Laser joining of ceramics in liquid phase
Transcrição
Laser joining of ceramics in liquid phase
Laser joining of ceramics in liquid phase Horst Exner, Anne- Maria Reinecke, Peter Regenfuß, Maren Nieher Laser Institut Mittelsachsen e.V. at Hochschule Mittweida (FH), University of Applied Sciences Abstract The only way to join ceramic bodies- even with complicated geometries- effectively without deterioration of their excellent properties is by a process with a liquid or a liquid like phase or a diffusion bonding process. We report the successful employment of laser beams for the joining of ceramic bodies to each other and to metal specimens. Our two-laser-beam welding method, an additive-free process in liquid phase, allows to join bodies of fusible ceramics, e.g. of alumina parts, resulting in a strength of 85 % of the bulk material. Furthermore for the first time we present results on laser welding of alumina with niobium. By this method parts of various shapes can be joined in only a few minutes without furnaces. The achieved results in combination with the general advantages of laser material processing, like narrow seams, high process flexibility, high productivity and a high degree of automation, proof this technology ideally suited for industrial application. Introduction Ceramics are materials usually produced by a special sintering process. Depending on their composition this leads to special qualities like high temperature resistance, extreme hardness, high chemical resistance and a lower density compared to metals. These excellent properties are the reason for the application of technical ceramics in the vast fields of electronics, automotive industry, aerospace, chemical industry and so on. Currently no technology exists which, within reasonable economical limits, produces joints of satisfactory quality between ceramic parts and preserves the excellent properties of the material (1-5). Metal brazing and adhesive bonding principally reduce the thermal and chemical stability of the ceramic system (6,7,8). These disadvantages originate from the presence of an additional material (glue or solder) with completely different properties than those of the ceramic. This means, a critical weak point is generated at the joint. Furthermore, metal brazing usually is possible only after the improvement of the wettability for the solder by metallisation of the ceramic surface. This process is time consuming and very expensive. On the other hand high quality joints can be achieved by diffusion welding (9). The joining mechanism is based on diffusion processes at high temperatures. This means, diffusion welding needs a process time of about one hour. The preparation of the material is very expensive (requires a high surface smoothness) and a high bearing pressure is necessary (therefore it is not suitable for joining small parts). In addition both, diffusion welding and brazing, demand a vacuum atmosphere. Experimental set-up Alumina specimens (α- Al2O3) and metal parts with a length of 20 … 30 mm, a width of 5 or 10 mm and a thickness of 0.7 … 1.2 mm were used in our experiments. Because of the ratio of thermal conductivity to thermal expansion of alumina, a fast and locally restricted energy input by a laser beam will normally generate cracks in the material. Thus the material has to be preheated to minimise the thermal shock effect of the welding laser beam. To overcome the disadvantages of a furnace process, in our case the preheating is performed by a second laser beam (Figure 1) (10). fibre Figure 1: Experimental set-up CO2- laser beam A continuous wave (cw) 600 W CO2- laser beam scans the surface of the material. Because of its very high absorption at the wavelength of 10.6 µm, the material gets heated focussing optic within seconds. When the necessary preheating temperature is achieved the cw 1.2 kW Nd: YAG laser beam starts to weld the parts Nd: YAG- laser beam together. It penetrates about 0.8 mm deep into the alumina (11). This means that for the low ceramics thickness of our specimens the generation of a melting bath for welding is nearly independent of the thermal conductivity (12,13). By welding the ceramic to metals argon was used as shielding gas on the upper and lower side to avoid oxidation. scanning mirrors Table 1: Properties of the used material alumina niobium steel 1.3981 purity % 96,0 99,9 melting point °C 2050 2468 1460 boiling point °C 3530 3300 3023 thermal conductivity W(mK)-1 27 52 17 thermal expansion coefficient 10-6 K-1 7,5 7,5 6,1 Ni: 29,0 Si: 0,2 C: 0,03 Co: 18,0 Mn: 0,2 Fe: 52,57 The surface temperature of the alumina was adjusted by a pyrometer controlled power application of the preheating laser. The preset for the emission value was 0.75. The surface as well as cross-sections of the welded seams were examined by optical and scanning electron microscopy (SEM). In the case of alumina specimen bonded to each other the strength of the joint was determined by a 4-point- bending test. Results & Discussion Welding of alumina with alumina In the case of alumina we found that 100 % crack-free bonded materials were generated with a preheating temperature of 1500 °C. Investigations of the cross sections of the welded seams showed minimum porosities for a preheating temperature of 1600°C. Higher preheating temperatures resulted in a higher porosity. To increase the efficiency of the process the time for preheating and cooling was minimised to a limit which still guarantees a crack free result. Our final results are: Up to 1400 °C a heating rate of about 30 Ks-1 can be applied. Above this temperature obviously a morphological rearrangement takes