Keywords: Superplasticformingprocess; Forming tools; Finite element simulation; Thermomechanical analysis
The superplasticformingprocess is one of the most advanced manufacturing methods for producing highly complex thin-sheet components in a single operation. This process is widely used in aerospace industry. Superplasticforming (SPF) shows significant advantages as compared to conventional forming methods. Superplastic metals exhibit high ductility and very low resistance to deformation and are particularly suitable for forming processes that require very large deformation. Superplasticforming is usually completed within only one step and intermediate annealing is usually not necessary. This process allows the production of complex, deep-shaped parts with quite uniform thickness. Drawbacks of the process include the need of tight control of temperature and strain rate. Very long forming time makes this process impractical for high volume production series. In addition, the high cost of Ti–6Al–4V sheet metal  is a limiting factor for wide and generalized spreading of the technology. So, the majority of SPF production still remains in the aerospace, transport and architectural fields.
Ti6242 alloy is used in the aircraft industry for the manufacturing of parts that can be used for thermal protection in hot areas. Its in-service behavior at high temperature is greatly improved compared to Ti-6Al-4V alloy, which is the most conventional titanium alloy . Presently, the complex shapes are obtained by a SuperplasticFormingprocess (SPF), require high forming temperatures ( ≥ 900 ◦ C)
Keywords: Forming tools / thermal behaviour / Finite Element simulation / Experiments /
Schl!sselw"rter: Umformwerkzeuge / thermisches Verhalten / Finite Elemente Methode / Versuche /
Heat resistant cast steels or ceramic materials are currently used to manufacture SPF tools [1, 2]. They allow to form superplasti- cally titanium based sheets. The environmental conditions are severe as the superplasticformingprocess is carried out at a max- imal temperature close to 900 8C (inside the heating press) and at a minimal temperature depending on the location at the surface of the die close to 200 8C when the mould is pulled out from the heating press to replace the formed component by a new sheet . These thermal cyclic conditions are repeated as many times as necessary to produce the required number of parts to be man- ufactured during one campaign. They induce mechanical stresses and cyclic plasticity that lead to the failure of the die. These mechanisms are very difficult to evaluate from an experi- mental point of view. Numerical simulation seems to be an accu- rate way to reach such information in order to optimise the tool design  and to improve its lifetime . So, constitutive models can be developed to reproduce the mechanical behaviour of tools depending on the material used to manufacture the mould [5, 6]. However, the temperature fields have to be first determined pre- cisely during the whole formingprocess. Indeed, an accurate mechanical behaviour model will not be useful if the thermal gradients within the tool are not well assessed. In this investiga- tion, the thermal environment and behaviour of SPF tools are
Superplasticformingprocess (SPF) is an advanced process conducted at high temperature using moderate strain rates, typically used for shaping TA6V sheets for aerospace applications. Thermomechanical stresses on the forming dies due to successive forming cycles may result in the early degradation and even fracture of SPF tools through fatigue crack propagation. To reduce cost and extend service life, dies are generally weld-repaired and subsequently re-used in the typical severe conditions of SPF. The implementation of robust, easy processing welding techniques resulting in high quality repair able to sustain cumulative thermomechanical stresses is of utmost concern to SPF parts manufacturers. The paper focuses on the development of an automated TIG technique to weld repair high nickel, high chromium heat resistant alloys based on a complementary approach including thermal instrumentation, numerical simulation using Sysweld TM and metallurgical investigation; this former being performed on either as-received, repaired and repaired plus damaged materials.
Figure 3: Evolution, along the edge, of shear angle computed with several forming parameters of model
The computational model used in this numerical sensitivity study exploits a hybrid discrete approach for forming of woven composite based on the hypoelastic behaviour of the shell element. This model is implemented in a commercial FE code (ABAQUS/Explicit) via a user material subroutine VUMAT. The effect of the sensitivity of the shear angle to the process parameters (blank holder, punch velocity and the coefficient of friction) has also been, studied. This parametric study gives results in good agreement with the literature .
Shot peening is a cold-working process that is used mainly to improve the fatigue life of metallic components. Experimental investigation of the mechanisms involved in shot peening is very expensive and complicated. Therefore, the Finite Element (FE) method has been recognized as an effective mean for characterizing the shot peening process and several types of FE models have been developed to evaluate the effects of shot peening parameters. However, in most of the existing FE models, the shot peening sequence and impact location were defined a priori. It is therefore the purpose of this study to consider the random property of the shot peening process. A novel 3D FE model with multiple randomly distributed shots was developed combining a Matlab program with the ANSYS preprocessor. The explicit solver LS-DYNA has been used to simulate the dynamic impingement process. Several potential applications of this novel model such as: the quantitative relationships of the peening intensity, coverage and roughness with respect to the number of shots have been presented. Moreover, simulations with multiple oblique impacts have been carried out in order to compare with results from normal impingements. Our work shows that such a computing strategy can help understanding and predicting the shot peening results better than conventional FE simulations.
 H. Badreddine, K. Saanouni and A. Dogui “Prediction of damage in hydro bulging test using a non normal and non quadratic anisotropic plasticity formulation” Second International Congress Design and Modelling of Mechanical Systems, CMSM’07, 19-21 mars 2007 Tunisia.
