CREATION OF SURFACE GEOMETRIC STRUCTURES BY THERMAL MICRO-LINES PATTERNING TECHNIQUES

Soshu Kirihara, Satoko Tasaki

Joining and Welding Research Institute
Osaka University
11-1 Mihogaoka Ibaraki, Osaka 567-0047, Japan

Yusuke Itakura

Graduate School of Engineering
Osaka University
2-1 Yamadaoka Suita, Osaka 565-0871, Japan

ABSTRACT

Thermal micro lines patterning techniques were newly developed as novel technologies to create geometrical intermetallics patterns for mechanical properties modulations of metal substrates. Pure copper particles were dispersed into the photo solidified liquid resins, and these slurries were spread on aluminum substrates. Micro patterns with fractal structures of Hilbert curves and dendritic lines were drawn and fixed by an ultra violet laser scanning. The formed patterns on the substrates were heated in an argon atmosphere, and the intermetallic or alloy phases with high hardness were created through reaction diffusions. The mechanical properties and surface stress distributions were measured and simulated by a tensile stress test and finite element method.

INTRODUCTION

Fractal geometries with self-similarity can be applied to modulate various flows in engineering fields [1,2]. Geometric networks with the fractal structure of intermetallic compounds patterned on light metals can strengthen whole materials efficiently by controlling surface stress distributions intentionally. In our research group, three dimensional metal and ceramics lattices with dendritic structures have been created and inserted into various matrices successfully to control stress and heat distributions [3,4]. Considering the next generation, mechanical properties enhancements by novel surface treatments will be expected to contribute novel materials processing of rare metals free. In this investigation, micro patterns composed of copper aluminide had been created on pure aluminum substrates by using a laser scanning stereolithography and a reaction diffusion joining. Microstructures and composite distributions in the vicinity of formed alloy and metal interfaces were observed and analyzed by using an electron microscope. Load dispersion abilities of the network were evaluated by using conventional mechanical tests and compared with simulated and visualized profiles by using a numerical analysis simulation.

EXPERIMENTAL PROCEDURE

Self-similar patterns of Hilbert curve with stage numbers 1, 2, 3 and 4 of the fractal line structures were designed by using a computer graphic application, and these graphic images were converted into the numerical data sets by computer software. These patterns of 25×25 mm in whole size were composed of arranged lines of 400 μm in width. These graphic models were transferred into the processing apparatus as operating data sets. Metal particles were patterned on a metal substrate by using a stereolithographic system. The pure copper particles of 50 μm in average diameter dispersed into a photo sensitive urethane resin at 60 volume percent. The mixed resin paste was spread with 100 μm in layer thickness on an aluminum substrate of 30×30×2 mm in size by using a mechanically moved knife edge as shown in Fig. 1. An ultraviolet laser beam of 355 nm in wavelength and 100 μm in beam spot was scanned on the resin surface according to the computer operation. A two dimensional solid pattern was obtained by a light induced photo polymerization. Figure 2 shows the appearance of the stereolithographic system. Subsequently, the dendiritic fractal patterns with the self-similarity of stage number 3 were adopted as the geometrical patterns. The patterns of 20×80 mm in whole size were composed of arranged lines of 400 and 8000 μm in width and length. The mixed resin paste with the pure copper particles was patterned on the aluminum specimen of 20×80×2 mm in size of the parallel part by using the stereolithography. After removing uncured resin by ultrasonic cleaning in ethanol solvent, the sample was heated to dewax the resin and create a self-similar pattern composed of copper aluminides at 600 °C above eutectic temperature for 4 hs of holding time in an argon atmosphere. Microstructures and composite distributions were observed by a scanning electron microscopy (SEM) and an energy dispersive X-ray spectroscopy (EDS), respectively. Stress distribution in the patterned material during uniaxial tension tests was simulated by a finite element method (FEM) calculation.

Figure 1. Schematically illustrated system configurations of a laser scanning stereolithography.

