KIT, Multitel, SIRRIS & VITO

Transcription

1 COLLABORATIVE PROJECT Laser micro-joining of metals and thermoplastics using medium power and low-cost lasers KIT, Multitel, SIRRIS & VITO common issues that touch the lives of EU-citizens 1

2 TABLE OF CONTENTS 1. Introduction Involved partners Experimental plan Results Conlusion LIST OF FIGURES Figure 1. Laser textured stainless parts at various magnifications... 4 Figure 2. Designed clamping device... 5 Figure 3. Experimental setup of the laser welding system... 6 Figure 4. Manufactured demonstrator (PMMA/stainless steel)... 7 Figure 5. Experimental setup... 7 Figure 6. Typical curve obtained during the tests... 8 Figure 7. Tensile tested samples from pre-test campaign... 9 Figure 8. Results from nominal tests... 9 common issues that touch the lives of EU-citizens 2

3 1. Introduction The objective of Ecolaserfact Collaborative Projects is to demonstrate the capabilities of lasers for low-cost, high precision, flexible and eco-friendly micromanufacturing. Potential applications will cover a large spectrum ranging from photovoltaic to bio-medical technology, from laser additive direct fabrication to online control with 3D-laser metrology, from micro-welding to patterning and modification of thin films. The aim of the Project Laser micro-joining of metals and thermoplastics using medium power and low-cost lasers is to demonstrate the capabilities of different laser technologies in the field of joining dissimilar or hybrid materials. Laser welding of the separate materials has become common practice, even in industry. The laser welding of the combination of these materials is a relatively new domain. A direct joining between metal and polymer materials is desirable in many ways: it enables weight saving by replacing some of the metal parts in a component by light weight polymer materials, assembly of only 2 components without a glueing step is made easier, moreover no toxic primer nor chemicals need to be used in such case making it more ecological as well. From an economic point of view the use of plastics is preferred over metals. Mechanical joining is also suffering some drawbacks such as flexibility compared to direct joining. Less assembly steps will allow an improvement of productivity, hence making it more interesting to apply in industry. 2. Involved partners From Germany: From Belgium: From Belgium: From Belgium: (Leader) common issues that touch the lives of EU-citizens 3

4 3. Experimental plan The concept of this project is to combine different expertises of the involved partners to manufacture a joint demonstrator. An experimental plan has been set up as follows: 1. Clamping device (VITO): design and manufacturing of a proper clamping device in order to ensure excellent contact between the dissimilar parts 2. Laser Micro-structuring (Multitel): selection and production of laser textured samples in order to obtain a mechanical anchoring for the plastic material 3. Laser transmission welding (KIT): applying the set-up of a combination metal/polymer sheets using laser radiation in a transmission welding set-up 4. Evaluation of the welded samples (Sirris): tensile testing of the laser welded samples 4. Results Multitel was in charge of supplying laser textured metal sheets. The textured surface should enable mechanical anchoring of the thermoplastic material when it has been molten using laser radiation. For the texturing of the metal components (stainless steel) a 1030nm laser wavelength was used, with a pulse duration of 500fs at a repetition rate of 10kHz and 4W of average power. A mesh like structure was applied using a scanning head at a traveling speed of 30mm/sec. After optimisation of the process, a structure as depicted in Figure 1 is obtained. Figure 1. Laser textured stainless parts at various magnifications common issues that touch the lives of EU-citizens 4

5 VITO designed a generic clamping tool in order to provide optimal contact of the components during welding. This contact between the parts is crucial in transmission laser welding, since only the bottom part will absorb the laser radiation and heat up the upper part in an indirect way. Every area in the welding zone without sufficient contact will weaken the weld strength. Figure 2. Designed clamping device The clamping device (Figure 2) consists of a pneumatic cylinder moving up and down. The cylinder is equipped with a platform supporting the two components. The components are pushed with controlled pressure towards a transparent PMMA plate which does not alter the properties of the beam nor the laser power. KIT performed the laser transmission welding tests using a structured metal sample provided by Multitel and a transparent PMMA sample. Since the bottom layer of the welding parts is absorbing for stainless steel, no absorbing layer or additional material is required. The two parts were set under a constant clamping pressure (2 bars) and the laser beam was radiated through the cover plate and was absorbed by the lower part which was consequently heated. The used laser radiation source (FLS IronScan, Fisba Optik AG) is a high power diode laser with a maximum laser output power of 50W at a wavelength of 940nm. The laser beam was focused onto the sample surface by an objective lens (F-Theta objective, focal length 163mm). The laser focus diameter is about 1 mm. The beam was scanned over the sample surface via deflection mirrors up to speeds of 2000mm/s. A closed loop feedback control between laser and pyrometer (FLS PyroS, Fisba Optik AG) common issues that touch the lives of EU-citizens 5