 K.I. Manabe, M. Amino, Effects of process parameters and material properties on deformation process in tube hydroforming, J. Mater. Process. Techno. 123, 285-291 (2002).
Figure 6: Measured and calculated thickness profiles
As can be seen in Figure 5, the sine law is an acceptable approximation for the first step of the multi step approach: when forming the first, 50° cone, the points are translated quasi downwards. This corresponds to a very limited tangential strain. For the next steps, however, the sine law is no longer valid. Instead of a downward translation, the points are quasi rotated about the backing plate edge, which causes a more substantial tangential strain that increases towards the bottom of the part (up to 25%: see Figure 5). The closer to the bottom of the part, the larger the horizontal distance between two consecutive step sections becomes and the more the rotational motion transforms into a downward translation. The maximum strains in both radial and tangential directions occur at the lower wall edge in the final stage of the vertical wall forming. In contrast with single-step forming, where maximum thinning and failure typically occur 10 to 15mm below the backing plate level when forming a cone, the edge of the cone bottom is also the location where failure can be expected to occur first in a multi-step strategy.
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The present study aims to identify and characterize the development of microstructure and deformation characteristics of steel grades in semi-solid state which is affected by the change in morphologies of microstructure at high temperature. Thixoextrusion tests with different combinations of forming temperature and forming speed were performed. It was identified that several process parameters, such as initial billet and die temperatures or forming speed, affect thermal exchanges thereby influencing the microstructure evolution and material flow. Furthermore, 2D and 3D microstructure characterization was performed on the same sample which was partial remelted and quenched. Reconstructed 3D images were compared with the ones obtained with a Scanning Electron Microscope and an Energy Dispersive Spectrometry system. The good agreement between 2D SEM observations and 3D X-ray microtomography results makes these two techniques efficient to characterize steels in the semi-solid state.
During a polymer formingprocess, viscoelasticity is the key parameter. Viscosity part control the spreading of the material in the right shape and elastic part is connected to final part mechanical stability. Presently, only thermal sensors are available for rotational moulding. For thermoplastics processing, this is sufficient because for a given polymer, viscosity is a simple function of temperature (shear rate can be neglected in rotational moulding). But in reactive process, it is not the case. Reactive rotational moulding requires a new type of sensors giving a value directly connected to viscosity. In the literature, ultrasonic response has been applied to polymer transformation studies such as glass transition . This in situ analyses technique is evaluated for the polyurethane thermoset studied here. Liquid isocyanate/polyol melt is laid down a steel plate at an average thickness of 3mm. A 3.5MHz ultrasonic sensor is attached below the plate and the signal is emitted through the steel plate and the polymer. The amplitude of the first echo from the interface steel/polymer is plotted as a function of time.
The first phases of design (requirements specification, conceptual and embodiment design) aims at assessing requirements and functions in order to define the product structure breakdown and the associated CAD models (parts and assembly CAD models). In those phases of design, the process is based on some fundamental concepts such as FBS  and sometime axiomatic design  when the solution tends to provide independent relation among functional and structural parameters. Afterwards (cf. Figure 1), the detail design phase (CAM, FEA...) other “designers” are assessing this first solution and react by giving some new recommendations (i.e. information integration) for improving the design solution. A lot of collaborative decision-making processes are then beginning to finally converge to a common agreement. A lot of interesting concepts with respect to integrated design [5, 6] and advanced product modelling [7, 8] have also provided real advances in design methodology and information modelling. They give opportunity to really set relations as soon as possible among the whole product information related to its entire lifecycle. Nevertheless the process is still based on a “redo until right” action and those concepts could have even more benefits through tackling the following issues 2 :
about 10 −4 s −1 to 10 −3 s −1 . These test conditions confer to the alloy properties known as superplastic that
is characterized by grain boundary sliding mechanisms at the microscopic scale. However, this process remains expensive and the aircraft industry wants to decrease both the forming temperature and the cycles times. To increase the manufacturing rates, researchers explored new processes (such as hot forming and stamping) which provide lower operating temperatures and faster pressure cycles. But also research has been focused on developing more knowledge on the thermo-mechanical processing which has allowed to further refine microstructures and control better the superplastic mechanisms across a broader temperature
screw spindles. It allows on the one hand a vertical (Z) tool displacement and a local stamping of the sheet and, on the other hand, a horizontal tool displacement in the frame of the sheet (X,Y) leading to the incremental forming of the piece. The tool path was specified by the pilot control with a computer connected to the later through a proper routine. The sheet deformation was performed with a 10 or 20 mm diameter hemispherical punch made of X38CrMoV5 tool steel. A 400×400 mm 2 work area can be studied in the current configuration of the machine and a 100 mm depth of draw can be reached. In order to form the piece, a bottom-top forming direction was considered. As a consequence, its design allows us the easy use of CCD cameras to perform in situ measurement of the deformation of the sheet by 3D-DIC (see section 2.2). Furthermore, a multi-axial sensor sets up the pilot that enables to monitor the three dimensional force measurement induced in the sheet by the punch displacement. The friction coefficient can be evaluated from this force measurement. For all the experiments, an EN AW-5086-H111 grade of aluminium alloy sheet of 1 mm thick was considered. A film of fluid grease was applied to the blank to reduce friction.
