Key Engineering Materials Vols. 410-411

Abstract: This paper describes new developments in incremental, robot-based sheet metal forming (Roboforming). Roboforming is a dieless sheet metal forming process which ensures cost-effective manufacturing of prototype parts and small batches. An approach for increasing the part accuracy in Roboforming is presented. It is developed in a cooperative project funded by the German Federal Ministry of Education and Research called Roboforming. The project concentrates on the development of an industrial applicable system design. The use of standard components allows a modular and scalable set-up. A servo loop, consisting of sensors and a programming system, represents the basis of this design and shall guarantee higher part accuracies by measuring the deviations between a formed part and its target geometry. The deviations are used to derive corrected tool paths. The correction is performed by an adjustment vector for every point on the tool path. The theory for this strategy and first results are presented in this paper.

Abstract: High strength aluminum blanks can be obtained by grain-refinement due to an Accumulativ Roll Bonding (ARB) process, in which two sheets are iteratively brushed, stacked on top of each other and subsequently rolled together. The high shear stresses during the rolling cycles result in an ultrafine-grained microstructure with an average grain size ranging between 200 to 1000 nm. Whereas the grain-refinement causes a drastically increased strength of the aluminum material, the formability of the ARB-blanks made of industrially used aluminum alloys such as the AA6016 deteriorates to the same degree as the strength rises. In this context, a local heat treatment of the ARB-blank reducing the material’s strength and increasing its ductility in specific zones will allow to recover the blank’s formability again. The research work presented in this paper studies the microstructural effects of a short-term heat treatment on the mechanical properties of ARB-blanks made of AA6016. Experimental investigations including hardness measurements, tensile tests as well as microscopic analyses show that heat treatments of only several seconds already result in significant increases of the material’s ductility and decreases of the material’s strength. By applying these microstructural mechanisms in terms of a specific heat treatment layout, functional gradients of strength and ductility adapted to the succeeding forming operation can be setup significantly enhancing the ARB-blank’s formability.

Abstract: In this paper, the high temperature, deformation behaviour of beta titanium alloy Ti-20V-4Al-1Sn sheet is studied by performing uniaxial tension experiments at three different strain rates at high temperatures of 700°C, 750°C and 800°C. The stress-strain curves for these temperatures show strain rate sensitivity, yield point phenomena and continuous flow, softening patterns. Microstructures of deformed specimens at several representative deformation stages and different strain rates are studied using an optical microscope. Dynamic recovery does not occur at the early stage of deformation including the yield-point and the subsequent yield drop regime, but it is activated at a large deformation stage, where it is affected by both strain rate and strain. A viscoplastic, constitutive model, based on the assumption of rapid dislocation multiplication, is proposed to describe such high temperature, yield-point phenomena. In this modelling, the softening effect due to dynamic recovery is also considered. The stress-strain responses, predicted by the constitutive model, well capture the yield-point phenomena, strain rate sensitivity and subsequent continuous flow, softening behaviour of the beta titanium alloy.

Abstract: A micro-tensile testing has been developed to investigate the adhesion behaviour of the oxide scale thermally grown on AISI 441 stainless steel sheet oxidised at 800 °C in different atmospheres - synthetic air and water vapour. In the test, a sample was placed in a tensile testing machine sitting in the chamber of a scanning electron microscope at room temperature. Evolution of the failure of the oxide scale was monitored in function of the imposed strain. It was found that the scale formed on steel oxidised in synthetic air exhibited the drastically lower spallation ratio in function of strain comparing to the scale on steel oxidised in 20 %v/v H2O/N2. For the sample oxidised in water vapour, it was clearly observed that the scale was primarily failed by the crack perpendicular to the tensile loading direction, followed by the spallation due to the compressive stress generated by the Poisson effect. After the test, precipitates rich in Nb, Si, and possibly Ti were observed at the internal interface between scale and steel substrate. For the oxidised samples that the final polishing direction paralleled to the main sample axis, the strain provoking the first spallation of the samples oxidised in synthetic air and 20%H2O/N2 were 6.23 and 3.52 % respectively. The theoretical model was developed in our previous work to quantify the mechanical adhesion energy. These values were 357 and 68 J.m–2 for the steels oxidised in synthetic air and 20%H2O/N2 respectively.

Abstract: Cosmetic defects such as ‘hollows’ are the result of deviations in a skin panel. These deviations are usually too small and local to be detected by discrete measurements of the panel but become visually apparent after the application of paint. As a result, the perceived quality of a panel may become unacceptable and considerable time may be dedicated to minimizing their occurrence through tool modifications. This paper proposes that there are three aspects to the problem: the springback or buckling of the panel, the optics of the painted panel and the ability of an observer to perceive the defect. In particular, it will be argued that hollows cause optical distortions that inform the human eye of the presence of a defect. The paper then suggests that signal processing techniques, in particular the wavelet transform, provide a simple way of locating and quantifying the severity of these defects. The transform was applied to two physical parts and a simulation model.

