Papers by Author: Bert Verlinden

Abstract: A Ti-50.8at.%Ni wire produced using a co-drawing method and a commercial Ti-50.8at.%Ni wire were annealed between 450°C and 700°C. Grains with diameter less than 100nm were revealed by TEM for both wires before annealing treatment. However, the microstructural heterogeneity of the co-drawn wire is more obvious than that of the commercial wire. The transformation behavior and mechanical properties of both wires were found to be sensitive to the annealing temperature. Multi-stage martensitic transformation was observed in the co-drawn wire, compared with the one-stage A↔M transformation in the commercial wire after annealing at 600°C for 30min. Moreover, the ultimate tensile stress and plateau stress of the commercial wire were found to be higher than that of the co-drawn wire under the same annealing conditions. The differences of total elongation, plateau strain and pseudoelastic recoverable strain between the commercial wire and the co-drawn wire were also observed. The differences of the transformation behavior and mechanical properties between the commercial wire and the co-drawn wire are attributed to the microstructural difference between these two wires.

Abstract: In our recent work, a new integrated model was proposed to describe the back-stress evolution based on the dislocation substructure and texture. By relating the back-stress to the dislocation density in cell walls and in the cell interior, this model is able to capture the back-stress evolution of ECAP processed pure aluminium. In this paper, the model is used for another FCC material, namely copper. The aim is to check whether this model is able to predict the tension/compression asymmetry (due to the back-stress) of copper. The results show that this is indeed the case and it is also found that the strain rate ratio proposed in our previous work [1] is a function of the dislocation density ratio.

Abstract: In most papers dealing with tension and/or compression tests, the conventional yield stress is determined either by an offset method (usually 0.2% strain) or by back extrapolation from the stress-strain curve. In our experiments on ECAP’ed Aluminium a transient hardening saturation (THS) is always observed during the compression tests, but not during the tensile tests. This THS occurs at a significantly lower stress than the conventional yield stress. The aim of the present paper is to determine which the “real” start of yielding is. Two different experimental approaches have been adopted, confirming that the THS stage is exactly the yielding stage. This is not unimportant because it increases the tension-compression asymmetry and hence the back-stress and kinematic hardening. The reason for this different behaviour between tension and compression can be ascribed to a different change in strain path with respect to the ECAP deformation.

Abstract: To understand and model grain refinement in severe plastic deformation, some analysis of Nb single crystals has been carried out in previous work. To bridge the gap with normal polycrystalline materials, supplementary experiments on large polycrystals, deformed at moderate strains appear to be necessary to explain the grain subdivision step by step. In the present work, successive uniaxial compression tests have been carried out on a large grained Niobium polycrystal up to height reductions of 30% with small strain increments. Electron backscatter diffraction (EBSD) analysis was done after each compression step to characterize the evolution of orientation and microstructures. It is observed that a “rotation front” forms inside the grain and moves with increasing strain from one side to the other side of the grain. In one grain, this process results in a grain boundary affected zone in the vicinity of the grain boundary. Both static orientation evolution inside the grain and historical evolution of the average orientation have been studied, which indicates that the grain orientation rotates around one of the (110) poles at low strain.

Abstract: High-purity niobium single crystals were deformed by equal-channel angular extrusion (ECAE) at room temperature to an equivalent Von Mises strain of about 1.15. Deformed samples were annealed in vacuum from 500 to 800oC for 1 hour to investigate their microstructure evolution. The microstructure of deformed and annealed samples was characterized by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and Vickers microhardness testing. The deformed structure after one ECAE pass is rather inhomogenous and consists of parallel sets of coarse shear bands whose spacing varies from one region to another in the cylindrical billet. the microstructure within the shear bands consists of elongated subgrains with sizes below 3 μm and lamellar boundaries. The remaining non-sheared regions display a coarser subgrain structure. Recrystallization is virtually absent in samples annealed at 500oC for 1 hour. Nucleation begins mostly within shear bands. The new grains with sizes ranging from 10 to 50 μm are arranged in clusters rather than being homogenously distributed. The recrystallized volume fraction also varies from one region to another indicating an inhomogenous distribution of stored energy. At 700oC, recrystallization is complete after annealing for 1 hour resulting in a structure with a mean grain size of about 100 μm.

