Surface roughness analysis of medical grade titanium sheets formed by single point incremental forming

Single point incremental forming (SPIF) process has proven benefits in terms of formability, flexibility, and low cost as compared to conventional sheet forming, although it has some issues such as dimensional accuracy and low quality of surface finish for some materials. The SPIF is considered as a potential method of producing customised medical parts such as craniofacial implants using a titanium sheet. This investigation aims to analyse the overall surface roughness of grade 1 pure titanium along the wall of the hyperbolic truncated cone formed by the SPIF process with different forming parameters (i.e., forming tool diameter, step size, and feed rate). Focus variation microscopy was used to measure the surface roughness experimentally along the wall of the truncated cone. Abaqus/Explicit was used to predict the equivalent stress and equivalent plastic strain along the wall of the truncated cone part from the top to the fractured region to evaluate the relationship between the stress, strain, and roughness distribution. It was found that the surface roughness changes with the deformed part height and rough surface could be produced in the region of high equivalent stress and low equivalent plastic strain. Such a surface roughness and equivalent stress and plastic strain correlation has a clear implication to the design and the surface quality of sheet parts made by SPIF

Evaluation of formability and fracture of pure titanium in incremental sheet forming

A forming limit diagram (FLD) is commonly used as a useful means for characterizing the formability of sheet metal forming processes. In this study, the Nakajima test was used to construct the forming limit curve at necking (FLCN) and fracture (FLCF). The results of the FLCF are compared with incremental sheet forming (ISF) to evaluate the ability of the Nakajima test to describe the fracture in ISF. Tests were carried to construct the forming limit diagram at necking and fracture to cover the strain states from uniaxial tension to equi-biaxial tension with different stress triaxialities - from 0.33 for uniaxial tension to 0.67 for equi-biaxial tension. Due to the fact that the Gurson–Tvergaard- Needleman (GTN) model can be used to capture fracture occurrence at high stress triaxiality, and the shear modified GTN model (Nahshon-Hutchinson’s shear mechanism) was developed to predict the fracture at zero stress or even negative stress triaxiality, the original GTN model and shear modified GTN model may be not suitable to predict the fracture in all samples of the Nakajima test as some samples are deformed under moderate stress triaxiality. In this study, the fractures are compared using either the original GTN model, shear modified GTN model or Nielsen-Tvergaard model with regard to stress triaxiality. To validate the ability of these models, and to assess which model is more accurate in predicting the fracture with different stress triaxialities, finite element (FE) simulations of the Nakajima test were compared with an experimental results to evaluate the applicability of the Nakajima test to characterise the fracture from ISF. The experimental and FE results showed that the shear modified GTN model could predict the fracture accurately with samples under uniaxial tension condition due to low stress triaxiality and that the original model is suitable for an equi-biaxial strain state (high stress triaxiality), whereas the stress triaxiality modified GTN model should be considered for samples which have moderate stress triaxiality (from plain strain to biaxial strain). The numerical and experimental FLCF of pure titanium from the Nakajima test showed good agreement with the experimental and numerical results of ISF

Finite Element Analysis of Sheet Metal U-Bending Using Free Form Surface

This paper deals with the optimization of tools geometry in sheet metal forming U-bending in order to reduce the equivalent strain, equivalent stress, and thickness change after forming. Free form surface method (Bezier curve, B-spline curve, and NURBS curve) is used to generate the die and punch profile; this technique is accurate to describing the die and punch profile. A 3D numerical simulation using the Ansys 12.1 FEM code was conducted to understand the effect of die profile in final product. The results show that the more uniform distributions in strain, stress, and thickness when using same profile radius for die and punch (Bezier curve)

Deformation and fracture characteristics of Al6092/SiC/17.5p metal matrix composite sheets due to heat treatments

In metal matrix composite (MMC) materials, the reaction between the metal matrix and reinforcement particles could change the composition of the matrix and the interface and lead to interfacial compounds. These intermetallic compounds may have either a deleterious effect to the mechanical properties or beneficial effect in enhancing the toughness and ductility of the composite. An aluminium 6092 alloy with 17.5% volume fraction silicon carbide (SiC) particles sheet manufactured by means of powder metallurgical method, heat treated to T6 condition, is used to obtain a fundamental understanding of the heat treatment effect on the fracture mechanism, the microstructural changes and the interface between the Al-matrix and SiC particles. Changes in the microstructure of the Al/SiCp and the topography of the fracture are investigated using scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM) to characterize the precipitate and intermetallic compounds formed at the Al/Sic interface. X-ray diffraction (XRD) is utilized to characterize the phase formation and to give confidence in the results of TEM and EDS. Tensile tests with different strain rates (8 × 10−5, 8 × 10−4, 8 × 10−3, 8 × 10−2 and 0.16 s−1) were carried out to study the toughness and to find a correlation between the strain rate and heat treatment. Under T6 condition, the results show that the mechanical property of this MMC is less ductile due to the formation of precipitations as a result of either the interaction between the Al and SiCp or from hardening precipitation treatment, e.g. Al2Cu, Al4Cu2Mg8Si7 and MgAl2. O-condition annealing can reduce the detrimental effect of the intermetallic compounds in the interface region and improve the toughness and ductility of the material by decreasing the intermetallic compound (Al2Cu). However, the Al/SiC sheet treated with O-condition annealing is more sensitive to the strain rate than the one treated with T6

Micromechanical Modelling of the Deformation and Damage Behaviour of Al6092/SiC Particle Metal Matrix Composites

