SURFACE ALLOYING OF ALUMINUM WITH COPPER USING CO 2 LASER

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1 5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT Guwahati, Assam, India SURFACE ALLOYING OF ALUMINUM WITH COPPER USING CO 2 LASER Woldetinsay G.Jiru 1 *, Mamilla R. Sankar 2 and Uday S. Dixit 3 Department of Mechanical Engineering, Indian Institute of Technology Guwahati s: 1* woldetinsay@iitg.ac.in, 2 evmrs@iitg.ac.in, 3 uday@iitg.ac.in Abstract Aluminium and its alloys have high demand in industries and service applications due to their light weight compared to their strength when alloyed with different metals like Cu, Mg, Ni, Cr, Zn and others for enhanced longer service life. In this work, commercially available 99% pure aluminium was alloyed with copper powder of 10µm particles size, which was melted by CO 2 laser. Three different methods were used for uniform placing of 95% copper powder and 5% aluminium powder on aluminium substrate. The result was examined by Vickers hardness test. SEM and FESEM were used for studying surface and subsurface defects. Defect free aluminium alloy with improved microstructure and enhanced mechanical property was achieved. Keywords:Surface alloying, CO 2 laser, Copper powder, Aluminium 1 Introduction Nowadays materials with special surface properties such as high corrosion, high wear resistance and enhanced hardness are highly demanded for advanced applications. Producing such alloys is not easy and their demand drastically increases the cost of the parts. Failure or degradation of engineering components due to mechanical, chemical or electrochemical interaction with the surrounding environment is most likely to initiate at the surface due to the high intensity of external stress and environmental attack at the surface. Surface engineering is a method of altering the properties of the surface in order to reduce the degradation over time. Laser surface alloying is one method to modify the surface properties of a material without affecting its bulk property [Elijahet al. (2009)]. Generally, laser surface modification is used to change either the surface composition or microstructure of a material to give it certain desired properties. The benefits of rapid solidification by laser power for surface modification are as follows: Fine grain structure due to fast solidification, Reduced micro segregation, Extended solubility of alloying elements. The intimate contact between the melt and the solid substrate causes a very fast heat extraction during solidification resulting in very high cooling rates of the order of 10 5 to 10 8 K/s. The high cooling rates to which this surface layer is subjected result in the formation of different microstructures from bulk metal leading to improved surface properties [Dubourget.al. (2002)].Figure 1 shows the schematic diagram of alloying used for this experiment. Figure 1 Schematic of laser alloying In view of the importance of aluminum for automobile, aerospace and many other sectors, this paper investigates the surface alloying of aluminum by copper. Three methods of alloying have been used. The mechanical properties and microstructures of surface alloyed samples have been compared. 2 Materials and Methods In the present work, commercially pure aluminum of 99% purity was alloyed with copper powder of 10 µm particle size and melting was done by continuous CO 2 laser power. During alloying 50% overlap is provided between consequent beads. After cooling, re-melting was carried out at orthogonal direction to alloyed layers. Sample dimension was 110 mm x 50 mm x 6 mm. Table1 shows substrate material composition tested by EDX. Table 1 Chemical composition of substrate material Element Cu Si Fe Al Composition (wt %) Balance 634-1

2 Three methods used for depositing the copper powder are as follows: a. Stirring of metal powder with Fevicol and then painting it on substrate to achieve 0.5 mm coating for samples 12, 13, 14, 15. b. First painting the substrate by Fevicol, then applying powder uniformly and finally compacting it to achieve total coating thickness of 1 mm for samples 4, 5, 6, and 9. c. First painting the substrate by Fevicol, then applying powder and blowing out excess powder to achieve total coating thickness of 0.5 mm for samples 1, 2, 3, 7, 8, 10 and 11.Table 2 shows the process parameters in different experiments. 3 Results and Discussion 3.1 Crystal size and lattice strain The crystal size and lattice strain developed due to thermal effect was examined for Sample 4 by XRD and the result is shown in Figure 2.Strain was calculated from [Williamsonetal.(1953)] Kλ β cosθ = + 4η sinθ (1) D Where β is the full width at half maximum, K is a constant, λ is the X-ray wave length of Cuk α radiation (λ = Å), D is average crystal size and η is lattice strain. The strain in the sample was as observed in inset of Figure 2 and average size of crystallite is nm. The crystal is cubic in structure as justified from ICDD#: , confirming the formation of Al 4 Cu 9 phase. Figure 2 X- ray diffraction pattern of Al 4 Cu 9 S. No. Laser power (kw) Table 2 Process parameters Process parameters for alloying Process parameters forre-melting Re- Stand off Scan speed Laser Laser Stand Scan melting distance (V) spot Power off Speed (SD)(mm) (mm/min) diameter (kw) distance (V)(mm/ Laser spot diameter (mm) (mm) (mm) min) S Yes S S S S No S Yes S No S Yes S S S S No S Yes S S

3 (a) µm µm Figure 3 Surface topography and corresponding line surface roughness of alloyed samples: (a) non uniform coated surface uniform coated surface Figure 4 Vickers hardness variation towards the alloyed area on the specimen 634-3

