Stability Test METHODOLOGY 1 Preparation of Nanofluids

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2.3 Thermal Properties Test

To recognize the thermal properties of the nanofluid, several tests need to be conducted such as thermal conductivity, viscosity and specific heat. The tests were conducted at three different temperatures at 6 ° C, 25 °C and 40 °C. In order to stabilize the samples temperature, the samples were placed inside the refrigerated water bath and this can minimize the effect of temperature variation. For thermal conductivity test, the KD2Pro thermal analyzer Decagon, USA was used to measure the nanofluid thermal conductivity. It was set on automatic mode and left on the flat table to avoid any movement or table vibration during the measurement process. Any movement of the sample would create the convection. For the viscosity test, the Brookfield rotational viscometer was used. Using multiple speed transmission and interchangeable spindles, a variety of viscosity ranges were measured. The viscosity of CNT-water nanofluids as a function of shear rate was measured. Besides that, nanofluids provide better fluid performance due to the higher shear rate at the wall which results in low viscosity there. Meanwhile, for specific heat test, it was conducted by employing the IKA C200 Bomb Calorimeter. The specific heat capacity determines the convective flow nature of the nanofluid and it necessarily depends on the volume fraction of the nanoparticles.

2.4 Heat Transfer Performance Test

Enhancement of heat transfer coefficient is much higher than the increase in the effective thermal conductivity [7]. They associated the possible reasons with the improved thermal conductivity, shear induced enhancement in flow, reduced boundary layer, particle re-arrangement and high aspect ratio of CNTs.

3. RESULTS AND DISCUSSION

Experiments were conducted to measure rheological properties of nanofluids in the particle volume concentration range of 0.2 - 1.0 and the temperature range of 6 °C to 40 °C. Based on Table 1 and Table 3, the thermal conductivity of nanofluids increase when temperature and volume concentration increase same like specific heat. But for viscosity based on Table 2, it will decrease when temperature is high. Table 1 Thermal conductivity for MWCNT nanofluids with different volume concentration and temperature Volume concentration of nanofluids wt Thermal conductivity reading at different temperature Wm.K 6 °C 25 °C 40 °C 0.2 0.738 0.750 0.758 0.4 0.744 0.752 0.760 0.6 0.748 0.755 0.762 0.8 0.751 0.760 0.766 1.0 0.753 0.763 0.770

4. CONCLUSIONS

From the results it was found that the thermal conductivity was increased with an increase of particle concentration and temperature increment. The increase in thermal conductivity enhancement shows 23.5 for the volume concentration of 1.0. Meanwhile, for the viscosity, it becomes much more dependent on the volume fraction of nanoparticles and base fluids also with the temperature. At low nanoparticle concentration, the viscosity of the base fluids are dominated and the viscosity increased when the temperature is decreased. This conditions vice versa when the concentration of nanoparticle and temperature is high. It was found that the specific heat of nanofluids decreased gradually with the increment in nanoparticle concentration however an increasing trend was found at elevated temperatures. Table 2 Viscosity for MWCNT nanofluids with different volume concentration and temperature Volume concentration of nanofluids wt Viscosity reading at different temperature Pa.s 6 °C 25 °C 40 °C 0.2 0.91 0.75 0.44 0.4 0.92 0.77 0.45 0.6 0.95 0.79 0.47 0.8 0.97 0.81 0.48 1.0 0.99 0.82 0.49 Table 3 Specific heat for MWCNT nanofluids with different volume concentration and temperature Volume concentration of nanofluids wt Specific heat reading at different temperature Jkg.K 6 °C 25 °C 40 °C 0.2 0.398 0.410 0.427 0.4 0.390 0.405 0.420 0.6 0.385 0.402 0.416 0.8 0.381 0.400 0.414 1.0 0.378 0.399 0.410 5. REFERENCES [1] L.S. Sundar and K.V. Sharma, “Thermal conductivity enhancement of nanoparticles in distilled water,” International Journal of Nanoparticles, vol. 1, no. 1, pp. 66–75, 2008. [2] Y. Ding, H. Alias, D. Wen R.A. Williams, “Heat transfer of aqueous carbon nanotubes,” International Journal of Heat and Mass Transfer, vol. 49, pp. 240, 2005. [3] D.P. Kulkarni, R.S. Vajjha, D.K. Das and D. Oliva, “Application of aluminium oxide nanofluids in diesel electric generator as jacket water coolant,” Applied Thermal Engineering, vol. 28, pp. 1774-1781, 2008. [4] X.Q. Wang and A.S. Mujumdar, “Heat transfer characteristics of nanofluids: A review,” International Journal of Thermal Sciences, vol. 46, no. 1, pp. 1-19, 2007. [5] P.C. Mishra, S.K. Nayak and S. Mukherjee, “Thermal conductivity of nanofluids,” International Journal of Engineering Research and Technology, vol. 2, no. 9, pp. 734-745, 2013. [6] A. Ghadimi, R. Saidur and H.S.C. Metselaar, “A review of nanofluid stability properties and characterization in stationary conditions,” International Journal of Heat and Mass Transfer, vol. 54, no. 17, pp. 4051-4068, 2011. [7] S.K. Das, S.U.S. Choi, W. Yu and T. Pradeep, Nanofluids Science and Technology. New Jersey: John Wiley Sons Inc.; 2007. __________ © Centre for Advanced Research on Energy Effect of process variables on the tensile shear strength of spot welds in 6061-T6 aluminum alloy S. Thiru 1, , Siti Hajar Ahmad Razin 2 , S. Hema 3 , I.S. Mohamad 2 1 Department of Mechanical Engineering, King Abdul Aziz University - North Campus Jeddah 21589, Kingdom of Saudi Arabia. 2 Faculty of Mechanical Engineering, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, 76100 Durian Tunggal, Melaka, Malaysia. 3 School of Engineering, Linton University College, 71700, Mantin, Negeri Sembilan, Malaysia Corresponding e-mail: thirukau.edu.sa Keywords: Resistance spot welds; weld strength; factorial design ABSTRACT – The changes in mechanical and metallurgical behavior of spot welded region generally occur throughout the spot welding process and these changes are very significant for the safety and quality of the welded joints. Due to the influence of current flow, the squeezing time and the load applied on electrodes have a vital effect on the mechanical properties. This paper presents the effect of such parameters on the tensile shear strength of single lap spot welded 6061-T6 aluminium alloy. Two level four factor fractional factorial design was employed to develop mathematical models. It was found that change in weld thickness and current are the primary parameters that control the tensile shear behavior of the spot welds.

