Study on Uniform and Varying Friction Conditions in Superplastic Forming

Abstract. A novel hybrid forming process involving the use of hot drawing along with superplastic forming (SPF) is studied here. The hot drawing stage helps in enhancing the formability and in fast deforming the sheet metal into a hollow shape with desired amount of material draw-in. During the subsequent SPF stage, gas pressure was applied onto the pre-formed part to complete the forming process at a targeted strain rate. With the hybrid process, titanium alloy Ti-6Al-4V sheets have been successfully formed in lab-scaled conditions at 800°C in 16 min. In this paper, finite element modelling (FEM) was used to demonstrate the effects of each stage (hot drawing and SPF) during the process. A plasticity model based on tensile test data was adopted as a material model for simulation. The pressure cycle which was predicted from the simulation has been used in the process to maintain the sheet forming at an average strain rate (e.g. 10-3 and 5×10-4 s-1). Material draw-in and thickness distribution were used to compare and optimise the process parameters. The simulations have shown the capability of the model to be used for the hybrid superplastic forming process. The influences of varying process parameters, such as punch geometry, blank size, blank-holder force, friction coefficient and pressure cycle, were investigated by the simulations. It was found that the punch geometry and blank size played significant roles on the thickness uniformity of the final part, from which an optimised hot-drawing system that could lead to minimum thinning has been designed by FEM method.

This article investigates superplastic forming (SPF) technique in conjunction with finite element (FE) simulation applied to dental repair. The superplasticity of Ti-6Al-4V alloys has been studied using a uniquely designed five-hole test with the aim of obtaining the modeled grain size and the flow stress parameters. The data from the five-hole test are subsequently put into the FE program for the simulation of a partial upper denture dental prosthesis (PUD4). The FE simulation of the PUD4 is carried out to set up appropriate input parameters for pressing due to the SPF process being fully automatic controlled. A variety of strain rates ranging from 2.4 × 10−5 to 1 × 10−3 s−1 are selected for the characterization of superplastic properties of the alloy. The Superflag FE program is used to generate an appropriate pressure-time profile and provide information on thickness, grain size, and grain growth rate distribution. Both membrane elements and solid elements have been adopted in the simulation and the results from both types of elements are compared. An evaluation of predicted parameters for the SPF of the prosthesis is presented.

Metrology and Microscopy Analysis of Multisheet Packs Manufactured via Superplastic Forming to Study Possible Diffusion Bonding

In this work, impact puncture tests (drop tests) have been used to both tune numerical models and correlate the performance of customised titanium cranial prostheses to the manufacturing process. In fact, experimental drop tests were carried out either on flat disk-shaped samples or on prototypes of titanium cranial prostheses (Ti-Gr5 and Ti-Gr23 were used) fabricated via two innovative sheet metal forming processes (the super plastic forming (SPF) and the single point incremental forming (SPIF)). Results from drop tests on flat disk-shaped samples were used to define the material behaviour of the two investigated alloys in the finite element (FE) model, whereas drop tests on cranial prostheses for validation purposes. Two different approaches were applied and compared for the FE simulation of the drop test: (i) assuming a constant thickness (equal to the one of the undeformed blank) or (ii) importing the thickness distribution determined by the sheet forming processes. The FE model of the drop test was used to numerically evaluate the effect of the manufacturing process parameters on the impact performance of the prostheses: SPF simulations were run changing the strain rate and the tool configuration, whereas SPIF simulations were run changing the initial thickness of the sheet and the forming strategy. The comparison between numerical and experimental data revealed that the performance in terms of impact response of the prostheses strongly depends on its thickness distribution, being strain hardening phenomena absent due to the working conditions adopted for the SPF process or to the annealing treatment conducted after the SPIF process. The manufacturing parameters/routes, able to affect the thickness distribution, can be thus effectively related to the mechanical performance of the prosthesis determined through impact puncture tests.Graphical abstract

Quick super-plastic forming has been developed as a high-volume, hot blow forming process for vehicle components, enabling higher volume applications than traditional super-plastic forming. It was chosen as the manufacture process for the side wall outer panel of London metro vehicle, which has complex shape and the requirement of high-volume. In this paper, the finite element simulation of super-plastic forming process for the side wall outer panel of London metro vehicle was conducted. The pressure-time relationships for a given optimum strain-rate and the thickness distribution are calculated. Based on the finite element modeling, the influences of key factors such as the frictional coefficient on the thickness distribution and pressure during the forming process were studied. The quick super-plastic forming has been successfully implemented for the side wall outer panel of London metro vehicle. Finally, experiments were performed and it is found that numerical results are in good agreement with experimental values.