4 3.2. Surface Roughness The surfaces of the alloyed layers were tested without polishing the surface. Taylor Hobson make CCI light, non-contact optical profilometer was used to check surface quality of alloyed samples. Figure 3 (a) shows Sample 2, where area of non-uniform coating resulted in void surface with larger peaks and valleys and Fig. 3 is Sample 9, with uniform deposited and alloyed surface with average roughness value (R a ) up to 1.9µm. 3.3 Micro hardness Analysis Micro-hardness test was conducted on thickness side of the samples by Vickers hardness test using 500gf and the results are plotted as Fig. 4. Maximum hardness achieved is 156HV at laser power of 1.7 kw laser beam diameter 5.39 mm and scanning speed of 500 mm/min. The re-melting was done in orthogonal direction to the alloyed direction at laser power of 1.6 kw and scanning speed of 800 mm/min. The re-melting was done after alloying improved hardness Pinto et al.[2003]. Samples 4 and 9 with 1 mm deposition thickness of Cu powder at resulted in improved hardness compared to other samples. 3.4 Microstructure and Morphology Analysis A pure metal normally solidifies at a constant temperature with the inter face between the solid and liquid media being planar for an alloy. Figure 5 shows morphology in solidified regions at different magnification analysed by SEM. It shows Cu growth in Al 4 Cu 9 inter metallic phase with a fine microstructure region [Porter and Easterling (1992)]. After initial cellular growth from the bottom of the molten pool, the (a) solidifying area developed lateral instability favourable for forming homogeneous structure. Figure 6 (a) shows uniformity of Cu growth into white Al primary grains and dark eutectic well distributed intermetallical 4 Cu 9 phase. Figure6 shows the grain boundary cracking that contain appreciable amounts of copper, when the cooling rate is insufficient to prevent precipitation on grain boundaries at higher temperature and hence is susceptible to stress corrosion cracking (SCC) and micro-crack initiation [Otterlooet al. (1995)]. Atoms of Cu lower their free energy when they migrate to lattice defects such as grain boundaries, phase boundaries and dislocation. In Figure 7 (a) marked region A is un-melted region. A regular growth of binary Al 4 Cu 9 eutectic morphology as a result of uniform and fast cooling time resulted in fine microstructure as shown by arrow in Figure 7 at higher magnification. Segregation to grain boundaries affects the mobility of the boundary and has pronounced effects on re-crystallization, texture and grain growth. Figure 8 shows the exsistance of gas pores or pockets within the alloyed area for samples first painted by adhessive and then copper powder deposition. The voids are formed due to gas entrapment during solidification. It is likely to occure when the rate of solidification is fast and there is no adequate time for the evolved gas to escape. Figure 5 Scanning electron microscopic morphology on Cu alloyed Al: (a) Sample 4 Sample

5 Figure 6 SEM result: (a) Sample 9 (P =1.8 kw, (a) SD = 30 mm, V = 400 mm/min and re-melting P =1.6 kw, SD=40mm and V = 800 mm/min). Sample 13 (P = 1.8 kw, SD = 40 mm, V= 600 mm/min, re-melting = 1.5 kw, SD = 40 mm, V = 800 mm/min). A Figure 7Laser parameters used for Sample 5: P = 1.7 kw, SD = 30 mm, V = 400 mm, no re-melting Porosity in the alloying Figure 8 SEM Morphology for Sample 11 showing porosity in alloyed area (P =1.8 kw, SD = 30 mm,v = 500 mm/min Remelting,P = 1.5kW SD = 40 mm,v = 400 mm/min) 634-5

6 (a) Figure 9 FESEM fractography test from powder of alloyed area: (a) sample 4 sample 9 Powder of alloyed region was tested by FESEM to investigate crack and defect at subsurface of alloyed regions. No defects were observed as a result of fine microstructure of Cu doped in aluminium forming Al 4 Cu 9 phase matrix with uniform and fine microstructure shown in Figure 9 for Samples 4 and 9. The alloyed region is free of defects. 4 Conclusions Surface alloying of commercially pure aluminium by 10µm copper powder resulted these findings; Uniformly deposited Cu powder by compacting resulted in uniformly surface alloyed layer with average surface roughness of 1.9 µm. At laser power of 1.7kw and scanning speed of 500mm/min alloying and re-melting done at lower power of 1.6kw orthogonal direction to alloyed layer resulted in improved micro hardness from 60HVto 156 HV. Formation of fine microstructure with Al 4 Cu 9 intermetallic phase shown figure 5(a) and, figure 6 (a) developed due to fast solidification and uniform deposition (allying) of cu powder on substrate material. First painting the substrate material and applying cu powder at higher laser power of 1.8kw resulted grain bounder cracking of alloyed regions which is favorable for initiation of stress corrosion cracking figure 8. Surface allying pure aluminum with 10µm cu powder resulted in defect free at subsurface regions of alloyed layers as investigated by FESEM figure 9 (a) and. Acknowledgments The authors acknowledge for the facilities procured under Department of Science and Technology FIST project number: SR/FST/ETI-244/2008.The authors also acknowledge for getting access to XRD Xpert facility procured under Ministry of Steels sponsored project number ME/P/SKJ/04 (PI: Dr. S Kanagaraj). References Dubourg,L., Pelletier, H., Vaissiere, D., Hlawka, F., and Cornet, A., (2002), Mechanical characterisation of laser surface alloyed aluminium copper systems, Wear, Vol. 253, pp Elijah, K.A.J., (2009), Principles of laser materials processing, John Wiley and Sons, Hoboken, New Jersy. Otterloo, J.L.D.M., Bagnoli, D. and Hosson, J.T.M.D., (1995), Enhanced mechanical properties of laser treated Al-Cu alloys: A microstructural analysis, Acta Metallurgica et Materialia, Vol. 43, pp Pinto, M.A., Cheung, N., Ierardi, M.C.F. and Garcia, A. (2003), Microstructural and hardness investigation of an aluminum copper alloy processed by laser surface melting.materials Characterization, Vol. 50, pp Porter, D.A., and Easterling, K.E., (1992), Phase Transformations in Metals and Alloys, 3 rd Edition, Taylor and Francis, CRC press, London. Williamson, G.K., and Hall, W.H. (1953), X-ray line broadening from filed aluminium and wolfram, Acta Metallurgica, Vol. 1, pp