Compensating for material springback during the forming process can be challenging. During forming, stress is applied to the material making it conform to the die geometry. The material will strain or displace across the die geometry, which consists of complex shapes, creating numerous strain patterns throughout the part. These multiple strain patterns trap residual stresses within the sheet metal, resulting in distortion and springback once the forces are released.
New Advanced High-Strength Steels are characterized by yield strengths several times that of typical mild steels. They require higher forming forces to reach the “yield” strength, allowing permanent deformation to occur in the shape of the die. Modifications to die geometry or process parameters are required to correctly compensate for these residual stresses, otherwise the stamping facility will battle repeatability issues, which results in scrap or tool and die downtime. Typical reaction to this variation is to ask for tolerances to be increased and forcing operations further downstream to address these issues. The use of part stiffening beads, stake beads, and die geometry compensation will help to correct springback issues.
Strategically placing geometric shapes along flange walls, flange angles and sidewalls can lock in residual stresses, reducing the angular changes or twist created by springback. These geometric changes must be approved through product design. Commonly used shapes are darts, beads, step flanges and offsets.
Figure 1: Step flanges, stiffening beads and offsets can help control springback in AHSS.
Step flanges (example 1) can be added to flange walls to compensate for angular changes due to springback. The added stiffness created by the part geometry locks in the residual stress, which would create a curl or twist in the flange. Stiffening darts (example 2) are also used to reduce the angular changes in sidewalls or flanges. Adding an offset (example 3) horizontally along a wall will reduce twist and add stiffness to the part and creating higher strain patterns, which are introduced at the bottom of the forming cycle.
The use of stake beads when forming Advanced High-Strength Steel is a common practice to manage the stamping’s residual stresses. The stake beads are added to the part addendum and come into play at the bottom of the stroke, resulting in increased sidewall strain; the effect is to lock in the plastic deformation, reducing the curl and angular change in the sidewall. Figure 2 illustrates an open hat section with and without stake beads.
Figure 2: Stake beads to minimize sidewall curl and angularity.
Training die makers to understand springback associated with Advanced High-Strength Steel, and how to manage springback through proven countermeasures enables more consistent stamping performance. The AHSS Application Guidelines Version 6.0 has a great deal of information on this topic. Section 3.C.3 (starting on Page 3-53) is devoted to Springback Management, addressing it in depth. If you haven’t already downloaded a copy, it is available free of charge at worldautosteel.org
And be sure to ask your questions here!
Every stamped part has springback. When sheet metal is plastically deformed into a part, the shape of the part always deviates somewhat from the shape of the punch and die after removal from the tooling. This dimensional deviation of the part is known as springback. Springback is caused by elastic recovery of the part, which can be illustrated simply on the stress-strain curves shown in Figure 1.
Figure 1: Schematic showing amount of springback is proportional to stress.
Unloading (by removing all external forces and moments) from the plastic deformation level A would follow line AB to B, where OB is the permanent deformation (plastic) and BC is the recovered deformation (elastic). Although this elastic recovered deformation at a given location is very small, it can cause significant shape change due to its mechanical multiplying effect on other locations when bending deformation and/or curved surfaces are involved.
In general, springback experienced in AHSS parts is greater than that experienced in mild or conventional HSLA steels, due in large part to the progressively higher initial yield strengths and the greater work hardening rate that significantly increases the yield strength during forming. In fact, springback for higher yield strength materials can be up to eight times that of traditional mild steels, as shown below in Figure 2.
Figure 2 – Springback angles for various steels.
The magnitude of springback is governed by the tooling and component geometry. When part geometry prevents complete unloading (relaxing) of the elastic stresses, the elastic stresses remaining in the part are called residual stresses. The part then will assume whatever shape it can to minimize the total remaining residual stresses. Creating a uniformly distributed residual stress pattern across the sheet and through the thickness will help eliminate the source of mechanical multiplier effects and thus lead to reduced springback problems.
Three common forms of springback are angular change, sidewall curl, and twist. Angular change is the angle created when the bending edge line (the formed part) deviates from the line of the tool. Sidewall curl is the curvature created in the side wall of a channel when sheet metal is drawn over a die/punch radius or through a draw bead. The primary cause is uneven stress distribution through the thickness of the sheet metal, generated during the bending and unbending process. Twist is caused by torsion moments in the cross-section of the part, developing because of unbalanced springback and residual stresses acting in the part to create a force couple, which tends to rotate one end of the part relative to another.