The present work collects some results of the three-years Research Program "BioForming", funded by the Italian Ministry of Education (MIUR) and aimed to investigate the possibility of using flexible sheet forming processes, i.e. Super Plastic Forming (SPF) and Single Point Incremental Forming (SPIF), for the manufacturing of patient-oriented titanium prostheses. The prosthetic implants used as case studies were from the skull; in particular, two different Ti alloys and geometries were considered: one to be produced in Ti-Gr23 by SPF and one to be produced in Ti-Gr2 by SPIF. Numerical simulations implementing material behaviours evaluated by characterization tests were conducted in order to design both the manufacturing processes. Subsequently, experimental tests were carried out implementing numerical results in terms of: (i) gas pressure profile able to determine a constant (and optimal) strain rate during the SPF process; (ii) tool path able to avoid rupture during the SPIF process. Post forming characteristics of the prostheses in terms of thickness distributions were measured and compared to data from simulations for validation purposes. A good correlation between numerical and experimental thickness distributions has been obtained; in addition, the possibility of successfully adopting both the SPF and the SPIF processes for the manufacturing of prostheses has been demonstrated.

Development of Thermomechanical Model for the Analysis of Effects of Friction and Cutting Speed on Temperature Distribution around AISI 316L During Orthogonal Machining

The role of friction on wear evolution is manifold since it interplays with lubrication regime, nominal contact point, and contact pressure distribution. Nevertheless, in the literature many wear models simulate wear assuming frictionless contact conditions to simplify the analyses. That assumption, physically not realistic, often appears as a contradiction, permitted in numerical simulations where friction and wear can be considered independent phenomena.This study aims to validate the frictionless assumption in wear models with steady nominal contact point, such as in many common configurations, e.g. pin on plate/pin on disc. Wear was simulated according to the Archard wear law for both non-conformal and conformal pin-on-plate contact pairs in reciprocating motion, assuming frictionless and frictional contact conditions, varying the coefficient of friction f in the range 0–0.4. Finite Element wear models were developed in Ansys® both with implicit and explicit kinematics. Results demonstrate that the effect of friction on contact pressure distribution and worn profiles and on their evolution is negligible (differences lower than 0.05%). Thus, wear can be predicted using models in frictionless conditions which allow to extremely reduce the computational costs that represent a limit of FE wear simulations. Additionally, a procedure with implicit kinematics was compared to the explicit one resulting valid and computationally convenient, especially in case of non-conformal contact.

Frictional forces at the die‐sheet interface during superplastic forming play an important role in controlling material flow. As such, lubricious coatings are routinely used to facilitate a low friction flow of sheet metal into the die cavity. They may also act as parting agents and/or oxidation barriers. The selection of coatings for the superplastic forming of Ti‐6Al‐4 V is limited by the requirement of thermomechanical stability at the high forming temperature, therefore, solid film coatings such as boron nitride are a common choice. However, little work has been published on coating performance during superplastic forming, particularly under true process conditions. This paper presents a method for the systematic assessment of superplastic forming coating performance by using a multi‐pocket die, which allows many coating types and combinations to be tested simultaneously during a single forming cycle. Sixteen coating combinations were evaluated in total and the resultant sheet thickness distributions compared, serving as an indicator of friction.

