Hans Vanhove

Manufacture of Accurate Titanium Cranio-Facial Implants with High Forming Angle Using Single Point Incremental Forming

One of the key application areas of Single Point Incremental Forming is in the manufacture of parts for bio-medical applications. This paper discusses the challenges associated with the manufacture of cranio-facial implants with extreme forming angles using medical grade titanium sheets. While on one hand, the failure wall angle is an issue of concern, the parts also need to be manufactured with accuracy at the edges where the implants fit into the human body. Systematic steps taken to overcome these challenges, using intelligent intermediate part design, feature analysis and compensation, are discussed. A number of case studies illustrating the manufacture of accurate parts in aluminium, stainless steel and titanium grade-2 alloy are discussed.

Accuracy Improvement in Single Point Incremental Forming Through Systematic Study of Feature Interactions

Previous studies have shown that feature detection and part segmentation are useful tools to generate compensated toolpaths for single point incremental forming leading to improvement in accuracy of manufactured parts. However, in most practical applications, features do not occur by themselves. Rather, they occur in combination with other features, and the presence of the neighbouring features influences the behaviour of the feature of interest. The final shape of the formed part depends on the interaction between the features. In this study, an attempt has been made to generate a complete taxonomy of common features relevant for incrementally formed parts. This taxonomy is then utilized to generate a matrix of feature interactions, and to classify them as feasible or not. From the subset of feasible feature interactions, a number of cases are analyzed to illustrate the effect of the interactions on the magnitude and nature of inaccuracies resulting in uncompensated parts. Strategies to use the knowledge of the interaction between these features to improve the accuracy of the manufactured parts are then discussed with the help of experimental case studies.

High-Speed Single Point Incremental Forming of an Automotive Aluminium Alloy

Incremental Sheet Forming processes have been characterized by their limited forming speed and accompanying lengthy production time. ISF has therefore been considered a process category suitable for small batch sizes or discrete part production only. The potential for greatly increasing the forming speed of incremental forming processes is studied here by means of axisymmetric incremental forming on a lathe. As an aluminium alloy commonly used in automotive applications, AA5182-O, is of interest for incremental forming at increased speed. In this paper the influence of an increasing feed rate on forming forces, temperature and formability is analyzed.

Obtainable Accuracies and Compensation Strategies for Robot Supported SPIF

Single point incremental forming (SPIF) is a flexible forming method allowing to shape sheet metal without the need for die. This process is particularly well suited for rapid prototyping and small series production. The classical setup for incremental forming is a specialised rig mounted on a standard milling machine. While this is an economic solution for small to medium size workpieces, large parts require the availability of a big and therefore expensive machining centre. A cheap solution consists in using a robot, which typically has a much better workable range to cost ratio. Unfortunately a robot is usually not a stiff machine, with the consequence that during forming, due to the forming forces, the deformation of the robot can be orders of magnitude greater than the accuracy of the process. In consequence the accuracy of the achieved part is significantly affected. To overcome this problem, a strategy for compensating the deflection of the robot at the level of the tool has been implemented. To support this strategy, two variables have to be examined: the forming forces on one hand and the stiffness of the robot on the other. In this paper it is demonstrated how, based on robot kinematics, the tool deflection can be computed from the knowledge of compliance of each joint in terms of angle deflection versus the moment load applied to the joint. Experimental results illustrate the effectiveness of this approach.

Electric Energy Consumption Analysis of SPIF Processes

Manufacturing processes, as used for discrete part manufacturing, are responsible for a substantial part of the environmental impact of products, but are still poorly documented in terms of environmental footprint. A thorough analysis on the causes affecting the environmental impact in metal forming processes, especially the innovative but very energy intensive sheet metal forming technologies required to form light-weight products, is nowadays necessary. Therefore, this paper presents an energy consumption analysis, including a power and time study, of Single Point Incremental Forming (SPIF) processes. First, the influence of the most relevant process parameters (e.g. feed rate, step down) as well as the material forming itself are analysed regarding the power demand. Moreover, a comparative study and related energy efficiency assay are carried out on two different machine tools. As the forming time proves to be the dominant factor for the total energy consumption, from environmental point of view, the overall results show many similarities with conventional machining processes. Finally, this paper reports on some potential improvement measures to reduce the SPIF energy consumption.

