Sheet metal products have been manufactured for centuries, in several sectors of industry. Different materials and processing routes have been developed over the years to overcome the difficulties in obtaining the desired in-service properties, lightweighting, reining in springback, in reducing the cost accompanied by enhancement in quality. Dent resistance, springback, light weight and spatial variation in sheet thickness in a formed product are the major issues which have been a challenge over the years. Moreover, sheet metal products need to be assigned generous tolerances and being compliant, inspection and assembly procedures have to be different compared to those for machined components. In addition, joining processes like welding can cause warpage and this impacts the dimensional accuracy of the sheet metal assembly.
Sheet metal working technologies have been continuously evolving to meet these challenges. In particular, lightweighting of sheet metal components have led to means of reducing material input, product designs, hybrid materials involving use of materials of lower density and development of high strength materials. Simultaneously, technologies like laser cutting and welding, flexible roll forming, hot stamping, hydroforming were developed to meet the challenges of enhanced accuracy and quality of making light weight designs.
As the quest for new materials and the technologies continues, variety of materials going into the light weight product have been increasing. End of life recycling of sheet metal products therefore becomes more complicated compared to designs using single material. Another advantage of using a single material, especially steel, is the ease of joining. A number of welding processes are available to weld a range of joint configurations, most of which call for an application of heat.
Multiplicity of materials call for diverse manufacturing technologies. For instance, sandwich sheets (for lightweighting), galvanised sheets (for corrosion resistance) lead to issues in welding and call for novel joining technologies. Mechanical joining technologies like rivetting, fastening, clinch joining, and similar solid state athermal processes (those not involving application of heat) are needed in such situations.
None of the solutions are free from problems. In fact, solution to one problem leads to many more. For example, high strength steels were developed so that the same dent resistance could be achieved by a thinner sheet enabling lightweighting. However, higher strength and lower sheet thickness meant greater springback, relatively lower ductility, higher forming forces and greater tool wear (reduced tool life), and a necessity to alter the product design to suit the characteristics of the high strength material.
Hot forming process overcomes these issues, since forming at a high temperature means better ductility of the sheet, lower forces in forming due to softening of the sheet at high temperatures, high in service strength and yet lower springback. Seemingly, most of the issues of high strength steel are taken care of. But then tool life (because of tools being subjected to mechanical and thermal fatigue damage), thermal stresses, surface finish and enhanced difficulty in trimming (impacting adversely, the tool life of trimming tools) are serious concerns and ‘cost raisers’. In other words, one set of problems get transformed into another. But then, every part is not expected to have the same set of product characteristics, like same level of fracture toughness (for crash behaviour), surface finish, same level of accuracy, etc. This makes some parts amenable to manufacture by hot stamping, while some others could be made with conventional technologies.
Given the increasing complexity of part geometries, together with ever expanding options from the availability of a spectrum of materials and processes, getting the part ‘first time right’ appears to be a difficult proposition, unless modern techniques of simulation are used. Simulation softwares have been continuously incorporating these advancements into their database to predict the strains and the potential locations of failure in the part being formed. Virtual manufacturing will take the centrestage in the near future, since one can get a feel for how close the formed part would be, to the geometry intended by the designer.
Virtual manufacturing of sheet metal parts will have to account for the variations introduced by the inevitable variations in material properties, tool wear, tool deflection under load and some inconsistency in springback. While different stakeholders are working on the consistency of the final product quality, each of the above mentioned factors will continue to bring in an element of uncertainty since the interaction material variations with almost every major parameter (like product geometry, tool design and tool geometry, sheet thickness, etc) impacts the final characteristics of the sheet metal part. Unless variations in material properties are curtailed, generous tolerances will have to be assigned to the dimensions and the philosophy of functional build will have to be adopted.
Failure criteria, which are important from the standpoint of process design and die design are another source of uncertainty, especially when it comes to virtual manufacturing. A large number of failure criteria applying to diverse materials are available. The Forming Limit Diagram/Curve (FLD/FLC) and the Forming Limit Stress Diagram are the most popular failure criteria to identify potential locations of failure in a formed part. Many of these criteria relate to the failure mechanisms that cause localisation and are therefore material specific. Tensile properties have often been correlated with the forming limits under biaxial conditions prevalent in a sheet being formed. A high value of strain hardening exponent is found to enhance the forming limits especially in stretching (stretch forming) while a high plastic strain ratio (normal anisotropy) promotes high drawability and is good for a drawn part. However, the available failure criteria are strain path dependent and the forming limits are subject to change depending on forming history. This often leads to unreliable predictions of the occurrence of failure and potential failure sites.
Strain distribution based criteria seem to overcome these issues. Since the strain distribution is the final outcome of the various parameters, the interactions of which are difficult to establish, this can be taken as the ‘signature’ of the various events occurring during forming of a part. A strain distribution based concept called ‘process signatures’ was proposed earlier, to infer on the various abnormalities in the process, based on the shape of the strain distribution ‘envelop’ called the process signature.
Quantities based on significant features of strain distribution in a formed part were recently defined as Strain Non-Uniformity Index (SNI) and the Constraint Factor (CF). These features are independent of the material, the geometry of the product and the processing conditions that were used to arrive at the given strain distribution in the sheet. Thus, a single definition of SNI and CF is used to establish their magnitude irrespective of whether the deformation was accomplished by cold press working, hot forming, superplastic forming, hydroforming, or, for that matter, any other process. It is possible to determine the critical value of the SNI for every ‘critical plane’ in a deforming product, and compare the current value of the SNI to ascertain if failure would occur in the part or not. Since this technique incorporates all effects, including those due to strain path variation, it is possible to realistically predict the locations of imminent failure, those of likely failure and the ‘safe’ regions in the formed part (Fig. 1 a, b). Such a method would achieve yet another purpose. Unlike many failure criteria which are using simple shapes and are thereafter applied to complex geometries, the SNI based criterion needs no such procedure.
To sum up, the product quality today suffers from certain sources of uncertainty as discussed above. The future lies in minimising these uncertainties so that the resulting sheet metal product will conform very closely to the design of the part. Robustness of the processing route would also help enhance greatly, the consistency of the part actually manufactured. This way, virtual manufacturing prior to manufacture of actual parts will make the designer’s dream come true!
Article authored by Prof P P Date, Dept of Mech Engg, IIT-Bombay