Annual Report Department

2.4Manufacturing processes

2.4.1Laser processing of sheet metal

Laser cutting remains a very complex process: besides the material characteristics, a lot of other parameters influence the process and the resulting cut quality. Therefore identifying the ideal process parameter settings remains hard and a guaranteed, constant high quality can not always be assured in industrial circumstances.

The research focuses on two main topics:

• 
  Process modelling and optimisation of laser cutting of thick plates: for such plates a very delicate balance between gas pressure, gas composition, laser power, cutting speed, focussing parameters and nozzle design needs to be respected. Therefore fundamental research of the process has been performed in order to enlarge this process window.
  
• 
  Adaptive control of the laser cutting process: Due to external influences it is impossible to always guarantee a good quality with fixed settings of the process parameters. A major strategy to deal with this is the introduction of an adaptive control which adjusts some of the control parameters in real-time in function of the observed quality.
  

The second, investigated laser manufacturing process is laser bending. This is a thermo-mechanical process, in which a temperature field is generated in the metal sheet by means of a moving laser beam. This results in an equivalent stress distribution and, as a result of this, in the bending of the sheet. Focus is mainly on the use of higher power laser sources with the aim to extend the known process window. Well-optimised process parameters and suitable control strategies have been identified for different material-thickness combinations. This helps to realize a productivity of the process that can be considered economically feasible for incrementally formed parts with complex contours.

Related projects: IWT Project 030413 "MonALaC", IWT Project 30262 "SEMPER"

Publications and reports: 2003PP017, 2003PP018

Scientific staff: J. Duflou, J.-P. Kruth, J. De Keuster, B. Calllebaut

2.4.2Electrical discharge machining: milling EDM

In electrical discharge milling (milling EDM), a rotating tubular electrode follows a programmed path in order to obtain the desired shape of a part. Compared to traditional sinking EDM, milling EDM with standard electrodes eliminates the need for customised shaped electrodes, thus reducing tool costs. It can be applied in the manufacturing of parts and moulds of limited size, which would require a large amount of electrodes when machined only with sinking EDM. Improvement of the process and the workpiece quality/accuracy is aimed at by the use of alternative electrode materials, machining methods and real-time process monitoring. The focus is on tool wear control, machining accuracy, surface quality and machining speed. Real-time tool wear compensation is investigated and implemented, based on indirect tool wear monitoring through on-line pulse analysis. The prospect of using milling EDM as a process for selective surface enhancement is explored: workpiece surface modification by material transfer from electrode or dielectric are investigated and compared with sinking EDM.

Related projects: Bilateral collaboration: Milling EDM, GOA-project 2002/06

Publications and reports: 2003PP078, 2003PP131

Scientific staff: J.-P. Kruth, B. Lauwers, Ph. Bleys

2.4.3Electrical discharge machining of ceramic materials and cermets

Ceramic materials and cermets are becoming more and more important in a variety of industrial applications. Because of their unsurpassed hardness and thus wear resistance they are the ideal material for stamping dies, cutting inserts, mechanical components, ... The research aims at the electrical discharge machining of electrical conductive ceramic materials and cermets for a variety of applications. The interaction between material composition, process parameters and obtained properties of machined parts is studied. Both Wire-EDM and Sinking-EDM are studied for three classes of ceramic materials (non-oxide ceramics, ZrO2 ceramics with a variety of conductive ceramic matrix materials and non-oxide-based cermets with a metal binder).

Related projects: EU STRP project NMP2-CT-2003-505541 "Moncerat", IWT GBOU-project "Spark", GOA-project: 2002/06.

Scientific staff: B. Lauwers, J.P. Kruth, W. Liu, B.Schacht, R. Verheyen, W. Eeraerts

2.4.4Electrical discharge machining: wire cutting

In EDM wire cutting a wire-electrode is used to cut complex contours in work pieces, e.g. for the production of moulds and dies. The research aims at the high quality and reliable wire electrical-discharge machining of complex parts with small radii and tight tolerances within a competitive time in comparison with conventional machining methods leading to a decreased production cost of parts. All aspects of the process are under investigation: new wires with high strength, alternative dielectrics that lead to lower surface deterioration and modern generator technology is looked at.