Die designers have engineered the dies to compensate for springback, and developed many effective countermeasures. They have applied the appropriate amount of restraining force to deform the material and configured the die geometry to lock in the plastic deformation that the material needs to form a quality panel. In the stamping operation, however, process variation can affect the forces applied to the material, resulting in unexpected springback, nonconforming parts, and potential fit-up issues during the assembly process. For these reasons, tool and die countermeasures and enhanced process recipe discipline are required for effective AHSS stamping productivity and quality.
BONUS! Dr. Stuart Keeler is a well-known name in the world of Metallurgy, having created the Forming Limit Diagram used widely today. Watch as Dr. Keeler talks about the origins of springback.
Dr. Stuart Keeler On Springback from worldautosteel on Vimeo.
A common problem in every stamping plant is trim edge burrs. As new materials have been introduced, special trim breakage (clearance) or entry amounts may be required. Researchers are still trying to understand the edge stretching limits of these new materials. Edge stretching limits are directly linked to the reduction of the work hardening exponent (n-value) due to the cutting operation. As the material is cut during the coil slitting, blanking, trimming, or piercing operations, the tensile stretching on the sheared edge reduces the amount of formability remaining in the material. Finding the proper trim breakage and trim edge condition is critical. New test studies help steel producers understand the maximum stretch limits of the material they produce.
The Hole Expansion Test (HET) is the accepted form of measuring edge stretching limits. The test is performed by punching a hole in the center of a flat blank which is then clamped down, while a conical punch is pushed up through the hole, creating a stretch-flanged edge (see Figures 1 & 2). The output is the ratio of final hole diameter/initial hole diameter. The hole in the blank can be produced by various processes, to simulate manufacturing conditions. Some of the best results are produced by utilizing milled edges, laser cutting, EDM, and water jet.
In the world of stamping operations, reduced formability of a trim or pierced edge can equate to downtime, scrap, or rework. Since the use of EDM and Waterjets are not practical solutions, we evaluate current methods and materials that are available. The intention is to provide information to the people on the shop floor who might deal with this issue on any given day. Worn or chipped trim steels, improper clearances, and worn punches need to be repaired and maintained. New Advanced High-Strength Steels have lower forming limits compared to mild steel, and the introduction of a worn tool will reduce that forming range significantly. The use of powder metallurgy or cutting steels can help improve the number of hits between preventive maintenance intervals significantly. Some surface treatments can also extend tool life, achieving the same relative tool wear as conventional mild steels.
Figure 1: Schematic of a typical hole expansion test (HET)
conical punch/die setup.
Figure 2: Schematic showing hole expansion capability for a 200 MPa mild steel for various punch conditions.
New grades such as Complex Phase (CP) steels have great strength, but also perform well in edge or stretch flange conditions because of bainite and grain refinement due to thermo-mechanical processing in the steel mill. Yet, the lack of proper tool maintenance can strip these steels of their performance advantage, as shown in Figure 3.
Figure 3: Hole expansion results for various AHSS grades, comparing effect of tool conditions.
Understanding the effects of tool wear rates, trim breakage, surface coating, and surface treatments will reduce downtime, scrap, and extend preventive maintenance intervals on trim and pierce dies. Providing training to the die makers on the newest materials, die components, and surface treatments available will help them make longer lasting corrections to stamping dies. Research and processes are evolving every day, resulting in new methods, products, and information for the successful stamping of Advanced High-Strength Steels.
Note: AHSS Application Guidelines Section 3.C.2 – Tool Materials and Die Wear contains more information that you may find helpful. Download the Guidelines free at www.worldautosteel.org.
- Figure 1: (Schematic): H. Mohrbacher, “Advanced metallurgical concepts for DP steels with improved formability and damage resistance” – NiobelCon bvba
- Figure 2: R. Hilsen et al, “Stamping Potential of Hot-Rolled, Columbium-Bearing High-Strength Steels,” Proceedings of Microalloying 75 (1977).
- Figure 3: Courtesy of C. Walch, voestalpine Stahl GmbH.
The mild steel currently being used for sheet metal stampings has higher n-values than High Strength Low Alloy Steel and Advanced High Strength Steel. The high n-value indicates that the material has a higher work hardening exponent making the steel much easier to stretch or form. The n-value describes how the material works together to resist localized fractures as stresses are applied. High strain patterns can be created in localized areas such as character lines and embossments. This strain pattern creates strain peaks or strain gradients. These strain peaks have much higher plastic deformation than areas on the rest of the material. The localized strain will cause the material to thin as it forms the character line or embossment. The die geometry does not allow the material to deform in stretch or draw modes, which means the material is in the plane strain mode of deformation on the Forming Limit Diagram (FLD). This deformation mode has the least amount of formability due to the location of the FLD₀anchoring point (See Figure 1).