Abstract: An optical monitoring system has been developed by the authors aiming at real-time monitoring of laser cutting of thick plates of mild steel. This optical system uses photodiodes and a NIR camera as sensors. The concepts and results of the photodiode-based monitoring system have already been reported [1,2]. This paper is dedicated to the development of the optical set-up for camera-based monitoring of the laser cutting process. Firstly, some general aspects concerning the sensing principles and the conceptual design of the physical layout of the camera set-up are discussed. Based on a detailed optical analysis of the proposed conceptual design, some relevant information with regard to the selection of the camera type was obtained. Aspects related to the acquisition and analysis of the images, are highlighted. The initially developed camera set-up is presented. Firstly the design of the corresponding optical path is discussed in detail, including the results of the performed optical analysis. Next the results of the verification tests, performed on this physical set-up, are presented. Finally some preliminary results are included, proving the superior capability of the NIR camera compared to the photodiode-based solution for monitoring purposes.

Abstract: One-component polyurethane coatings on galvanized sheet steel are widely used to suppress white rust of the car body. To fulfill the requirements of corrosion resistance and aesthetics, such coatings have to withstand sheet forming and assembling operations without being damaged. Damage - which in the wide sense encompasses pores, surface and interface microcracks - would mean an inacceptable reduction in corrosion resistance and aesthetics deterioration. Therefore, understanding of coating damage initiation due to sheet forming becomes a key issue for producers of coating materials and forming technologists. To study coating performance in a forming process, FE simulations can be employed. This requires, however, an appropriate constitutive model of the coating and accurately determined model parameters. The present work investigates a two-layer coil-coating system Coilprime C manufactured by the BASF Coatings AG. The system consists of a primer with a thickness of approximately 5 µm and an intercoat, which is about 20 µm thick, both being based on a polyurethane (PU) resin. Applicability of indentation for determination of mechanical properties of the intercoat is studied. The experiments were carried out with a Hysitron TriboindenterTM using a conospherical indenter. In the experiments, the intercoat exhibited viscoelastoplastic behaviour. In the FE simulations of the indentation test, the intercoat and the primer were assumed to be a single continuum with a constitutive behaviour described as a combination of hyperelasticity defined by the Arruda-Boyce strain energy potential and ideal plasticity defined by the rate-independent von Mises yield surface. The full deformation is modelled using a multiplicative split of the deformation gradient into hyperelastic and plastic components.

Abstract: The design of cold roll forming roll tools, roll geometry and the number of tool sets, can typically be referred to as experiential design. Within a multi-occupant design office there will normally, for the same sectional profile, be as many different designs as there are designers. This variation in design results in a lack of design consistency and hence makes the process of developing design understanding, increasing profile complexity, maintaining quality, and roll tool predictability very difficult. This paper contains an explanation of the approach taken within the Hadley Group to identify the discrete steps associated with roll tool set design. To implement methods to standardise these steps including strip width and clearance calculations. With appropriate design steps standardised an explanation is provided as to how elements of this design process is automated leading to a system of assisted design for roll tool sets. The results, including increased design capacity, of the assisted roll design after being incorporated into the design office are discussed.

Abstract: Metal Matrix Composites (MMC) are very interesting materials for applications in the automotive and aerospace fields, since they combine the lightness of Aluminium with the strength of the ceramic reinforcement. These materials are very difficult to join and conventional welding techniques are not applicable, whereas solid-state welding techniques, like Friction Stir Welding (FSW), could be a solution. However very hard tool materials will need to be chosen in order to overcome the problem of heavy abrasive tool wear. In this work the wear behaviour of coated and uncoated steel tools has been investigated in the Friction Stir Processing of extruded bars in A359 + 20%SiC. AISI 1040 steel was used as the tool material in the hardened uncoated and annealed and coated versions. In the latter case the steel was coated with multilayer PVD AlSiCrN or AlSiTiN. Coatings were fully characterised, in terms of thickness, hardness, adhesion and tribological behaviour. Further, tool wear analysis has been carried out using optical and electronic microscopy, with EDS analysis. The results gave some interesting information on the choice of tool in the FSW of MMC materials.

Abstract: The cold rolled forming process passes flat sheet metal through a series of roll tool sets resulting in complex final section profiles. The sheet metal is deformed both while in contact with the rolls and while in between the roll. Some of the variables in this process include the roll tool set and hence progression design and the designer; the metal type and its thickness; and the sectional profile and the associated tolerances required by the customer. When a customer orders a new profile the roll tool and hence system designers’, start by trying to determine if something similar has previously been rolled. If a similar profile can be identified then that becomes the starting point for the new process design which includes the roll tool sets and the number of sets. Typically previous designs have been identified from the collective design office memory or by searching through past drawings until something similar is found. When a previous similar rolled section is identified; unless the design is very recent it is often impossible to gauge a measure of design and hence rolling success. This paper describes how the authors have developed and implemented within the Hadley Group a searchable database of previously rolled profiles, thus enabling all similar profiles to be identified. In addition, the paper contains a description of how a measure of tooling design effectiveness has been developed and is used as part of the full design, pre-production rolling, and full production rolling process with the results being incorporated into the database, hence enabling an informed selection of the design starting point.