was used. The strain obtained at constant load capacity increases by ﬁfty percent and a similar value for the reduction in speciﬁc pressure was reported. It was also observed that the tendency of the blank forged to assume a barrel-like shape was reduced indicating that both the stresses and strains are more uniformly distributed. During their work Huang et al. (2000) proposed a ﬁnite element model (FEM) to study the effect of vibration during upsetting of an elasto-viscoplatic cylindrical sample of plasticine. Using a Herschel–Bulkley law for the viscoplastic domain and a Coulomb model of the friction forces, the FEM model implemented in ABAQUS was able to reproduce experimental results for vibra- tion in the range of 5–100 Hz, and amplitude varying from 5 to 40 m. An appreciable decrease in the stress necessary to maintain plastic ﬂow in comparison with that under static load is reported. In a later study, Huang et al. (2002) presented results while upset- ting a model paste with ultrasonic vibration superimposed during a static loading. When applying a constant die vibration amplitude of 10 m and frequency of 20 kHz, an immediate drop in the mean forming load was observed. When the ultrasonic oscillation appli- cation interval was longer, further reduction in the forming load was recorded. It was an indication that locally, in the vicinity of the boundary, temperature increases and yield stress reduces in the affected region. During their study, Daud et al. (2007) analyzed the effects of superimposed ultrasonic vibrations (f = 20 kHz, a = 8 m,
observed that there is reduction in drawing force and also smooth surface was obtained. The effect of applying radial and transverse ultrasonic vibrations on dies was investigated by Murakawa and Jin (2001) and compared with the effect of applying axial ultrasonic vibration and conventional process without vibration. It was ver- iﬁed that radially ultrasonic vibration assisted drawing process is effective in increasing the critical speed in ultrasonic wire drawing by 10 times that conventional axially vibrated dies. Hayashi et al. (2003) reported that the application of ultrasonic vibrations on the dies of tube and wire drawing results in reduction of the forming loads, ﬂow stress, friction between die and work-piece. Better sur- face qualities, reduction in wrinkling, cracking and higher precision were also reported. Studies also mentioned that the extent to which an oscillation may be advantageous depends upon the material, the drawing speed and the direction of the oscillation relative to that of the drawing. Ashida and Aoyama (2007) noted that there was reduction in friction, wrinkling and cracking when the vibration of frequency (11.4, 19.5 kHz) and amplitude 11.4 m was applied during the deep formingprocess.
On the other hand, industrial application of ultra- sonic vibration is limited by the power rating of ultra- sonic generators [ 17 ] and also low frequencies are usually recommended, in order to avoid large reactive current in piezoelectric actuator. Vibration impact forging and forg- ing with a superimposed vibration load (at 10–40 Hz fre- quency) gives a 50% increase in deformation and a similar reduction in speciﬁc pressure. At the same time, the ten- dency of the blank forged to assume a barrel-like shape is reduced, as a result of which both the stresses and strains are more uniformly distributed. During their research Ly et al. [ 18 , 19 ] have worked on low vibration assisted forg- ing process. Experimental setup incorporating piezoelec- tric actuator was developed to generate mechanical vibra- tions and it is placed under the lower die of Lloyd machine used for forging purpose. Norton-Hoﬀ law was used to de- scribe the viscoplastic behaviour of plasticine. Analytical model was developed to ﬁnd the gain in forging force re- duction and is compared with ﬁnite element simulations and experimental results. Khan et al. [ 20 ] use the same experimental setup and applied diﬀerent types of wave- forms during the upsetting process. It was observed that triangular vibration gave more forging force reduction as compared to sinusoidal vibration [ 21 ]. This gives a novel idea of using diﬀerent waveforms in vibration assisted forming processes [ 22 ]. Some studies have been performed in the domain of orbital forging combined with transla- tion [ 23 , 24 ] and has shown tendency of obtaining surface and volume eﬀects by changing frequency and amplitude of vibration.
J. Manuf. Mater. Process. 2019, 3, x FOR PEER REVIEW 3 of 15
Two types of 42CrMo4 steel bars were chosen each with different heat treatment to avoid any possible influence a chemical composition may have on turning process. Table 1 shows the main alloying elements as well as the mechanical properties of both steel products when subjected to hardness testing and tensile testing. Sample SB represents the bainitic structure obtained after an isothermal transformation using a salt-bath and sample QT represents the martensitic quenched and tempered bar. As intended, both samples showed similar mechanical properties. By having the same chemical composition and close mechanical behavior, we are limiting the number of variables to the microstructure only, and thus emphasizing its impact during high-speed turning. In addition to the chemical and mechanical aspects, characterizing the samples included measurements of the retained austenite. Many authors have studied its influence on machinability through its TRIP effect, so one cannot neglect the presence of an FCC phase when comparing samples. X-ray diffraction showed no presence of an FCC phase in either sample. The values %RA are presented in the table below.