1. INTRODUCTION

Employing light materials such as aluminium and magnesium alloys as an alternative for steel is the need of the hour for the ‘all-time’ competitive automotive industry to survive, in terms of the growing energy and environmental regulations of the modern world. Aluminum alloy 6061-T6 is a widely used structural material for automobile, air craft and maritime industries since it is highly weldable and has favorable mechanical properties [1-3]. Since the resistance spot welding RSW is the most widely used process for the fabrication of automobile case parts due to its simplicity, low-cost, high production rate and automation accessibility [4], it is critical to understand the behavior of the welding process control parameters on the mechanical properties of the aluminum 6061-T6 alloy. 2. METHODOLOGY 2.1 Development of Design Matrix After a series of trial runs, the experimental design matrix was developed in accordance with the design of experiments DOE methodology, by adopting a two level, four factor fractional factorial design 2 4-1 =8 of eight runs. For a factorial design of experiment, it is crucial to select the “individually controllable” process control parameters which are generally selected either from the published literature or the trial runs. For the present study the electrode force F, weld current I, weld time t and work piece thickness w were selected as the four main parameters that influence the tensile shear strength, the response parameter to be evaluated. All the direct and indirect parameters involved in the RSW process were kept constant. The upper limit highest level and the lower limit lowest level of a factor were coded as +1 and -1 or simply + and - respectively according to the equation 1. j jo jn j J X X X   1 Where, X j is the coded value of the factor; X jn is the natural value of the factor; X jo is the natural value of the basic level; J j is the variation interval and j is the number of the factors. The selected process control parameters, their units, symbols and the limits are given in Table 1. Table 1 Selected process control parameters and limits Parameter Unit Symbol Limits High Low +1 -1 Force kN F 1.8 1.2 Current kA I 26.6 23.8 Weld Time cycle t 8 3 Thickness mm w 1 0.71

2.2 Experimentation

In the present work, a pedestal spot welder, Lecco -Italia NKLTP 28 was used. Prior to the experiments, all the critical process control parameters were calibrated. Table 2 Design matrix for calculating the coefficients

S. No

β 1 β F 2 β I 3 β t 4 β w s T s T s T 1 + + + + + 30.63 34.79 32.71 2 + - + + - 76.29 78.35 74.24 3 + + - + - 51.79 52.52 53.26 4 + - - + + 42.03 43.85 40.21 5 + + + - - 76.29 84.65 67.93 6 + - + - + 51.41 55.00 47.81 7 + + - - + 28.96 34.27 31.61 8 + - - - - 61.25 55.60 66.90 For welding current - digital multimeter; for electrode force- SP-231N Spotron hydraulic weld force gauge and for welding time – Casio digital stop watch were employed. The experiments were performed as per