Incremental Forming of Titanium Ti6Al4V Alloy for Cranioplasty Plates—Decision-Making Process and Technological Approaches

Split-Plot I-Optimal Design Optimisation of Combined Oil-Based and Friction Stir Rotation-Assisted Heating in SPIF of Ti-6Al-4V Titanium Alloy Sheet under Variable Oil Pressure

The aim of this paper is to determine the optimal input parameters for the process in order to ensure the maximum formable wall angle is obtained in a conical frustum with a varying wall angle fabricated using Single Point Incremental Forming (SPIF). The test material was 0.8-mm-thick Ti-6Al-4V titanium alloy sheets, and the test used a tungsten carbide tool with a rounded tip with a radius of 4 mm. Complete workpieces were heated using hot oil with a temperature of about 200 °C, and in addition, the high rotation speed of the forming tool generated an amount of friction heat. The input parameters were tool rotational speed, feed rate, step size, and tool rotation direction. Various oil pressures were used to improve both the accuracy of the components formed and the friction heating process. On the basis of calculations performed by means of the response surface methodology, split-plot I-optimal design responses were obtained by means of polynomial regression models. Models were fitted using REstricted Maximum Likelihood (REML), and p-values are derived using the Kenward–Roger approximation. Observation of the fracture surface of Ti-6Al-4V drawpieces showed that the destruction is as a result of ductile fracture mode. Tool rotational speed and step size are the most significant factors that affect the axial force, followed by feed rate. It was also found that step size is the most significant factor that affects the in-plane SPIF force.

Single-Point Incremental Forming of Titanium and Titanium Alloy Sheets

Incremental sheet forming of titanium and its alloys has a significant role in modern manufacturing techniques because it allows for the production of high-quality products with complex shapes at low production costs. Stamping processes are a major contributor to plastic working techniques in industries such as automotive, aerospace and medicine. This article reviews the development of the single-point incremental forming (SPIF) technique in titanium and its alloys. Problems of a tribological and microstructural nature that make it difficult to obtain components with the desired geometric and shape accuracy are discussed. Great emphasis is placed on current trends in SPIF of difficult-to-form α-, α + β- and β-type titanium alloys. Potential uses of SPIF for forming products in various industries are also indicated, with a particular focus on medical applications. The conclusions of the review provide a structured guideline for scientists and practitioners working on incremental forming of titanium and titanium alloy sheets. One of the ways to increase the formability and minimize the springback of titanium alloys is to treat them at elevated temperatures. The main approaches developed for introducing temperature into a workpiece are friction heating, electrical heating and laser heating. The selection of an appropriate lubricant is a key aspect of the forming process of titanium and its alloys, which exhibit unfavorable tribological properties such as high adhesion and a tendency to adhesive wear. A review of the literature showed that there are insufficient investigations into the synergistic effect of rotational speed and tool rotation direction on the surface roughness of workpieces.

Emerging Trends in Single Point Incremental Sheet Forming of Lightweight Metals

Lightweight materials, such as titanium alloys, magnesium alloys, and aluminium alloys, are characterised by unusual combinations of high strength, corrosion resistance, and low weight. However, some of the grades of these alloys exhibit poor formability at room temperature, which limits their application in sheet metal-forming processes. Lightweight materials are used extensively in the automobile and aerospace industries, leading to increasing demands for advanced forming technologies. This article presents a brief overview of state-of-the-art methods of incremental sheet forming (ISF) for lightweight materials with a special emphasis on the research published in 2015–2021. First, a review of the incremental forming method is provided. Next, the effect of the process conditions (i.e., forming tool, forming path, forming parameters) on the surface finish of drawpieces, geometric accuracy, and process formability of the sheet metals in conventional ISF and thermally-assisted ISF variants are considered. Special attention is given to a review of the effects of contact conditions between the tool and sheet metal on material deformation. The previous publications related to emerging incremental forming technologies, i.e., laser-assisted ISF, water jet ISF, electrically-assisted ISF and ultrasonic-assisted ISF, are also reviewed. The paper seeks to guide and inspire researchers by identifying the current development trends of the valuable contributions made in the field of SPIF of lightweight metallic materials.

