Making threaded connections to thin metal sheets requires locally thickening of the sheet in order to provide enough thread length for a structurally sound connection. Shaped Metal Deposition processes like Gas Tungsten Arc Welding (GTAW) allow to locally build-up material in order to provide thickness for a sufficient length of thread engagement. This publication describes the research towards local thickening of a titanium sheet by means of pulsed Tungsten Inert Gas (TIG) droplet deposition, aimed at creating threaded holes for thin shelled bone fracture fixation plates. The influence of current, weld time and amount of filler material on droplet diameter and height is studied.

This study focuses on finding a toolpath strategy for accurately forming geometric details on a preshaped sheet metal part by incremental forming in multiple steps. The final thickness distributions and geometrical accuracy are analyzed for spiraling and dedicated feature toolpath strategies. The results are compared to forming the full part (base shape with details) in a conventional single stage manner. Forming the part in multiple steps did improve the accuracy of the part, by decreasing the underforming of the base shape compared to single stage forming. The observed overforming was highly influenced by the location of the detail. In terms of thickness distributions, the toolpath strategy highly influenced the location of the minimal thickness inside each detail. Here, the dedicated feature toolpath proved to be effective for achieving a more uniform thickness distribution.

Single Point Incremental Forming (SPIF), a numerically controlled sheet forming technique, excels in adaptability and cost reduction for small-scale production and customized parts. This study explores multi-stage SPIF, focusing on applications in the medical domain, particularly the fabrication of personalized medical implants. Addressing critical challenges related to formability and accuracy, this study emphasizes the significance of multi-stage forming and the importance of the design of intermediate shapes. Forming a cranial implant using one intermediate stage is studied, preliminary in pure Zinc as a cost-effective alternative for heated Ti. The intermediate shape is generated using automatic geometry adaptations such as translating and scaling the CAD model. Nine different experiments are conducted and compared in terms of final geometric accuracy and thickness distributions. The study shows that the proposed intermediate shapes for multi-stage forming significantly improve the geometric accuracy from 1.08 mm mean absolute deviation in traditional single stage forming to 0.41-0.59 mm. However, the used intermediate shapes have a strong but varying effect on thickness distributions throughout the part, showing the importance and sensitivity of the exact intermediate geometry. These insights provide a valuable understanding of the process, shaping the pathway for enhanced utilization of SPIF and setting the stage for future improvements.

Magnesium-zinc-zirconium (Mg-Zn- Zr) alloys, with their biocompatibility and biodegradability, exhibit great potential for biomedical applications. However, forming complex geometries poses a challenge due to the low formability of magnesium. Single Point Incremental Forming (SPIF) has emerged as a promising rapid manufacturing technique capable of producing complex-shaped, high-quality products. This preliminary study aims to investigate the feasibility of heat-assisted SPIF for Mg-Zn-Zr alloys, addressing formability at elevated temperatures, and the outcomes in terms of geometrical accuracy and material properties. Systematic parameter variations revealed that elevated temperatures and multistage toolpath strategies significantly improved formability, surpassing a 60 degrees maximum wall angle. Nevertheless, this enhancement led to increased surface defects and reduced strength during forming at elevated temperatures. The key finding highlights the need for a balanced combination of elevated temperature and maximum wall angle to optimize surface quality and strength in complex geometries.

Recent advances towards patient specific titanium sheet based medical implants introduce a new challenge for the fixation of these implants to bones. Mainly, the use of locking screws requires an implant thickness of approximately 2 mm for screw thread formation. Friction drilling is a hole-making process that displaces material to create a bushing below the sheet rather than extracting material. This experimental study explores the influence of axial force, rotational speed, and workpiece pre-heating temperature on the bushing height and thickness during friction drilling of titanium grade 2 sheets. The drilling parameters are optimized for both drilling at room temperature and at elevated temperatures for maximum bushing thickness with at least a bushing height of 1 mm. Subsequently, the samples are characterized for their microstructure and hardness, revealing preserved strength with a larger thermomechanical affected zone (TMAZ), a more gradual hardness gradient around the drill zone, and a significant reduction in microdefects in the bushing structure of the pre-heated sheets.

