Characterization of shape and dimensional accuracy of incrementally formed titanium sheet parts with intermediate curvatures between two feature types

Single point incremental forming (SPIF) is a relatively new manufacturing process that has been recently used to form medical grade titanium sheets for implant devices. However, one limitation of the SPIF process may be characterized by dimensional inaccuracies of the final part as compared with the original designed part model. Elimination of these inaccuracies is critical to forming medical implants to meet required tolerances. Prior work on accuracy characterization has shown that feature behavior is important in predicting accuracy. In this study, a set of basic geometric shapes consisting of ruled and freeform features were formed using SPIF to characterize the dimensional inaccuracies of grade 1 titanium sheet parts. Response surface functions using multivariate adaptive regression splines (MARS) are then generated to model the deviations at individual vertices of the STL model of the part as a function of geometric shape parameters such as curvature, depth, distance to feature borders, wall angle, etc. The generated response functions are further used to predict dimensional deviations in a specific clinical implant case where the curvatures in the part lie between that of ruled features and freeform features. It is shown that a mixed-MARS response surface model using a weighted average of the ruled and freeform surface models can be used for such a case to improve the mean prediction accuracy within ±0.5 mm. The predicted deviations show a reasonable match with the actual formed shape for the implant case and are used to generate optimized tool paths for minimized shape and dimensional inaccuracy. Further, an implant part is then made using the accuracy characterization functions for improved accuracy. The results show an improvement in shape and dimensional accuracy of incrementally formed titanium medical implants

Tool path generation for single point incremental forming using intelligent sequencing and multi-step mesh morphing techniques

A new methodology of generating optimized tool paths for incremental sheet forming is proposed in this work. The objective is to make parts with improved accuracy. To enable this, a systematic, automated technique of creating intermediate shapes using a morph mapping strategy is developed. This strategy is based on starting with a shape different from the final shape, available as a triangulated STL model, and using step-wise incremental deformation to the original mesh to arrive at the final part shape. Further, optimized tool path generation requires intelligent sequencing of partial tool paths that may be applied specifically to certain features on the part. The sequencing procedure is discussed next and case studies showing the application of the integrated technique are illustrated. The accuracy of the formed parts significantly improves using this integrated technique. The maximum deviations are brought down to less than 1 mm, while average absolute deviations of less than 0.5 mm are recorded

Single point incremental forming: An assessment of the progress and technology trends from 2005 to 2015

The last decade has seen considerable interest in flexible forming processes. Among the upcoming flexible forming techniques, one that has captured a lot of interest is single point incremental forming (SPIF), where a flat sheet is incrementally deformed into a desired shape by the action of a tool that follows a defined toolpath conforming to the final part geometry. Research on SPIF in the last ten years has focused on defining the limits of this process, understanding the deformation mechanics and material behaviour and extending the process limits using various strategies. This paper captures the developments that have taken place over the last decade in academia and industry to highlight the current state of the art in this field. The use of different hardware platforms, forming mechanics, failure mechanism, estimation of forces, use of toolpath and tooling strategies, development of process planning tools, simulation of the process, aspects of sustainable manufacture and current and future applications are individually tracked to outline the current state of this process and provide a roadmap for future work on this process

Overcoming challenges of prototyping with single point incremental forming through formability and geometric accuracy analysis

With recent developments in rapid prototyping technologies, the automotive industry has been able to move away from costly and inefficient methods of prototyping. In fact, rapid prototyping techniques now exist for nearly all the components in a car, meaning time and money is saved in product development. One exception to this trend, despite their ubiquity in automotive applications, is formed sheet metal components. Single point incremental forming (SPIF) is a sheet metal forming technique with a fast turnaround that uses little to no custom tooling. It is a promising method for filling the gap in rapid prototyping capability for sheet metal components. However, despite significant research over the last two decades, barriers to industrial viability still include the key issues of fracture occurring in the sheet metal, or a final part being rejected due to unacceptable dimensional error. These two issues are affected by SPIF process parameters, but the extent of their influences are not well understood. By investigating the effect of process parameters on material formability and geometric accuracy, this thesis seeks to address these issues. Case studies emphasise the impact of formability and geometric accuracy on prototyping automotive components with SPIF. Also emphasised is the importance of effective support walls and optimal design of the forming surface that is used to generate toolpaths for forming components. A systematic review of the literature regarding the first key issue, formability in SPIF, highlights significant inconsistencies in published research about the effects of process parameters. A hypothesis to explain this result presents the idea of non-linear effects and parameter interactions, which is supported by original experimental work. This shows the difficulty of empirical prediction of formability when, for example, a small change in one parameter may interact with another to significantly influence the outcome of the final part. Identifying and following safe formability limits will minimise the likelihood of fracture for the forming surface of a component. Research in this thesis looks at the thickness distribution of variable wall angle conical frustum (VWACF) parts as a basis for defining a safe formability limit. However, experimental results show this is not viable due to irregular trends in the thickness distribution close to the fracture point of the VWACF. The second key issue of geometric error in SPIF is approached by focusing on a single mode of error, namely `wall bulge', or springback in flat walls of components. Experiments studied how a variety of tool shapes and sizes affected its severity, and found a trade-off with `pillowing', another mode of geometric error. At the same time as flat-ended tools reduce pillowing in the base, the experimental results show an increase in the amount of bulging in the walls. The findings of this thesis demonstrate the impact that a single parameter change can have on multiple aspects of a component. Also highlighted are the complexities of the SPIF process that remain as barriers to industrial viability. This work contributes to overcoming these barriers and achieving efficient rapid prototyping of sheet metal components