Related projects: EU Growth project no. GRD1-2001-40502 "EDM2005", GOA-project: 2002/06.

Publications and reports: 2003PP038, 2003PP078, 2002PP072, 2003PP152

Scientific staff B. Lauwers, J.-P. Kruth, W. Liu, B. Schacht, R. Verheyen, W. Eeraerts

2.4.5Development of production machinery for "ThermHex" thermoplastic folded honeycomb

"ThermHex", as a novel thermoplastic honeycomb core, can serve as core material in sandwich constructions to be used e.g. in automotive applications. Common advantages of thermoplastic honeycombs are recyclability and good mechanical properties at low weight.

ThermHex adds to this the possibility for a high material output at considerable production cost, due to its novel continuous production concept.

At PMA, the production concept of ThermHex is being implemented. Task is to create machinery for the purpose of sample production and the simulation of industry scale production, which is envisaged in the future.

At the lab for lightweight structures, the three main production steps are investigated. Upon concept approval, lab scale production machinery has been developed, designed, manufactured and tested. In the year 2003, the approval of the possibility for production in industrial scale has been achieved. Experimental studies have been performed, resulting in processing frames, which are further worked out in the ongoing course of the project.

Related projects: EUREKA project E!2796, IWT project 020573 “Thermhex”

Publications and reports: 2003PP178

Scientific staff: D. Vandepitte, P. Bratfisch, W. Covens, E. Seghers

2.4.6Development of production machinery for "Torhex" paper honeycomb core

The objective of this project is the development of a lightweight sandwich panel to be used as a substrate for car interior panels. The panel is based on a TorHex paper sandwich core and fibre reinforced polyurethane skins. The formability of the sandwich construction has to be investigated to be able to produce complex shaped threedimensional components. The components should be ultra light weight, and they should have the required mechanical properties. Next to the effect of material selection on mechanical properties, the process for forming complex threedimensional sandwich panels will also be developed. The development will be verified with the prototype of a car door interior panel.

Related projects: IWT project no.030518 “Lightback”

Scientific staff: D. Vandepitte, W. Neirinck

2.4.7Rapid Prototyping: Stereolithography

Stereolithography is a layer-by-layer manufacturing process, based on photopolymerisation. The materials used are photo-curable liquid monomers, which polymerise under influence of UV-radiation of a specific wavelength. Past research involved the design of a new system for depositing liquid layers: the curtain recoating system. In this system the liquid is brought under pressure and subsequently squeezed out of a narrow slot. The liquid falls down as a thin sheet onto the build vat. A liquid layer is applied by moving the recoating system over the build vat. The curtain recoating system has been optimised in order to reduce the thickness of the recoated liquid layers. Currently, layers of 0.05 mm thickness can be recoated. This result has been achieved through research aiming at improving the dynamic response of a liquid curtain to applied acceleration trajectories. Firstly, the air shields have been redesigned. A configuration with two shields is used. Secondly, it has been shown that the curtain's dynamic response is significantly damped by the dynamics of the air present in the chambers, between the curtain and the shields. This damping has been reduced by breaking the curtain in a controlled way during acceleration, such as to short-circuit the airflow.

Related projects: TAP/PAT project PA-01-321 (OSTC)

Publications and reports: 2001PP074, 2003R016

Scientific staff: J.-P. Kruth, M. Gilio

2.4.8Selective Laser Sintering and Selective Laser Melting: technology

Selective Laser Sintering (SLS) and Selective Laser Melting (SLM) are Layer Manufacturing techniques, allowing building complex 3D parts out of powder material. These parts can be used as prototypes (Rapid Prototyping), but the techniques are used also for the production of tools (Rapid Tooling) and the direct production of functional parts (Rapid Manufacturing). While SLS can be used to build plastic or metal parts, SLM is used for metal parts only. The equipment used at PMA consists of a commercial SLS machine with a 100 W CO2 laser, a self built SLS/SLM machine with a 300 W Nd:YAG laser, and a furnace for postsintering or infiltration of SLS ‘green parts’. Research focusses mainly on SLS and SLM of metal powders.