This paper discusses the thickness distributions calculated from surface strain measurements using stereo Digital Image Correlations (DIC) for parts produced with Single Point Incremental Forming (SPIF). The research is carried out on six benchmark cones and pyramids with each convex, straight and concave walls. The accuracy of the thickness calculations, under the assumption of material incompressibility and using the formula for the Green-Lagrange strains, is compared to the thickness distributions measured with a fringe projection scanner. The thickness estimations based on the measured strains proved to be representative for the measured thickness distributions with a mean error of 0.0182 mm, which corresponds to a relative error of 1.47 % of the mean measured thickness. However, errors of up to 0.1688 mm were found in areas of high wall angles and curvatures, corresponding to a relative maximal error of 13.69 % of the mean measured thickness. Hence, the DIC measurements are well suited for characterizing the thickness. Using the thicknesses calculated from the DIC measurements to find the minimal thickness as an indicator of part failure, is possible with relative errors that have an average overestimation of 2.87% of the minimal measured thickness.

While Incremental Sheet Forming (ISF) is approaching accuracy levels suitable for industrial take-up for specific applications, limited forming angles are still a great concern, leaving many applications out of reach. In this paper a two sided strategy for multistep incremental forming is presented, aiming at increased uniform wall thickness. By sequentially forming steeper wall angles, alternating passes between front and back side of the sheet, wall angles up to 105.5° were successfully reached in AA3103 with a blank thickness of 1.5mm. A resulting minimal thickness of 0.4mm and thickness range of 0.2mm was achieved for the 105.5°part.

Although Incremental Sheet Forming has overcome a number of challenges, its notorious lack of accuracy and need for high formability withholds this process from wide industrial adoption. This study aims to improve accuracy and formability through the use of an elliptical tool. The increased freedom of combining a large curvature in one direction and small curvature in the other allows to tailor the contact area between tool and sheet. An initial explorative study is described in this paper. A 3° increase in maximum forming angle of an AA5754 aluminium pyramid, is observed when using an elliptical tool while it is rotating around its axis. When using the tool in a lengthwise fixed orientation with respect to its forming direction, a 41% reduction in bulging of the wall is noted compared to a standard hemispherical tipped tool. This increased accuracy is accompanied by a reduction of forming forces and twisting.

Single Point Incremental Forming owes its flexibility to lack of dedicated tooling needed to create a variety of different geometries. This is possible because of its computer controlled nature: the toolpath that is executed will determine the shape of the produced part. By altering the toolpath in a smart way, the process can be optimized towards different criteria: speed, accuracy, surface quality. This paper describes a new methodology to automatically create toolpaths based on feature geometry. It can be used to add local features to a preformed shape, or to improve the surface quality of complex freeform parts. The paper describes the slicing algorithm and toolpath generation. The robustness of the procedure is demonstrated and a first test case shows an increase in surface quality without compromising on accuracy.

Surface registration, in which a mapping between surfaces is calculated, is a powerful tool to create patient-specific musculoskeletal and statistical shape models. However, to create these models, surface registration must assure a one-to-one correspondence between both surfaces. To enhance this process, a surface registration framework that uses a combination of manual and automatic landmark detection is presented in this study. In addition, an extensive parameter study of the framework for a femur and a clavicle is conducted. The average correspondence quality of nine femur landmarks decreased from 15.6 mm using automatic point detection to 6.1 mm by combining manually and automatically indicated landmarks. For the clavicle, the average distance of five landmarks decreased from 2.8 to 0.9 mm. Combining manual and automatic landmark detection clearly improved correspondence quality. Results confirmed the applicability of the proposed registration framework for femur and clavicle, although with different parameter settings.

Incremental sheet metal forming in general and Single Point Incremental Forming (SPIF) specifically have gone through a period of intensive development with growing attention from research institutes worldwide. The result of these efforts is significant progress in the understanding of the underlying forming mechanisms and opportunities as well as limitations associated with this category of flexible forming processes. Furthermore, creative process design efforts have enhanced the process capabilities and process planning methods. Also, simulation capabilities have evolved substantially. This review paper aims to provide an overview of the body of knowledge with respect to Single Point Incremental Forming. Without claiming to be exhaustive, each section aims for an up-to-date state-of-the-art review with corresponding conclusions on scientific progress and outlook on expected further developments.