Heat-assisted incremental sheet forming of TI-6AL-4V sheets

Single point incremental forming (SPIF) is a sheet forming technique that deforms sheet materials incrementally to a designated shape. The process has shown high ability to deform low-strength materials for good geometrical accuracy and formability at room temperature. Deforming high-temperature alloys, such as Ti-6AI-4V, requires integrated heat sources to increase the ductility of the metal sheets for deformation. However, the integration of heating results in unpredictable thermomechanical behaviours on the formability, geometric accuracy, thickness distribution and surface quality. Considerable research efforts have been in developing in different heating methods and designing novel tools and analytical modelling to resolve the limitations. The current challenge remains to improve the localised and stable heating and functional tool design to reduce the thermal expansion and friction at the tool-surface contact area and the analysis of relationship between thermal and mechanical effects. This PhD research aims to develop and improve the induction heating-assisted SPIF system for Ti-6AI-4V sheets in tool design, lubrication, tool path optimisation and numerical analysis. Total four research methods include the study of microstructural and mechanical properties for low temperature (600 ℃ and 700 ℃) deformation using Zener-Hollomon parameter (Z-parameter). A novel tool design with water-cooling lubricant system to assist lubricant service. A combination of crystal plasticity finite element simulation method (CPFEM), representative volume element (RVE) and cellular automata (CA) to predict the grain orientation, crystal texture and grain size evolution in experimental scale and microstructure. A radial basis function (RBF) artificial neural network to optimise the tool path for improvements of geometric accuracy and surface quality above beta-transus (950 ℃) temperature

Heat-assisted incremental sheet forming of TI-6AL-4V sheets

Single point incremental forming (SPIF) is a sheet forming technique that deforms sheet materials incrementally to a designated shape. The process has shown high ability to deform low-strength materials for good geometrical accuracy and formability at room temperature. Deforming high-temperature alloys, such as Ti-6AI-4V, requires integrated heat sources to increase the ductility of the metal sheets for deformation. However, the integration of heating results in unpredictable thermomechanical behaviours on the formability, geometric accuracy, thickness distribution and surface quality. Considerable research efforts have been in developing in different heating methods and designing novel tools and analytical modelling to resolve the limitations. The current challenge remains to improve the localised and stable heating and functional tool design to reduce the thermal expansion and friction at the tool-surface contact area and the analysis of relationship between thermal and mechanical effects. This PhD research aims to develop and improve the induction heating-assisted SPIF system for Ti-6AI-4V sheets in tool design, lubrication, tool path optimisation and numerical analysis. Total four research methods include the study of microstructural and mechanical properties for low temperature (600 ℃ and 700 ℃) deformation using Zener-Hollomon parameter (Z-parameter). A novel tool design with water-cooling lubricant system to assist lubricant service. A combination of crystal plasticity finite element simulation method (CPFEM), representative volume element (RVE) and cellular automata (CA) to predict the grain orientation, crystal texture and grain size evolution in experimental scale and microstructure. A radial basis function (RBF) artificial neural network to optimise the tool path for improvements of geometric accuracy and surface quality above beta-transus (950 ℃) temperature

Numerical and experimental studies of asymmetrical Single Point Incremental Forming process

The framework of the present work supports the numerical analysis of the Single Point Incremental Forming (SPIF) process resorting to a numerical tool based on adaptive remeshing procedure based on the FEM. Mainly, this analysis concerns the computation time reduction from the implicit scheme and the adaptation of a solid-shell finite element type chosen, in particular the Reduced Enhanced Solid Shell (RESS). The main focus of its choice was given to the element formulation due to its distinct feature based on arbitrary number of integration points through the thickness direction. As well as the use of only one Enhanced Assumed Strain (EAS) mode. Additionally, the advantages include the use of full constitutive laws and automatic consideration of double-sided contact, once it contains eighth physical nodes. Initially, a comprehensive literature review of the Incremental Sheet Forming (ISF) processes was performed. This review is focused on original contributions regarding recent developments, explanations for the increased formability and on the state of the art in finite elements simulations of SPIF. Following, a description of the numerical formulation behind the numerical tools used throughout this research is presented, summarizing non-linear mechanics topics related with finite element in-house code named LAGAMINE, the elements formulation and constitutive laws. The main purpose of the present work is given to the application of an adaptive remeshing method combined with a solid-shell finite element type in order to improve the computational efficiency using the implicit scheme. The adaptive remeshing strategy is based on the dynamic refinement of the mesh locally in the tool vicinity and following its motion. This request is needed due to the necessity of very refined meshes to simulate accurately the SPIF simulations. An initially mesh refinement solution requires huge computation time and coarse mesh leads to an inconsistent results due to contact issues. Doing so, the adaptive remeshing avoids the initially refinement and subsequently the CPU time can be reduced. The numerical tests carried out are based on benchmark proposals and experiments purposely performed in University of Aveiro, Department of Mechanical engineering, resorting to an innovative prototype SPIF machine. As well, all simulations performed were validated resorting to experimental measurements in order to assess the level of accuracy between the numerical prediction and the experimental measurements. In general, the accuracy and computational efficiency of the results are achieved