The self built machine has been equiped with an infrared heating system to investigate the influence of powder preheating on residual stresses remaining inside the produced parts and on the occurrence of cracks. A new laser scanner software (PMA LaserScan) was developed in order to test the influence of the scanning strategies on the part properties and the residual stress.

A statistical analysis (Design of Experiments) was performed on the DTM machine in order to evaluate the contribution of each process parameter to the quality of the sintered parts after SLS.

Related projects: FWO project no. G.0216.00, GOA project 2002/06, TAP/PAT project PA-01-321, CRIF/WTCM project 5P.

Publications and reports: 2001PP032, 2003PP003, 2003PP117, 2003PP084, 2003PP119, 2003PP150

Scientific staff: J.-P. Kruth, B. Lauwers, J. Van Vaerenbergh, P. Mercelis, S.Kumar, S. Campanelli

2.4.9Selective laser sintering and selective laser melting: materials

Different iron based materials were tested using mainly the self-built machine. The influence of melting point and surface tension lowering elements was investigated by testing pure Fe, Fe-Cu-Fe3+xP-Ni, and Fe-C mixtures.

The microstructure, density, hardness and surface finish were determined. It was possible to build parts of these materials having a density up to 92% by melting and partially evaporating the material. The process parameters had to be carefully selected in order to avoid the formation of cracks due to the high thermal stresses.

The properties of the parts (specially hardness, surface finish and bending strength) were generally better when the material is partially evaporated than when it is not, due to plasma or plume formation generating a beneficial recoil pressure. The influence of preheating and scanning strategy on the densification was studied. Design of experiments was used to test the influence of some powder properties and processing parameters on the resulting material properties.

Related projects: GOA project 2002/06

Publications and reports: 2001PP032, 2003PP003, 2003PP119, 2003PP144, 2003PP146

Scientific staff: J.P. Kruth, J. Van Vaerenbergh, P. Mercelis, S. Campanelli, S. Kumar, L. Froyen (MTM), M. Rombouts (MTM)

2.4.10Selective laser sintering and selective laser melting: simulation

Due to various parameters occurring in SLS and SLM, analytical modeling of these processes is nearly impossible. In order to be able to simulate the processes using Finite Element methods, parameters like powder conductivity and powder absorptance need to be determined. Powder conductivity at room temperature was investigated experimentally by using the photopyroelectric method at the department of physics. The amount of thermal radiation between particles as a function of the temperature can be obtained from the extinction coefficient of the powder bed. The latter was measured experimentally using an own build setup. A relation between the measured extinction coefficient and the specific surface area of the different powder is found. The surface area is determined by image analysis and laser diffraction. The normal and hemispherical reflectance of different powder materials was also measured experimentally. These results are compared with the results obtained by modeling the radiation transfer in powder beds. These results serve as input data for the Finite Element modeling of SLS.

Related projects: GOA project 2002/06

Publications and reports: 2003PP076, 2003PP146

Scientific staff: J.-P. Kruth, A. Gusarov, M. Rombouts (MTM)

2.4.11Selective laser sintering and selective laser melting: medical applications

In medical and dental applications SLM can be applied as a Rapid Manufacturing technique to digitize the production of an implant and to replace the labor-intensive and time consuming lost-wax or milling processes. Research mainly focuses on implants for multiple tooth replacement. Based on the measurement data of the patient’s mouth, the design of the implant is made in a CAD-environment. Using the CAD model, the frameworks need to be built with SLM out of mouth persistent and biocompatible materials like Ti or CoCr alloys.

Different scanning possibilities were investigated (X-ray tomography and laser scanning) and the design strategy of the framework within the CAD environment was developed. Several dental frameworks were laser sintered in steel to test the whole production trajectory and analyze the achievable accuracy.

Future research will focus on the difficulties in Selective Laser Melting of Ti and CoCr biometals like cracks, distortions, porosities, surface roughness and a brittle microstructure.

