Forming Strategies and Solutions

• Notch-free Flanges on Channel Section Ends
• Balancing Splits and Wrinkles in Curved Sections
• Lightweighting with 3rd Generation AHSS

 

The information provided on this page represents generalizations and may not be directly applicable to every forming situation. OEMs factor in many considerations when choosing grades for each application, including but not limited to the cost and availability of products with the targeted strength level and coating consistent with the manufacturing and engineering strategy of the specific OEM.

 

Notch-free Flanges on Channel Section Ends

Joining longitudinal cross members to other parts typically is facilitated by an end flange joint, as shown on the left side of Figure 1. The corners of these flanges are highly strained, and are therefore at risk for edge cracking when higher strength sheet steels are used.  Notching the section corners (Figure 1, center) avoids these high strains and associated edge cracks, but also reduces the stiffness provided by a continuous flange in these sections. A notch-less end flange, such as shown in the right image in Figure 1, retains the desired structural benefits.

 

One potential forming approach that minimizes the edge strains^N-32 incorporates a developed blank edge where the edges of the sheet steel are cut to the precise dimensions to achieve the targeted contour of the formed part with no additional post-forming trimming. Additionally, the sheet is supported using a lower pad throughout the forming process, as indicated in Figure 2.^N-32

 

At the beginning of the forming process, the lower pad is held at a position higher by its vertical stroke (S_lp) than the upper surface of the punch, and during the forming process, the width center of the blank is maintained at that height.

After the upper pad bottoms to the die surface of the die, the upper pad and the die move as one unit, and the lower pad moves down. At bottom dead center of the press stroke, the level at which the blank was held becomes equal to the punch surface, and the forming of the first process ends here.  

The referenced study^N-32  tested steels with 980 MPa and 590 MPa tensile strength.  With a testing setup similar to what is shown in Figure 2, conventional approach resulted in edge cracks, but the developed method successfully formed crack-free parts.  A lower pad vertical stroke S_lp was optimized to minimize the thickness decrease seen in the flange. At this value, the thinning strain using the 980 MPa steel was essentially the same as that of the 590 MPa steel. 

 

 

Balancing Splits and Wrinkles in Curved Sections

When a high-strength steel sheet is formed into a curved shape, like what might be seen at the top and bottom of a full B-pillar reinforcement or in the L-shaped corner section of an A-pillar reinforcement where the dash transitions to the rocker, wrinkles can appear at the top flat portion. To suppress these wrinkles, a corrective approach of applying greater back tension, either by beads or binder force, risks fracture in these higher strength grades. Figure 3 shows these typical at-risk parts.

 

The developed method includes a pad to restrict the blank at the top flat portion to prevent the wrinkles and use of bending rather than drawing to prevent fracture. Material flow control accelerates the sheet metal flow into the regions at greatest risk of fracture.

Figure 4 shows a generic T-shape part, and Figure 5 presents a schematic of the developed forming method.

 

 

 

Metal forming simulation of a typical conventional forming approach showed that the strain map in the top section has a strain ratio (minor true strain/ major true strain, ϵ₂/ϵ₁) of much less than -1, and is therefore in the compression zone of the Forming Limit Diagram (the black points in Figure 6b).

The developed approach changes the metal flow path to one that is much closer to a strain ratio of -1, and thus transitions away from the wrinkling risk (the black points in Figure 6a).

In the area of edge fracture risk adjacent to the door opening section, the developed process minimizes the strains, keeping them below the forming limit and removing the fracture risk (the gray points in Figure 6).

 

Application of this method is shown to keep the edge strains essentially independent of material strength, in tests ranging from 590 MPa to 1470 MPa tensile strength.

More details about this process can be found in Citations N-32  and T-55.

  

 

Lightweighting with 3^rd Generation AHSS

Material for this section comes from Citation H-75. 

DH steels, or High Ductility Dual Phase steels, are a type of 3^rd Generation advanced high strength steel that has approximately 5% better elongation compared to conventional dual phase steels of the same strength level.  This extra ductility may help justify a substitution of the current approach with a higher strength grade at reduced thickness to achieve comparable performance with lighter weight and a minimal cost impact since less metal will need to be purchased.

Two front structure parts are evaluated in Citation H-75.  For front longitudinal components, the incumbent base materials are HSLA260 and HSLA300, at thicknesses ranging from 1.35 mm to 2.6 mm, as indicated in Figure 7. 

 

A contender substitute grade is DH800, a 3^rd Generation AHSS with an 800 MPa minimum specified tensile strength.   DH 800 has 140 MPa and 180 MPa higher yield strength than the 260LA and 300LA baseline grades it might replace, as indicated in Table 1.

 

Crash performance of the baseline HSLA approach was assessed. By switching to DH800, comparable crash performance was achieved with a 23% gauge reduction in each component, representing a 5 kg savings.  At that stage, stamping feasibility was assessed with forming simulations which indicated similar performance between all grades. One of the sections is shown in Figure 8.

 

In another front-end structure, DP800 base material was compared to DH1000. The property requirements are reproduced in Table 1, with the parts shown in Figure 9.

 

As before, the targeted thickness was determined as that which gives equal crash performance as the incumbent grade.  The analysis considers the strengthening associated with work hardening.  Replacing the baseline DP800 with DH1000 showed a downgauging potential of 17% per part, representing a 3 kg weight savings.  Figure 10 presents a comparison of forming simulation results.

 

The extra formability that the 3^rd Generation AHSS grades offer results in an improved deep drawing capability which is necessary to obtain good parts at higher strength levels. Incorporating these higher strength grades may allow for thickness reduction while maintaining crash performance and potentially being cost-neutral.