To enhance the performance and design of metal matrix composites, it is extremely important to gain a better understanding of how the microstructure influences the deformation and damage behaviour of metal matrix composites under different loading conditions. Finite element (FE) analysis can be used to collect certain micromechanical information of composites that is difficult to obtain from experiments. In this work, the effect of the distance between the SiC particles and the loading conditions on the deformation and damage behaviour of Al6092/SiC particle composites is investigated under different strain rates (i.e., 1x10-4 , 2x10-4 , and 4x10-4 s-1). A program is developed to generate the 2D micromechanical FE model with 17.5Vol. % SiC particles. Based on the scanning electron microscopy (SEM) images, the FE model contains four SiC particle sizes (3.1, 4.46, 6.37, and 9.98 μm) with various percentages, which are randomly distributed in the micromechanical Al6092 alloy matrix. User-defined field (USDFLD) subroutine was developed and implemented through Abaqus/Standard based on maximum principal stress and Rice-Tracey triaxial damage indicator to evaluate the formability of the aluminium matrix composite (AMC) and to predict the brittle and ductile fracture of the SiC particles and the aluminium matrix, respectively, under tensile and shear loads. The results showed that the distribution of SiC particles in Al matrix has a significant effect on the mechanical properties of Al6092/SiC 17.5 particle composites. The formability and damage behaviour of composites improve as particle distance increases and strain rate decreases under tensile and shear loading. The fracture initiation toughness of fine SiC particles is higher than that of coarse SiC particles

PEEK based cranial reconstruction using thermal assisted incremental sheet forming

Compared to conventional sheet forming operations, incremental sheet forming (ISF) is a flexible forming technique that can achieve higher formability in terms of localized deformation. Due to excellent mechanical properties and X-ray penetration, polyether-ether-ketone (PEEK) is an ideal alternative to titanium alloy and stainless steel in orthopedic applications. In this study, a 3-axis desktop manufacturing system has been fabricated to investigate the temperature-dependent formability of PEEK in terms of manufacturing the cranial plate by using the ISF technique. Meanwhile, the forming force, temperature distribution, geometrical accuracy, and thermal properties were obtained and analyzed. The findings indicate that the ISF technique provides technological and economic advantages in cranial reconstruction by using PEEK

Modelling of ductile fracture in single point incremental forming using a modified GTN model

Understanding the deformation and failure mechanisms in single point incremental forming (SPIF) is of great importance for achieving improved formability. Furthermore, there will be added benefits for more in depth evaluation of the effect of localised deformation to the fracture mechanism in SPIF. Although extensive research has been carried out in recent years, questions still remain on the shear and particularly its effect to the formability in SPIF processes. In this work, a modified Gurson–Tvergaard-Needleman (GTN) damage model was developed with the consideration of shear to predict ductile fracture in the SPIF process due to void nucleation and coalescence with results compared with original GTN model in SPIF. A combined approach of experimental testing and SPIF processing was used to validate finite element results of the shear modified Gurson–Tvergaard-Needleman damage model. The results showed that the shear modified GTN model improved the modelling accuracy of fracture over the original GTN model under shear loading conditions. Furthermore, the shear plays a role under meridional tensile stress to accelerate fracture propagation in SPIF processes

Fracture characteristics of PEEK at various stress triaxialities

Polyether-ether-ketone (PEEK) is an alternative to metal alloys in orthopaedic applications. It gives significant advantages including excellent mechanical properties and non-toxicity. In this work, a set of specimens with different notched radii were selected to examine the effect of triaxial state of stress on the fracture behavior of PEEK. Fractographic analysis via scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) further elucidated the fracture micromechanisms. Distinct fracture patterns were identified under different stress triaxialities. In addition, the microstructural inclusion properties in PEEK specimen such as inclusion size and chemical composition were analysed and determined. Finite element simulations were carried out to evaluate the correlation of observed fracture characteristics with different stress triaxialities

Hot tensile fracture characteristics and constitutive modelling of polyether-ether-ketone (PEEK)

The effects of strain rate and deformation temperature on the deformation behaviors of polyether-ether-ketone (PEEK) were studied by uniaxial tensile tests with the temperature range of 23–150 °C and strain rate of 0.01–1 s−1. The effects of deformation temperature and strain rate on the hot tensile deformation behavior and fracture characteristics were investigated by scanning electron microscope (SEM) and discussed in detail. SEM experimental results suggest that fracture morphology is not strain rate sensitive but temperature sensitive. Based on the tensile results, the Johnson-Cook and modified Johnson-Cook constitutive models were established for PEEK. Furthermore, a comparative study has been made on the accuracy and effectiveness of the developed models to predict the flow stress. The results show that the original Johnson-Cook model reflects the deformation behavior more accurately throughout the entire test temperature and strain rate range under uniaxial tensile conditions

Review on the influence of process parameters in incremental sheet forming

Incremental sheet forming (ISF) is a relatively new flexible forming process. ISF has excellent adaptability to conventional milling machines and requires minimum use of complex tooling, dies and forming press, which makes the process cost-effective and easy to automate for various applications. In the past two decades, extensive research on ISF has resulted in significant advances being made in fundamental understanding and development of new processing and tooling solutions. However, ISF has yet to be fully implemented to mainstream high-value manufacturing industries due to a number of technical challenges, all of which are directly related to ISF process parameters. This paper aims to provide a detailed review of the current state-of-the-art of ISF processes in terms of its technological capabilities and specific limitations with discussions on the ISF process parameters and their effects on ISF processes. Particular attention is given to the ISF process parameters on the formability, deformation and failure mechanics, springback and accuracy and surface roughness. This leads to a number of recommendations that are considered essential for future research effort