The utility of finite element modeling in optimizing superplastic metal forming is dependent on accurate representation of the material constitutive behavior and the frictional response of the sheet against the die surface. This paper presents work conducted to estimate the level of precision that is necessary in constitutive relations for finite element analysis to accurately predict the deformation history of actual SPF components. Previous work identified errors in SPF testing methods that use short tensile specimens with gauge length-to-width ratios of 2:1 or less. The analysis of the present paper was performed to estimate the error in predicted stress that results from using the short specimens. Stress correction factors were developed and an improved constitutive relation was implemented in the MARC finite element code to simulate the forming of a long, rectangular tray. The coefficient of friction in a Coulomb friction model was adjusted to reproduce the amount of material draw-in observed in the forming experiments. Comparisons between the finite element predictions and the forming experiments are presented.

GA-based optimization to control the thickness distribution in components manufactured via superplastic forming

Conventional superplastic forming has been applied in automotive and aerospace industries for a few decades. Recently, superplastic forming combined with mechanical pre-forming process has been reported to be capable of forming non-superplastic AA5083 at 400 °C to a surface expansion of 200 % [1]. In this paper, finite element modeling (FEM) was used to develop the combined forming process by using the non-superplastic material AA5083-O. The simulation follows the experimental sequence and was divided into two phases (mechanical pre-forming and superplastic forming). A conventional creep equation based on tensile test data was adopted as a material model for the simulation. The pressure cycle and forming time was simulated according to the actual process route. The thickness distributions obtained from simulation validated the capability of the model to be used for this case. The influence of different parameters, such as holder force, friction, and punch depth was investigated by comparing the final sheet thickness and level of material draw-in. It was found that the punch depth played a significant effect on the uniformity of thickness distribution, from which a more uniform formed part can be obtained by using the punch with higher depth during mechanical pre-forming phase.

Optimisation of the superplastic forming of a dental implant for bone augmentation using finite element simulations

Superplastic alloys, such as Ti–6Al–4V, are a group of polycrystalline materials which can undergo large elongation in special conditions. By this specific characteristic, superplastic forming can be carried out to manufacture products with complicated shapes. Superplastic forming, in comparison with other similar processes, has a good capability to produce parts with uniform thickness; however, in superplastic blow forming, the final part does not have a uniform thickness distribution, and the prediction of the part thickness has a great significance to the process designers. Numerical methods are used to predict thickness distribution in final part and to improve it by suggesting a precise pressure–time diagram. In this regard, the constitutive equation plays a significant role in predicting the process performance. In the present study, superplastic blow forming has been simulated for a cone-shaped part via using commercial finite element code Abaqus and implementation of a proper constitutive equation by user subroutine UMAT. The constitutive equation considers grain growth hardening effects in addition to strain rate effect. A new method is also proposed to predict pressure–time diagram when UMAT subroutine is used. The effects of the process parameters on this diagram have also been investigated. Comparison between the findings of this research and those conducted by other researchers revealed that considering grain growth hardening has a significant role in the improvement of the accuracy of thickness distribution predictions.

Linear and nonlinear finite element analyses are used to examine the effects of friction, test geometry, and fixture compliance on the perceived toughness as obtained from threeand four-point bend end-notched flexure tests. To this end, a newly developed ‘direct energy balance approach’ is used to obtain the ‘true’ energy release rate for any given specimen, test geometry, and coefficient of friction. Finite element analyses are also used in a simulated compliance calibration technique, which is combined with experimental results from fixture compliance tests to obtain a perceived toughness, i.e., the value that would be obtained by experiment. By varying the different parameters, the individual and combined effects of friction, test geometry, and fixture compliance on the ratio of the perceived to true toughness is obtained. The approach is applied to two graphite/epoxy materials for which toughnesses by the three(3ENF) and four-point bend end-notched flexure (4ENF) tests are obtained experimentally for a range of geometries. These experiments produced larger perceived mode II toughnesses, GIIc, by the 4ENF than the 3ENF test, and GIIc values from the 4ENF test were observed to decrease with increasing outer span length. The finite element simulations were shown to accurately recreate the perceived values of GIIc obtained from these tests. Moreover, the finite element simulations indicate that the true toughness values are essentially constant for a given material. These findings are used to make some general recommendations for choosing 3ENF and 4ENF specimen and test geometries, as well as to discuss the relative advantages and disadvantages of the 3ENF and 4ENF test methods.