Materials Science Forum Vol. 1183

Abstract: Adaptive thermal management is a prerequisite for multi-stage tailored forming of hybrid steel-aluminium blocks, as each section of material must remain within its forming temperature window and the joining zone must be protected from excessive thermal stress. This study defines a control-oriented process space for a combined induction heating and dual-fluid spray cooling concept developed in the Collaborative Research Center (SFB) 1153 “Tailored Forming.” A three-phase test program is applied: Phase A quantifies and evaluates the influence of air pressure p_air, water pressure p_w, and nozzle distance d on the cooling performance and the formation of an axial gradient using standardized regression coefficients. In phases B and C, a reference setting is applied to rotationally friction-welded 20MnCr5/EN AW-6082 blocks in a cold-start and preheated state, which are representative of multi-stage forming processes. The results show that p_air and p_w dominate both the cooling capacity and the formation of gradients, while d plays a subordinate role in the range investigated. The relationships remain qualitatively consistent for hybrid blanks and preheated conditions when the heating program is adapted to the aluminium and joining zone boundaries. The derived actuator ranking forms the basis for closed-loop temperature control in volatile, multi-stage tailored forming chains.

Abstract: In order to achieve a long service life for highly stressed parts such as shafts for drivetrains, a combination of a bainitic microstructure with compressive surface residual stresses is beneficial. While a bainitic microstructure offers a good balance between strength and toughness, compressive residual stresses especially near the surface have a positive impact on service life. Research has shown that this is due to a shift of the crack initiation towards the core and a reduced crack growth . These properties can be achieved by hot forming as an established method for manufacturing highly stressed parts followed by an adapted cooling strategy. As this general approach was demonstrated for a simplified process in a prior study , the present article is dedicated to the functionalisation for hot forming processes. In detail, a customised spray cooling is presented for a hot impact extrusion process whereby shafts made of AISI 4140 are cooled down from the forming heat in a single step with adjusted cooling rates. In a finite element-based process design, different cooling strategies were investigated and adequate heat treatments to achieve the combined properties were identified. Following this process design, shafts are formed via hot impact extrusion and spray cooled according to the cooling strategy for experimental validation of the numerical model. Additionally, shafts with air cooling are produced as a reference. During forming, force-displacement curves are measured, which are used for the validation of the numerical hot impact extrusion simulation. The resulting plastic strain and temperature distribution significantly influence the following cooling simulation. The final microstructure as well as hardness values of the produced shafts are determined and compared for the varying cooling strategies.

Abstract: In cold bulk metal forming, coatings based on zinc phosphate are commonly used for lubrication. This has a negative impact on the environment, negatively affects human health, and requires significant pre-and post-surface treatments. Powder metallurgical (PM) components are a promising alternative to zinc phosphate coatings due to the process related porosity of the workpiece which acts as lubricant reservoir. During the forming process, the lubricant stored in the pores is released and lubricates the tool and workpiece surfaces. For an efficient process design of such components, finite element method (FEM) is an effective tool to analyse forming and friction behaviour. To this end, a realistic material model is essential for accurate simulation results. Hence, in this work, the flow behaviour of PM semi-finished products is characterised by means of compression and tensile tests. The results indicate that the material exhibits different behaviour under compression and tension. In compression, the material demonstrates higher yield strength and flow stresses compared to tension. Additionally, inhomogeneity of the material distribution can be observed, characterised by a denser core and more porous outer regions. The porous outer regions make it suitable for storing lubricant for application in forming processes.

Abstract: Ring rolling (RR) is a widely used process for producing seamless rings, but its complex thermo-mechanical behavior often requires costly experiments or FEM simulations. This study presents a novel analytical method for predicting torque and energy in RR that explicitly accounts for the fishtail effect, a lateral deformation of the ring cross-section. The approach combines a slab-based mechanistic model with a regression linking fishtail deformation to the kinematic ratio between mandrel feed and ring rotation. Validation was performed via FEM simulations on an industrial AISI 1045 steel case, covering thirty conditions with varying feed rates and rotational speeds. Results show that conventional models ignoring fishtail can overestimate errors by over 60% for torque and 50% for energy, whereas the proposed method reduces errors below 15% in most cases. These findings highlight the importance of including fishtail effects, offering a fast, reliable, and efficient tool for early-stage RR process design and optimization.