place and around 1500°C the critical temperature is reached and the highest stresses occur. So it was necessary to decelerate the increase of temperature to reduce these stresses until we arrived at equilibrium. Above these point we could raise the temperature again at a constant rate and the welding can be performed with the second laser beam (Nd: YAG). Altogether, the complete preheating process takes at least 100 seconds. After completion of the welding controlled cooling is recommended in the temperature range above 1500°C. Below that temperature no cracks occurred during further cooling in an environmental atmosphere. The maximum acceptable temperature gradients perpendicular to the seam were determined as 70 x 10³ Km-1. That means, at distances further than 20 mm off the weld the temperature is already below 500 °C. Therefore conventional metal clamps suffice as a holding device. Figure 2: Cross section of a cw laser welded Al2O3 seam (butt welding) 100 µm The laser welded specimens in figure 2 shows a very homogeneous structure. Average grain size is about five times that of the initial material. The packing is dense and approaches that of the base material. Furthermore it shows a concentration of pores at the former border liquid / solid. They arise from vaporisation of impurities, and/or from agglomeration of pores inherent in the initial material. Path energy is often considered as a process parameter. It is defined as the ratio of laser power over welding velocity. However, even at constant path energy, different solidification structures may be obtained. The heat gradient arises from the balance of heat flow in the material which is reached as a function of both, the rate of laser energy transfer into and the dissipation of it inside the material as well as the loss of heat due to thermal conductivity. On one hand, if the heat gradient is shallow, the solidification of the bath starts at its borders by the formation of columnar crystals oriented toward the centre. Impurities will concentrate in the regions that solidify at the lowest temperatures. Simultaneously a contraction occurs at both solidification fronts and leads to hot cracks which are similar to those observed in metal welding. On the other hand at too high laser powers and velocities heat dissipation takes too much time. This results in welding seams with big grains, up to 100 µm, in the centre. Impuri- ties will be mostly vaporised. The borders of the seam show columnar crystals up to a length of some 100 µm. Figure 3: SEM view of the surface morphology of an optimal welding seam At a well-balanced ratio between energy input and energy losses due to thermal conductivity solidification in a homogeneous and nearly isotropic manner (Figure 3) is possible. The cross-section of these joints is comparable to those shown in figure 2. The crystal growth is limited to the threefold of the original crystal size. These joints are also gas-tight. The quality shown is the optimum that could be achieved experimentally. The strength of the bonded specimens was determined by a 4- point- bending method. Two pieces measuring 30 x 7 x 0.8 mm³ were welded together in the cw mode. The resulting strength (σ) of the weld was compared with the virgin material (see table 2). 1 F/N m σ/ MPa σrel/% cwwelded 34 11 183 85 Virgin Alumina 51 3,9 191 100 Table 2: Strength of the welded alumina Fracture probability . 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0 0 cw 10 Referenz 20 30 40 50 60 70 Fracture load, N Figure 4: Fracture probability vs. fracture load The investigation of more than 20 specimens allows a statistical evaluation. For an industrial application the relation between fracture probability and fracture load is of special importance (figure 4). The slope of the curves is a measure for the predictability of a material failure. The cw welded alumina shows a reproducible smaller scatter of fracture load compared to virgin alumina. It is probable that the concentration of pores at the transition from the seam to the base material weakens the material and induces a local failure point corresponding with the higher inclination of the curve. Welding of alumina with metals In this part of the report we can present the first results and parameters which enable to weld these materials of absolutely different composition. Within a window of parameters: a welding power of 100…140 W and a welding velocity of 0,45…0,65 mms-1, the materials get welded completely in a relatively tight manner. Because of nearly the same thermal expansion coefficients, no cracks occurred during the cooling process. Nevertheless, as a result of insufficient shielding gas atmosphere, the niobium showed superficial changes. These are due to oxidation of the highly reactive metal. The development of a black, compact layer, sometimes superimposed of a loose whitely yellowish layer, was already reported by other authors and analysed as Nb2O5 and NbO (14). At lower preheating temperatures between 1250…1500°C (at the beginning of the investigations due to the cooling effect of the shielding gas) big crystals of a size up to 1,5 mm could be obtained within the black compact layer (figure 5). scin-deep oxidised niobium niobium crystals welding seam welding seam alumina alumina 1 mm a) b) Figure 5: formation of crystals on the surface of niobium after the welding process In figure 5 a) and b) welding joints between Al2O3 and niobium with different surface oxidation are presented. The SEM view (figure 6) of a cross section through the joint zone yields the following information: As expected from the good mutual wetabilities (15), a joint