Figure 2. An appearance photograph of the used laser scanning stereolithography equipment.

RESULTS AND DISCUSSION

The copper aluminide micro pattern with the fractal structure of Hilbert curve was formed successfully on the substrate. Figure 3 shows the formed fractal polyline of the number 3 in fractal stage. The micrometer order geometric structure was composed of fine intermetallics lines of 450 μm in width. The part accuracy of these microline patterns was estimated as 10 % approximately. The copper aluminide composite was formed widely comparing with the designed line width, though the reaction diffusion between the copper and aluminum. The EDS measurement results suggested that copper concentrated in the micro network and formed intermetallic phase of CuAl2. Microscopic defects were not found in the formed intermetallics layers though the SEM observations. The smooth interfaces were obtained between copper aluminide and the aluminum substrate. During the heat treatment at the high temperature, eutectic reaction with liquid phase formation occurred between the pure copper particles and the pure aluminum substrate. After the solidification of molten alloys, the dual phase microstructure of the intermetallics and alloys composites can exhibit the higher mechanical strength. The stress distributions on the patterned surfaces were visualized for the Hilbert curve of stage number 3 through the numerical simulation as shown in Fig. 4. The required mechanical properties of Young’s modulus were defined along the compositional analysis and the phase identifications. The stress intensities concentrate into the vicinity of fixed edge and are distributed along the patterned lines and the corners with the higher hardness.

Figure 3. A copper aluminide Hilbert curve patterned on an aluminum substrate.

Figure 4. A stress distribution on the fractal pattern of intermetallics visualized by using FEM.

According to the designed dendritic model as shown in Fig. 5-(a), the real fractal pattern composed of the pure cupper dispersed urethane resin were created clearly as shown in Fig. 5-(b) by using the laser scanning stereolithography. The formed line width was measured as 400 μm. The part accuracies of geometric patterns were verified within 5 %. Through the heat treatments, the copper aluminide networks with the self similar patterns were formed by using the reaction diffusion joining as shown in Fig 5-(c). The mechanical properties of the patterned sample were measured through the tensile test as shown in Fig. 6. A small fracture crevasse is formed in the central position of the specimen perpendicularly to the intermetallics line connecting with two dendritic patterns. The tensile stresses were considered to be focused effectively by the both dendritic patterns and concentrated into the center connecting line. Through the dynamically monitoring for this artificial fracture source, real time materials life estimations will be realized effectively. The stress distributions on the patterned surface were simulated and visualized for the dendritic lines of stage number 3 through the FEM calculation as shown in Fig. 7. In the calculation process, the tensile strengths were loaded for the both edge of the specimen model. The red and blue colored areas indicate the higher and lower intensities of the bending stress, respectively. The stress intensities are concentrated into the vicinity of the center region and distributed along the patterned lines. The crossing points of the intermetalics lines with the higher hardness show the stress concentrations. The simulated and visualized results have good agreements with the measured results as shown in Fig. 6. The self-similar patterns can include the more numbers of sides and nodes in the limited area comparing with the periodic arrangements of the geometric polygon figures. Therefore, the dendritic fractal line patterns are considered to be able to disperse the mechanical stresses intentionally on the surface areas of the substrates.

Figure 5. The dendrite patterns formed by the stereolithography and reaction diffusion.

Figure 6. A tensile specimen with the dendritic fractal pattern after mechanical test.

Figure 7. A simulated and visualized stress distribution on the patterned test specimen.

CONCLUSIONS

Geometric networks with fractal structures of Hilbert curves and dendritic lines were created to modulate mechanical properties intentionally through computer aided designing and manufacturing. Microlines composed of urethane resin with copper particles dispersion were patterned successfully on pure aluminum substrates by using a stereolithography. The patterned samples were heated in an argon atmosphere to create intermetallics lines of copper aluminides through reaction diffusion between the copper and aluminum. Cracks and pores were not observed in welded interfaces by...