Abstract: A fine blanking (FB) has grown up as a manufacturing means to fabricate the precise automotive parts from the thick sheet metals. During this net-shaping process, each part product had fully burnished surfaces through a single stroke. The punch for this FB, were highly loaded enough to distort the punch edge and to deteriorate the cost competitiveness and sustainability. A chamfered SKH51 tool steel punch was designed by controlling the chamfer widths and angles. A massively nitrogen supersaturated SKH51 punch was also prepared to investigate its effect on the suppression of punch edge distortion. AISI304 stainless steel work was used to describe the variation of punch distortion and blank qualities with increasing the number of strokes.

Abstract: Friction plays an important role in flat rolling processes for the force and power demands,kinematics and final product quality. In search for a method of in-situ characterization of the coefficientof friction (COF) by non-contacting measurements, a method for determination of the COF from highaccuracy forward slip measurements, i.e. by laser doppler velocimetry combined with a comprehensiveevaluation without simplifications is the method of choice. The evaluation must comprise the volumeflux equilibrium at the neutral point and the roll gap exit, as well as the connection of the neutral pointand COF by von Karman’s ODE. This iterative procedure involves multiple solutions of the ODE aswell as the nonlinear volume flux relation. In the present work, this problem is addressed by a physicsinformed neural network (PINN), providing a rapid connection between the forward slip and the COFbased on the mathematical rolling theory. The contribution shows that we can solve von Karman’sODE inversely by a PINN, enablind detection of the COF and the neutral angle from the measured forward slip.

Abstract: Cold rolling forces are strongly affected by lubrication and material properties, and maypotentially be used to estimate material property variations along the coil. The 1D slab method iscommonly used to estimate rolling forces as it is computationally inexpensive. By model definition,the pressure distribution as modelled in the slab method has a single peak or friction hill in the rollbite. However, it has been observed in several studies that multiple local peaks can appear in thepressure distribution in the roll bite. Additionally, material anisotropy also affects the contact pressuredistribution and steady state roll force. In the present work, a 2D plane strain finite element rollingmodel is used for a detailed sensitivity study of the multitude of parameters that affect the verticalpressure distribution and the steady state roll force. The considered model parameters are materialanisotropy, entry sheet thickness, roll radius, tensions, coefficient of friction and roll gap.

Abstract: Open-die forging is an incremental bulk metal forming process for producing large, safety-relevant components such as turbine and generator shafts. Besides achieving the target geometry, the process improves mechanical properties through grain refinement and the elimination of casting-related defects. With the increasing use of high-alloy steels, precise process control is required to prevent surface and internal cracking caused by material damage. However, predictive models for damage evolution under the thermo-mechanical conditions of open-die forging remain limited, particularly with respect to high-temperature recrystallization and the incremental process character with inherent pause times. In this work, a recrystallization-sensitive damage model was developed and validated for open-die forging. The parameters of the Lemaitre damage formulation were determined for the cold work tool steel D2 (1.2379, X155CrVMo12-1) using hot tensile tests over the relevant forging temperature range. Dynamic recrystallization kinetics were characterized by hot compression tests and described using an Avrami-type JMAK formulation, while static recrystallization behavior was analyzed by stress relaxation experiments and also modeled with JMAK kinetics. These results enabled the quantification of recrystallized fractions as functions of strain, temperature, strain rate, and dwell time. To link microstructural evolution with damage development, tailored recrystallization states were generated in dilatometer experiments and examined metallographically with respect to void formation and healing. The extended model was implemented in a finite element framework and validated through open-die forging experiments on demonstrator geometries, showing its capability to predict damage initiation under industrially relevant conditions.

Abstract: This work analyses the influence of batch-to-batch variability on both strain hardening and ductile fracture behaviour of a 42CrMoS4 steel under cold forging conditions. Mechanical testing combined with full-field strain measurements and finite element simulations is used to characterise material response and fracture under different stress states. Batch-dependent hardening laws are identified, and ductile fracture initiation is described using the Hosford–Coulomb criterion, calibrated independently for each material batch. The identified fracture strains and fracture surfaces exhibit a measurable variability between batches, even for similar stress-state conditions. The results provide quantitative evidence of batch-dependent material behaviour relevant for forming simulations