6 enables during the welding process a continuous measurement and adjustment of the surface temperature. The sample rate of the monochromatic pyrometer system was 1ms. If the temperature increases, e.g., because of dust particles or absorbing layer inhomogeneity the laser power is lowered within several milliseconds. Therefore process temperature can be controlled in the range of 120 C up to 700 C. The accuracy of temperature control was within ±5 C verified by pyrometer measurements. Figure 3 depicts the laser welding setup with the clamping tool provided by VITO. Figure 3. Experimental setup of the laser welding system For laser welding, two parameters are decisive for the result: the welding time and the welding temperature. After a parameter variation the following process parameters were used for the final fabrication of the demonstrators. For joining of metal and polymers, two different temperatures were chosen to further investigate the bond strength. Two temperatures, 130 C and 150 C, were used for manufacturing and further investigation. A welding time of 33s/cm² was applied here. common issues that touch the lives of EU-citizens 6

7 Figure 4. Manufactured demonstrator (PMMA/stainless steel) Figure 4 depicts a sample that was manufactured to be shown at conferences and exhibits to demonstrate laser processing capabilities. In order to characterize the efficiency of the welding process, tensile tests were carried out on plastic-on-metal welded parts by Sirris. Tensile tests were conducted with a universal mechanical testing machine (Figure 5) Instron 4507 (capacity 200kN). The machine is calibrated by Schütz. The 10kN load cell calibration certificate is valid until July The crosshead speed and displacement calibration certificates are valid until November Figure 5. Experimental setup (Instron 4507 machine, shown in a generic configuration) common issues that touch the lives of EU-citizens 7

8 Figure 6. Typical curve obtained during the tests A pre-test campaign was carried out, based on samples available in quantity (Figure 7). One of the prerequisites was to find the correct glue to interface the samples to the grips of the tensile test machine, for the movement is perpendicular to the welding plane. Different glues were tested in a preliminary phase: Cyanoacrylates, Acrylic Adhesiv bi-component and Epoxy Adhesives. Adhesive failure was observed on the glass side for every adhesive material, with the exception of the Loctite 3430 (bi-component) material. This latter was chosen for the nominal trials. The parameters for the testing equipment are based on this information and pretests chosen as follows: Acquisition (pts/s ): 25 Humidity ( % ): 50 Temperature ( C): 23 Tests Speed: 10mm/min Distance between grips: 7mm common issues that touch the lives of EU-citizens 8

9 Figure 7. Tensile tested samples from pre-test campaign Nominal tests were then carried out, based on the protocol consolidated from the pre-tests. In any case, the failure occurred at the welding interface, indicating that the weld is weaker than either of the two components. However the weld is indeed stronger than non-textured samples, which can not be welded at all using transmission laser welding. The tensile strength is not very high at this moment in testing. This can be due to the direction of tensile testing perpendicular to the welding plane. A tensile test direction parallel to the welding plane would probably yield higher tensile strengths. This was also found in literature. Sample 1 Sample 2 Load (@ Failure): 80.9daN Failure at the welding interface Load (@ Failure): 68.23daN Adhesive failure on the glass side, but the failure was initiated at the welding interface too (see picture) Figure 8. Results from nominal tests common issues that touch the lives of EU-citizens 9

10 5. Conlusion The involved partners have shown an integrated and combined laser processing of materials in order to join dissimilar materials i.e. metal and polymer (Stainless steel and PMMA). Using laser texturing of metals and combining this with a consecutive laser transmission welding step, metal and thermoplastic components can be joined directly without any glues, chemicals or extra mechnical assemby step. The joint transnational cooperation on a single device led to the production of a demonstrator highlighting mutliple laser technologies for micro-manufacturing and acting as excellent example of laser micro-manufacturing for SMEs. common issues that touch the lives of EU-citizens 10