resulted. At the peak temperatures of welding, considerable masses from both specimens had merged into at least one liquid phase forming the volume of the seam. During solidification the system had passed through several phase transitions. Concluding from the number and arrangement of the solid phases a composition gradient across the seam persisted throughout the whole process. Presently we are investigating the compositions and congruency of the various precipitates, taking into account the incomplete equilibration of the system during the process. Reactions between the two metals niobium and aluminium , between their oxides or between each metal with each of the oxides have been reported (14,16,17). Between pure niobium (bright area in Fig. 6, left side) and pure alumina (dark area in Fig. 6, right side) two new regions, separated by a sharp border line, were generated. In the region adjacent to the Niobium side of the border line (left) black round spots were identified as alumina balls incorporated in particles of nearly pure Niobium. The contrast between the grey areas and pure niobium suggests a higher portion of heavier weight elements than detected. In the region on the alumina side (right), niobium could be detected in the interstices between the alumina crystals which had acquired the threefold of the initial crystal size. For this area the notation “cermet” is appropriate, as it denotes a compound consisting of at least one ceramic component along with at least one metal component. Figure 6: SEM view of the cross section of the welding zone between niobium (light) and Al2O3 (dark) Figure 7 displays the hardness of the joint which is considered a measure of its quality: The greyscale of the background of the figure corresponds to different regions. Within the pure niobium region (white) we determined a hardness of about Hv= 250 HV(0,1) corresponding to that of the initial material. Within the light grey region it arises to about Hv= 950. This points to an embrittlement or a measurement on the fine distributed alumina areas. The hardness becomes drastically higher within the middle grey region with a maximum of Hv= 2332. At a distance of 3 mm from the seam the hardness approaches that of the initial alumina. vickers hardness, HV . 2500 greyscales Background Elements white light grey middle grey dark grey Nb Nb, Al, O Al, O, Nb O, Al 2000 1500 1000 500 0 -4 -3 -2 -1 0 1 2 3 4 distance, mm Figure 7: Hardness HV (0.1) along the transition from alumina to niobium across the joint 5 mm Nevertheless, these first results encourage to further investigations of laser beam welding of alumina to metals. Therefore, to remove from the very expensive metal niobium, our future work will be related to the welding of alumina to steel. Our first results allow optimism (figure 8); they will be discussed and presented later. Figure 8: Photo of a welding seam between alumina and steel 1.3981 (perm alloy) Summary For the first time extensive investigations are reported on the bonding of alumina to itself and to metals, by laser radiation, with our two-laser-beam method (Nd: YAG and a CO2- laser beam). The preheating of the material, necessary to achieve crack free joints, was performed by the CO2- laser beam. This technology is very well suited for laser welding processes. Particularly when compared with the alternative method of preheating in a furnace, the advantages are: high processing speed high flexibility, processing under conditions of normal atmosphere temperature fields can be generated and varied very quickly controlled temperature gradients instead of overall heating save energy the assemblies can be clamped by conventional devices direct observation of the process is possible. On the other hand, in the case of alumina-alumina joints, material thickness is presently limited to about 2-4 mm because of its low thermal conductivity. It could be established, that the wavelength of the Nd: YAG laser beam as well as the continuous mode are prerequisite for a homogeneous solidification structure of the welded seam. The reasons are the absorption behaviour of alumina and the energy profile typical for a fibre guided laser beam. The high quality of the joints between alumina is confirmed by a bending strength of 85% of that of the base material. The above results give rise to expectations that the properties of the joints at high temperatures and/or in a corrosive atmosphere are close to those of the base material. Furthermore, the first experiments to join alumina with metals, turned out very promising, and will be investigated and reported further. The authors thank the Saxon Ministry of Science and Art SMWK (Contract No.: 4-7542.5/) for the financial support of a national innovation scholarship. References /1/ /2/ /3/ /4/ /5/ /6/ /7/ /8/ /9/ /10/ /11/ /12/ /13/ /14/ /15/ /16/ /17/ Lugscheider, E.; Boretius, M.; Tillmann, W., Aktivlöten- Eine Verbindungstechnologie für oxidische und nichtoxidische Ingenieurkeramiken, Schweißen und Schneiden 43, Heft 7, 1991, S. 390- 394 Fernie, J.A., Ceramic joining, Proc. of the Ceramic Joining Symp. USA (1996) pp. 414 Cordes, R.; Suthoff, B., Reibschweißen mit fast allen Werkstoffen; Konferenz- Einzelbericht: DVS- Berichte, Band 162 (1994) S.: 194- 197 Nicholas, M.G., Joining of Ceramics, Chapman and Hall, London, New York, Tikyo, Melbourne, Madras (1990) Rashid, H., Hunt, K.N., Evans, J.R.G., Joining Ceramics before firing by ultrasonic welding, J. Europ. Ceram. Soc., 8, 1991, pp. 329-338 McDermid, J. R., Drew, R.A.L., Thermodynamic brazing alloy design for joining silicon carbide, J. Am. 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