Enhanced Formability of Age-Hardenable Aluminium Alloys by Incremental Forming of Solution-Treated Blanks

The influence of thermal pre-treatment on the formability of a precipitation-hardening aluminium alloy AA-2024 has been studied for three different heat treatment conditions: annealed (O-temper), solution treated and quenched (W-temper) and solution heat treated, quenched and then cold worked (T-temper).The maximum draw angle has been determined and the geometrical accuracy of specific SPIF formed parts has been compared. It is found that the maximum forming angles of the blank formed in O-temper and W-temper conditions show a respective 41% and 32% increase to those of the T-temper condition (initial blank).The hardness of the material reduces significantly after annealing, while SPIF parts formed from W-temper blanks regain their initial hardness after natural aging.

In-Process Hardening in Laser Supported Incremental Sheet Metal Forming

The effect of localized laser hardening on the dimensional accuracy of incrementally formed steel sheets has been studied. By dynamically heating by means of laser beam scanning (500W Nd:YAG) the temperature of the sheet reaches the austenization temperature and by subsequent self-quenching a hard martensitic structure will form. Using FE modeling, a laser power setting of 202 W, scanning velocity of 600 mm/min and beam diameter of 6 mm were selected as optimum processing parameters for transformation hardening. Hardness tests were performed in order to investigate the hardness profile along the depth and width of the laser hardened zone. Experimental results reveal that generation of a selectively hardened martensitic band, formed by transformation hardening, can increase the accuracy of the incrementally formed part.

Formability Enhancement in Incremental Forming for an Automotive Aluminium Alloy Using Laser Assisted Incremental Forming

The aim of this study is to establish general guidelines for minimizing the number of tests required to determine optimum process parameters in terms of formability for laser assisted single point incremental forming (LASPIF). An automotive aluminium alloy (AA5182-O) is selected and the room temperature failure angle of this material is determined experimentally. The straining behaviour as well as sheet thinning of the test part (at its maximum forming angle) is studied using an experimentally validated finite element model. From the thinning rate of the sheet metal and the shape of the contact zone between tool and sheet it is concluded that continuous straining of the sheet on the wall region of the contact area is responsible for extra thinning and failure. Based on the size and position of the contact zone, different laser tool positioning strategies have been used to achieve the highest forming angle. It is concluded that due to an elongated shape of the contact zone in steep wall angle parts and considering a small deviation of the forming robot, the selection of a large spot diameter is necessary in terms of maximum obtainable wall angle. It has been observed that the maximum forming angle is still achievable using a large forward offset. It is concluded that the partial stress-relief annealing of the deformed geometry during the approach of the forming tool, is responsible for this formability enhancement.

Tool Directionality in Contour-Based Incremental Sheet Forming: an Experimental Study on Product Properties and Formability

The Incremental Sheet Forming (ISF) process offers a large variety in tool path strategies to obtain a particular final product shape. As fundamental understanding of the relevant deformation modes in ISF is growing, the selection of the tool path strategy may be shifted from trial-and-error towards more fundamentally based knowledge of the process characteristics. Truncated cones and pyramids have been fabricated by both unidirectional (UD) and bidirectional (BD) contour-based tool path strategies, considering different wall angles and materials (Mn-Fe alloyed aluminum sheet and low carbon steel sheet). It is shown that the induced through-thickness shear along the tool movement direction is clearly non-zero for UD, in which case the sense of tool movement is the same for all contours, while it is close to 0 for BD, due to the alternating tool sense during consecutive contours. Furthermore, the heterogeneity in product thickness, as observed for the UD strategy in [1,2], is avoided by using the BD strategy. It is verified that this difference in deformation may affect the mechanical properties in the walls of pyramids by means of tensile testing, but the results are material-dependent. For the aluminum alloy, the re-yield stress along the tool movement direction is smaller for BD in comparison to UD, and the fracture strain in large wall angle products is higher. For the steel, no statistically significant differences in mechanical properties between UD- and BD-processed parts are observed. Finally, for both materials a (slightly) higher limiting wall angle has been repeatedly measured using the BD tool strategy. In light of these results, the bidirectional tool path strategy is to be preferred over the unidirectional one, as thickness distribution and formability are more favorable, while both strategies require similar resources and processing time.

Incremental forming of aluminium alloys in cryogenic environment

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.