Single Point Incremental Forming is a flexible process that is well-suited for small batch production and rapid prototyping of complex sheet metal parts. The distributed nature of the deformation process and the unsupported sheet imply that controlling the final accuracy of the workpiece is challenging. To improve the process limits and the accuracy of SPIF, the use of multiple forming passes has been proposed and discussed by a number of authors. Most methods use multiple intermediate models, where the previous one is strictly smaller than the next one, while gradually increasing the workpieces' wall angles. Another method that can he used is the manufacture of a smoothed-out "base geometry" in the first pass, after which more detailed features can be added in subsequent passes. In both methods, the selection of these intermediate shapes is freely decided by the user. However, their practical implementation in the production of complex freeform parts is not straightforward. The original CAD model can be manually adjusted or completely new CAD models can be created. This paper discusses an automatic method that is able to extract the base geometry from a full STL-based CAD model in an analytical way. Harmonic decomposition is used to express the final geometry as the sum of individual surface harmonics. It is then possible to filter these harmonic contributions to obtain a new CAD model with a desired level of geometric detail. This paper explains the technique and its implementation, as well as its use in the automatic generation of multi-step geometries.

The flexible nature of the Single Point Incremental Forming process implies that it is well-suited for small batch production and rapid prototyping of complex sheet metal parts. To overcome the process limits and to improve the accuracy, methods that alter the tool paths used to generate the part, like multi-step tool paths or compensated tool paths, have proven their effectiveness. However, their practical implementation in the production of complex freeform parts is not straightforward. Part of the challenge lies in deforming the geometric models without compromising the integrity of the surfaces. When using STL models, offsetting the surfaces often introduces anomalies like intersecting facets and flipped surface normals. This paper explores the use of Non-Rigid-Registration based morphing to produce regular and smooth STL surfaces. This is a technique used in medical imaging to identify anatomical correspondences between different meshes or images acquired, used to compare different images (one subject) or study anatomical variability (multiple subjects). The adaptation of the technique is explained and its use is demonstrated in the automatic generation of multi-step and compensated surfaces.

Shallow sloped single point incrementally formed (SPIF) parts typically show undesirable geometric deviations from the designed shape due to accumulated unwanted bulging. In this study, the effect of global heating by forced-air warming on the geometric accuracy of low-angled parts with a conical geometry is investigated. A finite element (FE) model was developed to simulate the SPIF process at elevated temperatures. The simulation results show that the sheet metal is subjected to significantly higher levels of stress in cold forming condition with respect to parts formed at elevated temperatures. Due to the increase in strength caused by strain hardening, a larger area of the sheet (both in the wall and bulge regions) is affected by the tool during forming at room temperature. Softening of the material by heat-assisted forming reduces the strain hardening and extended straining of the part which results in a reduction of the bulge height and of the under-forming of the cone base.

This study examines the possibility of applying lasers for the formation of laser-affected bands in hardenable steel sheets, with a specific focus on how the formation of these hardened bands can improve the accuracy of the single point incremental forming process (SPIF). For this purpose, the process parameters for the hardening process have been chosen using finite-element (FE) modeling. The results of the modeling have been validated by temperature field measurements obtained from IR camera observations. The microstructural analysis of the laser-affected zones has been performed using optical microscopy (OM) and scanning electron microscopy (SEM). These investigations confirm a phase transformation to a martensitic structure during laser scanning, and microhardness (HV0·1) results show a hardness increase by a factor of about three in the laser-affected region in comparison to that of the base metal (BM). Finally, using a laser assisted single point incremental forming (LASPIF) setup, hardened bands have been generated for preprocessing and intermediate processing during the different phases of a SPIF procedure. Geometric accuracy studies show that appropriate use of hard martensitic bands can increase the process accuracy through significantly reduction of an unwanted sheet deformation, and has the potential to eliminate the need for a backing plate.