Figure 1: Benefits of Uniformed Strain Distribution.
What does that mean for your stamping process? Mild steel has the ability to reach a high strain gradient due to higher n-values. High strength steels do not have the ability to reach the higher peak gradients due to lower n-values and less stretchability. These high strain areas will be more susceptible to a greater amount of thinning and/or fractures. If changes in the stamping process occur, such as reduced lube quantity, greater thinning can occur, at times exceeding minimum thickness and resulting in metal fracture. These concerns can be minimized through a better understanding of material capabilities, specific geometry effects, and the use of process recipe discipline. For example, Figure 2 compares the instantaneous n-value for Dual Phase steel, a member of the AHSS family, to HSLA steel. The early n-value increase reflects enhanced local formability, which is observed in stamped parts, contrary to what the typical stress-strain curve does not show the early n-value increase, which reflects enhanced formability in local regions of stamped parts. Other AHSS grades don’t show this tendency but have been developed with greater concentrations of bainite or finer dispersion of martensite within a ferrite matrix; both effects result in better localized forming.
Figure 2: Instantaneous n-values versus strain for DP 350/600 and HSLA 350/450 steels.
Training die makers to understand these effects, while managing die geometry, will have a dramatic effect on the rework, downtime, and scrap associated with a conversion to AHSS products. The use of FLD₀ and formability analysis should identify areas of concern on the stamped part, but should also be coupled with hole expansion testing, or 2-D tension tests to more fully explore the formability condition. When trouble areas have been identified, there should be a review of the analysis and part with T&D managers, die makers, and quality personnel to formulate a corrective action plan. This plan should have specific and measurable direction, buy in, and understanding of the impact that die changes will have to the existing process.
Watch a video of renowned metallurgist Dr. Stuart Keeler explaining AHSS Instantaneous n-value:
Keeler On N-Value from worldautosteel on Vimeo.
Figure 2 Image provided courtesy of Dr. Stuart Keeler.
Figure 1: Laser Welding is commonly found in these vehicle subsystems.
Laser welding is finding its way into more vehicle applications due to inherent weld strength, adaptability to complex weld geometries, and lower part distortion (Figure 1). Automotive applications use a variety of welding joint designs for laser welding in both lap joint and seam butt joint configurations as shown in Figure 2. For example, laser butt-welding is used for welding tubes in roll-forming production lines as an alternative method for high frequency induction welding. Seam welds on butt joints need less power from the machine than lap joints due to the smaller weld fusion area, producing less distortion and a smaller heat affected zone (HAZ). Butt joint configurations are more cost efficient, however, the fit up for seam welds can be more difficult to obtain than those of lap joints.
Figure 2: Common seam and joint types for laser welding of automotive applications.
When seam welding butt joint configurations, a general guideline for fit-up requirements include a gap of 3-10% the thickness of the thinnest sheet being welded, and an offset of 5-12% thickness of the thinnest sheet. Conversely, lap joints can require a gap of 5-10% the thickness of the top sheet being welded (Figure 3).
Laser welding is often used for AHSS lap (overlap) joints, but of course use different parameters compared to seam butt joint configurations. This type of weld is either a conventional weld with approximately 50% penetration in the bottom sheet or an edge weld. Welding is performed in the same way as for mild steels, but the clamping forces needed for a good joint fit-up are higher with AHSS than for mild steels. Lap joints tend to provide a larger process window, which can compensate for some of the manufacturing difficulties with AHSS, including springback and part distortion.
To achieve good laser-welded overlap joints for Zn-coated AHSS, a small intermittent gap (0.1-0.2 mm) between the sheets is recommended, which is identical to Zn-coated mild steels. In this way, the Zn does not get trapped in the melt, avoiding pores and other imperfections. An excessive gap can create an undesirable underfill on the topside of the weld.
Figure 3: Fit-up requirements for butt joint and lap joint configurations in laser welding.
Studies have shown laser welding Zn-coated steels can be done without using a gap between the overlapped sheets. This is accomplished using dual laser beams. While the first beam is used to heat and evaporate the Zn coating, the second beam performs the welding. The dual laser beam configuration combines two laser-focusing heads using custom-designed fixtures.
AHSS grades can be laser butt-welded and are used in production of tailored products (tailor-welded blanks and tubes). The requirements for edge preparation of AHSS are similar to mild steels – in both cases, a good quality edge and a good fit-up are critical to achieve good quality welds.
If a tailor-welded product is intended for use in a forming operation, a general stretchability test such as the Erichsen Olsen cup test can be used for assessment of the formability of the laser weld. AHSS with tensile strengths up to 800 MPa show good Erichsen test values (Figure 4).
