sides, the greater the reduction.
(4) Tensile failure mode of tailor welded blanks generally has two kinds: First, when the weld is consistent with the tensile direction, because the weld is lower in plasticity than the parent material, the weld part is often pulled off; the second is when welding. When the seam is perpendicular to the stretching direction, the thin side base metal tends to be excessively thinned and cracked.
In addition, Tailored Blanks must not only pay attention to its forming performance, but also take into account the influence of different material thickness differences on subsequent processes. For example, if the material thickness difference is large or the performance difference is large, the weld line should be avoided. Through the hole punching position (easily lead to small holes bent or broken) and so on.
2.Welding seams of tailor welded blanks and their process countermeasures
Weld movement is the fundamental factor that distinguishes tailor-welded plates from ordinary plate production and is fundamental to the success of product design and process analysis. According to the drawing process theory and related experimental demonstrations, the following conclusions can be drawn: The direction of weld movement and the distance of movement mainly depend on the ratio of material strength on the two sides of the weld, the position of the weld, and the distribution of tensile blank holder force.
Since the amount of movement of the weld can only be reduced and cannot be eliminated, it is necessary to set a thick open surface at a certain range on the side of the weld where unequal thickness of the weld surface occurs. The open space is open to the thin side of the material, and the open area of ​​the pressure surface is correspondingly large. The corresponding position of the die punch is correspondingly small.
Using the clamping device can significantly improve the degree of weld movement, making the amount of weld movement reduced by 72.6% to 84.9%. In the actual body panel design, it is also possible to design reasonable ribs and other structures near the weld where the product structure is possible. Through the die structure, the weld seam is first formed to control the movement of the weld seam.
In addition to taking necessary technological measures to control and reduce weld movements, it is also necessary to use alloy inserts and other measures to control the wear and picking of the profile in the moving zone of the weld.
3. CAE analysis of weld migration
With the development of computer technology, it has been possible to apply CAE methods to more accurately analyze the rule of weld movement of tailor welded plates, which provides a reasonable basis for product design and lays a solid foundation for the rationalization of manufacturing processes. Fig. 4 shows the CAE analysis results of the mid-sized car side panels inner panel welding. It clearly reflects the movement of welds at each location and the overall drawing conditions.
Figure 4 Overall stamping CAE analysis results for tailor welded blanks
Determination of Position of Weld-weld for Side Panels of Medium Size Cars and Its Effect on Cost
Figure 5 is a side view of the cab of a medium truck. It is composed of a side corner panel with a material thickness of 0.75 mm and a side wall panel with a material thickness of 1.6 mm. The boundary line is the illustrated curve. The initial process was to form two parts separately and weld them together. Corresponding stamping layout and material use effect according to this scheme are shown in Figure 6. It can be seen that the disadvantage of this scheme is that the material utilization rate is extremely low, which is 36.2% and 43.9%, respectively, and the part production process is also poor. Taking into account the two parts of the assembly relationship, preliminary judgments such as the use of tailor-welded production will effectively improve product manufacturability and reduce costs.
Figure 5 Cab Side Cabs
Fig. 6 Production layout and material utilization effect of parts
1. Selection of weld location
Whether the design intent of the laser welding product can be best achieved through the process, the key link is whether the position selection of the welding seam is successful. The setting of the weld location not only needs to consider the function and structure of the product, but also comprehensively considers the requirements of the stamping process and the requirements of the cost. The organic combination of the three will result in an optimal design structure and overall vehicle quality.
Based on the requirements of the drawing theory and the research results of tailor-welding technology, the following basic principles for selecting the weld location can be obtained:
(1) The selection of welds must first meet the requirements of product structure and function.
(2) The weld is a straight line and it is best to ensure that the square plate is welded after the process is completed.
(3) Welds are to be prevented from passing through areas with large draw effects, in particular to prevent welds from passing through the forming R area in parallel.
(4) Due to the difference in blanking gaps with different material thicknesses, it is necessary to avoid the welds passing through the punched holes.
Taking the side wall shown in Figure 5 as an example, the initial design structure of the product is a curved fold line. With this curve, the weld seam is the current tailor-made welding technique that cannot be satisfied at present. At the same time, the curve may show 0.75 mm and 1.6 mm material thickness at the small hole due to the movement of the delayed weld at the A/B position. This can easily lead to bending or breaking of the punch punch, thus requiring the repositioning of the weld seam.
The preferred weld location according to the requirements of the weld setting principles (1) and (2) is shown in Figure 5 by the thick red line. The main disadvantage of this weld is that the delay of the weld to the thick material will still cause different material thicknesses in hole A, and at the same time, due to the large convex shape of convex undulations at point A, it may cause cracking. In combination with the positional relationship of the two holes A and C in FIG. 5, the tailoring line is adjusted to the position shown in FIG. 7, basically satisfying the related requirements.
Fig. 7 Weld seam position that basically meets relevant requirements
2. Influence of cost factors on weld position
Figure 8 shows the side of a model of Dongfeng. The fitting line at the preliminary design stage is shown by the red line. The results of the CAE analysis for this scenario resulted in severe cracking along the weld line (Figure 9). The main reason for this is that the point A is close to the R angle and the deformation is severe.
Figure 8 side of a model and its welding line
Figure 9 CAE results of initial weld locations
Taking into account the styling characteristics of the piece, the weld line that fully meets the process requirements is shown by the blue line in Fig. 8, and the CAE effect is good. Comparing the two weld lines of FIG. 8 , it can be seen that the 1.6 mm thick material zone, which fully meets the process requirements, is significantly increased, and thus the weight of the parts is increased and the material consumption is also significantly increased, so this solution is also not the best result.
Can the analogy of Figure 7 be set as shown in Figure 10? If implemented in accordance with this plan, it will meet the product requirements as much as possible and greatly reduce the heavy material area, thereby reducing product costs.
Process Analysis Figure 10 The greatest risk to the program is that cracks in the weld zone may occur in the step and bevel areas (CAE analysis also confirms this result) and that design changes need to be made to the zone. According to the analysis, the product design department has made ramps and transitions for the corresponding areas. Figure 11 shows the CAE analysis results after product optimization to meet the factors of products, processes, and costs.
Figure 10 Analogical Assembled Welding Line
Figure 11 CAE results to meet process and cost after optimizing product design
Conclusion
Tailored blanks as a new process means to bring new product solutions to the body manufacturing and bring new opportunities for quality improvement and cost reduction. At the same time, due to the particularity of its process implementation, it is necessary to closely cooperate with related personnel such as product design, stamping process, and mold manufacturing, so as to finally produce qualified products that satisfy both structural and performance requirements, and are reasonably stable and cost-effective.
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