The expanding demand for discrete freeform parts in bio-medical applications emphasizes the need for flexible production processes like Single Point Incremental Forming. This paper presents the specific challenges encountered during the design and fabrication of clavicle implants using medical grade titanium sheets. The fracture fixation and bone aligning task of these implants call for specific accuracy distribution, while the distinct geometry and post forming heat treatment influence production accuracy. The design potential for these implants considering forming angles of titanium grade 2 and soft tissue interactions are discussed. A generic case for the design, forming, heat treatment and trimming of a Titanium clavicle fixation plate is presented.

This research is aimed at enhancing the poor room temperature formability of heat-treatable aluminum alloy AA2024-T3, without deterioration of its post-forming properties. For this purpose, the influences of different heat-treatment conditions as well as warm forming on the single point incremental forming formability and post-forming properties of this material were investigated. Thermal pre-treatments were consisting of annealing (O-temper), solution treating and quenching (W-temper), and solution heat treating, quenching, and then cold working (T-temper). The formability results as well as forming forces of pre-heat-treated sheets were compared to those of the warm forming process results carried out using a laser-assisted single point incremental forming (LASPIF) setup. The post-forming properties of SPIF-formed parts were analyzed by hardness testing. The maximum forming angles of the blank formed under O-temper and W-temper conditions showed, respectively, 41 and 32% increases compared to the one under T-temper condition. LASPIF forming of this material at a temperature of about 360 °C resulted in 41% improvement in the maximum forming angle with respect to parts formed at room temperature from the T-temper sheet. The hardness of the material reduced significantly after annealing, while SPIF parts formed from W-temper blanks and under LASPIF condition regained their hardness after natural aging. The fracture surface characteristics of the failed parts showed that voids nucleate at the interface between intermetallic particles and matrix, and a dimple rupture fracture mode was identified under all heat-treatment conditions. Under O-temper condition, due to precipitation of particles along the grain boundary, an intergranular dimple rupture was observed. Finally, Energy dispersive x-ray (EDX) and electron backscattered diffraction (EBSD) were used to investigate the possible effects of the heat treatment and the deformation on the changes in the composition of intermetallic and second-phase particles, grain size, and texture.

Single point incrementally formed parts with a low wall angle geometry typically exhibit a manufactured geometry that significantly deviates from the design surface due to accumulated unwanted bulging deformation. Development of the bulge on the bottom of the part might result in wrinkling of the sheet at the bulged region which leads to higher forming forces and can even cease the forming process. In this study, the geometric inaccuracy of low angled parts is investigated by means of both Finite element analysis and an experimental campaign on a conical geometry. Deformation mechanisms in shallow sloped parts have been studied in detail and the tool-sheet contact area has been characterized both for low and high angled geometries. In a second phase, the laser assisted single point incremental forming process and its potential for improving accuracy are investigated. To obtain suitable process parameters for a warm forming condition, a transient heat transfer analysis is developed to simulate the laser movement on the conical geometry. Based on the simulated and experimentally determined tool-sheet contact zone, different laser spot positioning strategies have been used while the accuracy of the part and forming forces were measured. It has been observed that overforming of the cone wall is due to the continuous deformation of the sheet outside the contact zone which changes into underforming upon laser treatment. By selection of a proper laser positioning strategy a reduction of 42 % in bulge height is observed. This shows its effect in reducing radial forming forces.

Incremental Sheet Forming processes suffer from stringent forming limits, restricting the range of producible geometries. Through in-process cooling of the sheet to cryogenic level, this paper explores the potential of altering material properties benefiting the formability and residual hardness of different aluminium alloys. Global cooling of aluminium sheets with liquid nitrogen and dry ice allows to reach temperatures of 78K and 193K respectively. Extended with experiments at room temperature (293K), these tests form a base for comparison of surface quality, formability and residual hardness. As an aluminium alloy commonly used for its high strength to weight ratio, but suffering from limited formability compared to draw-quality steels, AA5083-H111 is of interest for cryogenic treatment. AA1050-H24 is included in the test campaign as a base for commercially pure aluminium.