Metodologia avançada para simulação de processos de estampagem incremental

Doutoramento em Engenharia MecânicaThe framework of the present work supports the numerical analysis of the Single Point Incremental Forming (SPIF) process resorting to a numerical tool based on adaptive remeshing procedure based on the FEM. Mainly, this analysis concerns the computation time reduction from the implicit scheme and the adaptation of a solid-shell finite element type chosen, in particular the Reduced Enhanced Solid Shell (RESS). The main focus of its choice was given to the element formulation due to its distinct feature based on arbitrary number of integration points through the thickness direction. As well as the use of only one Enhanced Assumed Strain (EAS) mode. Additionally, the advantages include the use of full constitutive laws and automatic consideration of doublesided contact, once it contains eighth physical nodes. Initially, a comprehensive literature review of the Incremental Sheet Forming (ISF) processes was performed. This review is focused on original contributions regarding recent developments, explanations for the increased formability and on the state of the art in finite elements simulations of SPIF. Following, a description of the numerical formulation behind the numerical tools used throughout this research is presented, summarizing non-linear mechanics topics related with finite element in-house code named LAGAMINE, the elements formulation and constitutive laws. The main purpose of the present work is given to the application of an adaptive remeshing method combined with a solid-shell finite element type in order to improve the computational efficiency using the implicit scheme. The adaptive remeshing strategy is based on the dynamic refinement of the mesh locally in the tool vicinity and following its motion. This request is needed due to the necessity of very refined meshes to simulate accurately the SPIF simulations. An initially mesh refinement solution requires huge computation time and coarse mesh leads to an inconsistent results due to contact issues. Doing so, the adaptive remeshing avoids the initially refinement and subsequently the CPU time can be reduced. The numerical tests carried out are based on benchmark proposals and experiments purposely performed in University of Aveiro, Department of Mechanical engineering, resorting to an innovative prototype SPIF machine. As well, all simulations performed were validated resorting to experimental measurements in order to assess the level of accuracy between the numerical prediction and the experimental measurements. In general, the accuracy and computational efficiency of the results are achieved.O presente trabalho assenta na análise numérica do processo de Estampagem Incremental por Único Ponto (SPIF) recorrendo ao refinamento adaptativo da malha através do Método dos Elementos Finitos (FEM). Nomeadamente, a atenção é dada à redução do tempo de cálculo baseado no esquema de integração implícito em combinação com um elemento finito do tipo “sólidocasca” predefinido. O principal motivo da escolha do tipo de elemento finito deve-se à sua formulação possibilitar a atribuição de um número arbitrário de pontos de integração na direção da espessura combinado com a utilização de um único modo de deformação acrescentada e integração reduzida no plano. Além disso, as vantagens incluem a utilização de leis constitutivas tridimensionais, análise automática de contacto em dupla face e espessura, uma vez que é um elemento hexaédrico de 8 nós. Inicialmente, uma revisão da literatura relacionada com o processo de estampagem incremental (ISF) é apresentada evidenciando as contribuições recentemente desenvolvidas, explicações do aumento da formabilidade do material em ISF e com maior ênfase o estado-de-arte das simulações numéricas pelo FEM do processo SPIF. Seguidamente, é apresentado a descrição dos conceitos teóricos que suportam e foram utilizados ao longo desta pesquisa, resumindo tópicos de mecânica não-linear relacionada com o código LAGAMINE, formulação de elementos finitos e leis constitutivas. O principal objetivo do presente trabalho é a aplicação do método de refinamento adaptativo combinado com um elemento finito sólido-casca, a fim de melhorar a eficiência computacional usando o esquema de integração implícito. A estratégia de refinamento adaptativo é baseada no refinamento dinâmico da malha localmente na proximidade da ferramenta e acompanhando o seu movimento. Este requisito é devido à necessidade de malhas muito refinadas para simular com precisão as simulações SPIF. A malha inicialmente refinada requer enorme tempo de cálculo e uma malha grosseira leva a resultados inconsistentes devido a problemas de contato. Neste sentido, o refinamento adaptativo evita o refinamento inicial total da malha e consequentemente melhora a performance computacional da simulação. Os testes numéricos realizados são baseados em casos estudo e em testes experimentais realizados na Universidade de Aveiro, Departamento de Engenharia Mecânica, recorrendo a uma máquina protótipo inovadora construída propositadamente para SPIF. Todas as simulações realizadas são validadas recorrendo às medições experimentais, de modo a avaliar o nível de precisão entre a previsão numérica e as medições experimentais. Em geral, a precisão e a eficiência computacional dos resultados são alcançados