Figure 4: Hardness and stretchability of laser butt welds with two AHSS sheets of the same thickness (Erichsen test values describe the stretchability.)
The hardness of the laser welds for AHSS is higher than for mild steels (Figure 5). However, good stretchability ratios in the Erichsen test can still be achieved when the difference in hardness between weld metal and base metal is only slightly higher for AHSS compared to mild steels. If the hardness of the weld is too high, a post-annealing treatment (using HF-equipment or a second laser scan) may be used to reduce the hardness and improve the stretchability of the weld.
Figure 5: Improved stretchability of AHSS laser welds with an induction heating post-Heat treatment (Testing performed with Erichsen cup test)
Contributed by Menachem Kimchi, Ohio State University
Imagery and work thus represented is provided as follows:
Figure 2 and 3: Courtesy of TRUMPF
Figure 4: H. Beenken, “Joining of AHSS versus Mild Steel,” Processing State-of-the-Art Multi-phase Steel; European Automotive Supplier Conference, Berlin (September 23, 2004).
Figure 5: Courtesy of ThyssenKrupp Stahl.
Arc welds are normally used for vehicle components where the loads are high, for example in shock towers and engine cradles. Conventional arc welding processes (GMAW, TIG, and plasma) can be used as effectively for AHSS as with mild steels. The same shielding gases can be used for both, and arc weld strength can often be equivalent to the base metal with shorter welds (although increasing the length of the weld usually achieves greater weld strength). By adjusting the number and length (that is the total joined area) of welds, the fatigue strength of the joint can be improved. Fatigue strength of arc welds is generally superior to spot welds.
Figure 1: Martensite content compared to tensile strength.
Despite the increased alloying content used for AHSS, there are no increased arc welding imperfections compared with mild steel. The strength of the welds for AHSS increases with increasing base metal strength and sometimes with decreasing heat input. Depending on the chemical composition of AHSS [for example, mild Steels and DP steels with high martensite content and strength levels more than 800 MPa], the strength of the weld joint may be reduced in comparison to the base metal strength due to small soft zones in HAZ (Figure 1). For CP and TRIP grades, no soft zones occur in HAZ due to the higher alloying content for these steels in comparison to DP and mild steels.
Higher strength filler wires are recommended for welding of AHSS grades with strength levels higher than 800 MPa. It should be noted that higher strength fillers are more expensive and, more importantly, less tolerant to the presence of any weld imperfections. When welding AHSS to lower strength or mild steel, it is recommended that filler wire with 70 ksi (483 MPa) strength be used. Single-sided welded lap joints are normally used in the automotive industry, but due to the unsymmetrical loading and the extra bending moment associated with this type of joint, the strength of this lap joint is lower than that of the butt joint.
Figure 2: Joint design tolerance.
For automotive applications, a design gap tolerance (G) of 0-0.5 mm is allowed for all weld joints, as illustrated in Figure 2. An edge trim tolerance (Et) of ±0.5 mm is required where the edge is part of the weld joint, shown in this same figure.
Figure 3: Edge location tolerance for fillet weld in a lap joint.
The variation in edge location causes variation in alignment of the electrode wire with the weld joint, as shown in Figure 3. Misalignment of the electrode may cause poor weld shape, improper fusion and burn-though. To control this variable, the trim tolerance at the weld joint must be held to ±0.5 mm and the electrode must maintain a root joint alignment tolerance of ±0.5 mm.
Figure 4: Maximum GMAW welding gap.
A tolerance stack-up review must be performed on all GMAW joints. The worst-case maximum designed gap including tolerance stack-up shall not exceed what is listed in Figure 4. It is preferable to target the smallest possible gap (the thickness of the thinnest sheet or 1.5 mm, whichever is smaller).
Figure 5: Reducing weld stress concentrations.
High-stress areas defined by CAE analysis and/or functional testing should be reviewed for weld optimization. Figure 5 illustrates techniques used to reduce the fillet weld stress concentration, which results in improved weld performance. These techniques include placing the weld start/stop away from corners and other high-stress areas, avoiding abrupt weld line direction changes when possible, etc.
Figure 6: Intermittent fillet weld spacing.
Intermittent Welds – Intermittent welds can be employed as a method to reduce heat input and distortion (maintaining gap control), but they also introduce weld starts and weld stops, both of which are stress risers. Weld start/stops of intermittent welds should be placed away from high stress areas. Intermittent welds are specified by the center-to-center distance (i.e., pitch) and weld length, as shown in Figure 6.
Contributed by Menachem Kimchi, Ohio State University
Imagery and work thus represented is provided courtesy of Auto/Steel Partnership and AET Integration.