Influence of Lubrication and Cooling on the Quality of Single-Point Incremental Forming Parts of Polycarbonate Sheets

Without ensuring high productivity, single-point incremental forming allows obtaining cavities in sheet-type workpieces by rotating and moving a rotary forming tool along a predetermined path. The process can be used in the case of both metal and plastic sheets. The heat generated in the processing area is expected to cause different elongations and contractions, affecting the final dimensional accuracy of the surfaces obtained. A full factorial experiment with three independent variables at two levels was used to investigate the correlations between the values of some of the process input factors and the results of thermal processes developed at the contact between the rotating tool and the workpiece. Experimental research was performed in dry single-point incremental forming, using lubricants and, respectively, with the generation of a decrease in temperature by the use of coolants. Empirical mathematical models were determined, and they confirmed the influence of the values of considered input factors on the thermal processes developed at the contact between the tool and the workpiece material. Temperatures of up to 147 °C were recorded in the processing area of the plastic workpiece.

Central Composite Design Optimisation in Single Point Incremental Forming of Truncated Cones from Commercially Pure Titanium Grade 2 Sheet Metals

Single point incremental forming (SPIF) is an emerging process that is well-known to be suited for fabrication in small series production. The aim of this paper was to determine the optimal input parameters of the process in order to minimise the maximum of both the axial and the in-plane components of the forming force achieved during SPIF and the surface roughness of the internal surface of truncated-cone drawpieces. Grade 2 pure titanium sheets with a thickness of 0.4 mm were used as the test material. The central composite design and response surface method was used to determine the number of experiments required to study the responses through building a second-order quadratic model. Two directions of rotation of the forming tool were also considered. The input parameters were spindle speed, tool feed rate, and step size. The mathematical relations were defined using the response surfaces to predict the surface roughness of the drawpieces and the components of the forming force. It was found that feed rate has an insignificant role in both axial and in-plane forming forces, but step size is a major factor affecting axial and radial forming forces. However, step size directly affects the surface roughness on the inner surfaces of the drawpieces. Overall, the spindle speed −579 rpm (clockwise direction), tool feed 2000 mm/min, and step size 0.5 mm assure a minimisation of both force components and the surface roughness of drawpieces.

On the Free-Surface Roughness in Incremental Forming of a Sheet Metal: A Study from the Perspective of ISF Strain, Surface Morphology, Post-Forming Properties, and Process Conditions

Due to absence of any supporting die, the free surfaces in incremental sheet forming (ISF) experience uneven deformation. This results in rough surfaces, possibly leading to the reduced service life of components. Therefore, it is necessary to analyze and quantify the effects of the application of strain on the free-surface roughness. Moreover, in order to control roughness, both on the free surface and the opposite contact surface, the nature of correlation between the two types of roughnesses needs to be identified by classifying the significance of different process conditions. The present work is a fundamental study to address these points. A series of specimens are produced by subjecting a metallic sheet to a range of ISF strains (13% to 98%). These specimens are then subjected to a number of characterization tests, namely roughness, uniaxial tension, and residual stress tests. The results reveal that the mean free-surface roughness increases non-linearly as the normal strain (stretching + bending) on the free surface increases (where strain state on the surface is as follows: ɛ1 = 0, ɛ2 > 0, γmax = ɛ2 and 1 and 2 are principal directions). The roughness also increases, although linearly, with the post-forming sheet strength, residual stress, and forming force, thereby showing that strain hardening has a direct influence on the roughness in a way that sheet strengthening is achieved at the cost of surface quality. The surface morphology reveals that the free surfaces contained orange peel, slip lines, and micro-voids, with density increasing with strain application, thus indicating the possible influence of tensile stresses on free surface deformation and roughening at an increasing degree with strain. Further analysis of roughness results discloses that the free-surface roughness and the contact-surface roughness are inversely related, because the responses of the two to ISF processing were mutually exclusive. Based on the obtained results, future research directions are also discussed.