Related projects: GOA project 2002/06

Publications and reports: 2003PP084, 2003PP144, 2003PP145

Scientific staff: J.-P. Kruth, B. Vandenbroucke, M. Savalani, J. Van Vaerenbergh, P. Mercelis

2.4.12Selective Laser Sintering and Selective Laser Melting: Aerospace applications

SLS and SLM offer two main advantages that make the techniques interesting for aerospace applications: an unlimited geometric complexity together with a cost reduction for single part or small series manufacturing. However, in order to certify the process for aerospace applications, full dense structures are required; neither micro cracks nor porosities are allowed. Therefore the technology needs to be further improved. The aim is to produce parts in Ti-6Al-4V, a widely used alloy.

A first case study - the production of lightweight structures - was preformed using glass filled nylon powder.

Related projects: GOA project 2002/06, TAP/PAT project PA-01-321, CRIF/WTCM project 5P.

Publications and reports: 2003PP119

Scientific staff: J.-P. Kruth, M. Van Elsen

2.4.13Flexible methods for sheet metal forming

The first flexible forming method studied is Single Point Incremental Forming (SPIF). The applied principle can be compared to cavity milling, but the cutting tool is replaced by a smooth, hemispherical cup and the raw material by a metal sheet . The forming limits achieved by this process in some cases far exceed what is possible in mass production techniques such as deep drawing. However, the limits of the process have not yet been explored and only a limited range of materials have been tested. The control of the process remains a problem. The material behaviour has not yet been studied in detail and forming mechanisms has therefore not been clearly identified. Due to a lack of appropriate process models, the CNC programming can be an iterative procedure, requiring multiple attempts to achieve the desired shape. Systematic tolerance control strategies on a process planning level have also not yet been investigated.

An extensive series of tests, conducted on the different selected materials, will provide insight into the influence of different process parameters (tool geometry, feed rate, step size, spindle velocity, clamping force, etc…). Experimental design methods will be used to efficiently identify dominant control parameters. Geometrically simplified incremental forming tests will be defined for this purpose. The results will be analysed using dimensional measurement methods and multi-linear regression techniques. Failure limits will be experimentally identified for different materials, thicknesses and sample geometries.

Laser Forming, another forming method on which research is being conducted, is a flexible way to plastically deform sheet metal. It is a thermo-mechanical process, in which a temperature field is generated in the metal sheet by means of a moving laser beam. This results in an equivalent stress distribution and, as a result of this, in the bending of the sheet towards or away from the laser beam, depending on the mechanism employed. This technique can be useful in the field of 3D rapid prototyping.

The study consists of an intensive experimental exploitation of the process-material interaction: an extensive series of tests, conducted on the selected materials, will provide insight in the influence of the different process parameters (power, pulse frequency, duty cycle, focus distance, velocity, etc…). The process limits (melting, etc…) will be evaluated. Special test parts will be designed to explore the creation of local 3D form features with flat surrounding sheet along the boundary condition.

As a second step, process models will be conceived that will be gradually refined and tuned to deal with increasingly complex part geometries.

Related projects: IWT Project 30262 “Semper“

Scientific staff: J. Duflou, J.-P. Kruth, A. Szekeres, B. Callebaut

2.4.14 Process models for robot based force controlled high efficiency deep grinding

Within today casting industries, fettling and finishing of complex shaped castings is still a major cost element and is often manually performed. This research fits in an EU project which aims the development of an affordable and flexible system for the automated fettling and finishing of medium/large castings (up to 1500 kg) made in small numbers. Robot based force controlled grinding has been selected as the process to finish the stubs after removing the feeders and risers by robot based oxy fuel cutting. Process models, describing the relation between material removal rate, surface qaility and the process parameters set (force, feed rate,…), are used to be integrated into the off-line robot programming system.

Various stub grinding experiments have been peformed on casted blocks made of steel ASTM A216 WCB. Different cup-stones with varying composition and characteristics have been investigated. The influence of the applied force, feed rate and spindle speed on the material removal rate and surface quality has been studied.

Related projects: EU Project no. GRD1-200-25135 Autofett

Scientific staff: B. Lauwers, H. De Baerdemaeker