Abstract: Over the past few years both sheet metal process planning and production planning issues received increased attention. For process planning of the laser cutting process, nesting algorithms are developed in order to decrease the waste material. Additionally, algorithms are available for path planning, i.e. determining the best sequence for cutting the different parts. Production planning is mainly performed based on the ability to fill a sheet. For air bending, process planning focuses on bend sequencing and tool selection, while production planning optimization aims at minimizing time consuming setups between the different production layouts at the press brake. However, when integrating both processes, the benefits from individual optimization counteract one another: good nestings at the laser machine can create additional setups at the press brake, hence increasing the makespan. An integrated approach is proposed to verify whether this problem can be solved by already taking into account possible setups at the press brake in the early nesting stage. Integration of both processes aims at an optimal combination of parts on a sheet and minimization of the setups at the press brake. In this paper, an overview of a modeling effort addressing both goals is proposed. When combining parts on a sheet, preference is given to parts requiring the same production layout at the press brake. If this is impossible, production layouts with low changeover times are preferred. Industrial cases are used to verify the applicability of the proposed model. The results are compared to a reference approach where nesting is performed with dedicated software and planning for air bending is based on an operator’s experience. Compared to this reference approach, a makespan reduction and a setup time reduction can be observed. The planning is generated almost instantaneously and no additional sheets are required compared to the reference approach. Future research will focus on expanding the model and verifying its applicability on a larger data-set.

Abstract: An experimental AA5182 sheet was cold rolled 80%, and tensile specimens removed with orientations 0, 45 and 90 degrees to the rolling direction. Room temperature monotonic tensile tests were performed on the specimens in different recovered states obtained by isothermal annealing at 230°C from 0.1 to 10h. The tests were instrumented to measure instantaneous plastic strain ratio, and unloaded just after incipient necking, but before failure if possible. With annealing the flow curves are characterized by lower strains to the onset of jerky flow, the reappearance of yield point elongation, decrease in work hardening, and increasing ductility. The recovery in substructure was described using a constitutive parameter proportional to the mean slip distance.

Abstract: The mechanical properties of fine-grained Aluminium AA1050 produced by ECAP at room temperature, have been investigated under various deformation modes. Because ECAP leads to an unstable microstructure, some samples were annealed at 300°C for 10 minutes in order to stabilise the microstructure. For mechanical testing, cylindrical samples were used in three types of monotonic deformation tests: axisymmetric compression, uniaxial tension and simple shear by torsion. The influence of the deformation mode on the yield locus and strain hardening behaviour has been studied and will be discussed for both hot rolled AA1050, ECAP samples without annealing and annealed ECAP samples. To achieve a better understanding of the strain hardening, some preliminary tests with a change in strain path were also performed. The results of tension tests followed by compression will be reported.

Abstract: Two Aluminium alloys, type AA5182 and AA5182+1.2wt% Cu, have been studied. The second alloy in solution treated condition is 18% stronger than the first one. During ageing at 150°C or 200°C it shows a characteristic fast increase in yield strength during the first minutes of ageing, followed by a 'plateau'. Both materials have been deformed in an ECAP die (4 and 8 passes) at 200°C and the microstructure, hardness and mechanical properties in compression at room temperature have been investigated. Although in none of the two materials a true sub-micron grain size was obtained at 200°C, a fair combination of strength and strain hardening was observed. The AA5182+Cu alloy, when ECAP’ed after a solution treatment and quenching, shows an increase in strength of about 20% compared to the AA5182 reference alloy. A post-ECAP annealing at 200°C does not lead to a further increase in hardness or strength. An analysis of the substructure and the mechanical properties during ECAP led to the conclusion that the precipitates formed during ECAP at 200°C do not directly contribute to the higher strength of alloy AA5182+Cu, but they contribute indirectly by slowing down the recovery.

Abstract: The substructure of a single grain in an electron backscatter diffraction (EBSD) data map is studied, focusing on the influence of the grain boundary configuration on the misorientation to the average grain orientation of data points close to the grain boundary. For most grain boundary segments a certain degree of linking between the misorientations to the average orientation of the grain exists and large deviations from the average orientation of the grain are observed close to the triple junctions of the boundary segments. Changes of the misorientation over one boundary segment are analysed and possible explanations for these variations are discussed. It is suggested that the variations of the misorientation over the boundary segment can be attributed to the requirements of stress equilibrium and strain compatibility. Also the tendency of the grain boundary to lower its surface energy might have a significant influence on the misorientation profile and therefore on the subdivision behaviour of the grains.