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Structural life enhancement on friction stir welded AA6061 with optimized process and HFMI/PIT parameters
Article in The International Journal of Advanced Manufacturing Technology · June 2017 DOI: 10.1007/s00170-016-9697-7
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The user has requested enhancement of the downloaded file. Structural life enhancement on friction stir welded AA6061 with optimized process and HFMI/PIT parameters
Yupiter H. P. Manurung, Mohamed Ackiel Mohamed & Azrriq Zainul Abidin
The International Journal of Advanced Manufacturing Technology
ISSN 0268-3768
Int J Adv Manuf Technol DOI 10.1007/s00170-016-9697-7
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Int J Adv Manuf Technol DOI 10.1007/s00170-016-9697-7
ORIGINAL ARTICLE
Structural life enhancement on friction stir welded AA6061 with optimized process and HFMI/PIT parameters
Yupiter H. P. Manurung1 & Mohamed Ackiel Mohamed2 & Azrriq Zainul Abidin3
Received: 5 September 2016 /Accepted: 3 November 2016 # Springer-Verlag London 2016
Abstract This novel study presents an unconventional ap- 65, 25 and 10% for air pressure, impact frequency and indent- proach to find the best governing process parameters of high er diameter, respectively. Secondly, using subsequent post- frequency mechanical impact technique based on multi- weld mechanical treatment, the life cycle number can be ex- objective optimization method. In this investigation, the tended up to 12 times on friction-stirred weld. Finally, based post-weld mechanical treatment is aimed to enhance fatigue on the experimental confirmation test, the proposed method resistance of structural friction-stirred weld subjected to fluc- can effectively estimate the structural life and surface hardness tuating loads by obtaining nominal sub-surface hardness. The within the acceptable range of relative error. experimental study was conducted on aluminium alloy AA 6061 with thickness of 6 mm under varied parameters centred Keywords Fatigue life . Friction stir welding (FSW) . High on indenter diameter, air pressure and impact frequency. The frequency mechanical impact (HFMI) . Pneumatic impact investigation began with obtaining optimum parameters for treatment (PIT) . Optimization single response by using conventional Taguchi method with L9 orthogonal array. Next, advanced optimization approach by means of multi-objective Taguchi method attempts to con- 1 Introduction sider the multiple responses simultaneously which are sub- surface hardness and structural life. As the final results, the The friction stir welding (FSW) process is witnessing a optimum value was acquired by calculating the total normal- growth in a wide range of industrial applications due to the ized quality loss and multiple signal-to-noise ratios based on minimal governing parameters and many other advantages as unequal desirability. The significant level of the parameters a solid state welding compared to the commonly used fusion was evaluated by using analysis of variance. Furthermore, welding process. However, tensile residual stress remains to the second-order model for predicting the objectives was de- be significant concern due to its extensive clamping and stir- rived by applying response surface methodology. It can be ring process which can lead to lower fatigue resistance partic- summarized that, first, the affecting parameters to obtain su- ularly in structures subjected to fluctuating loads triggering a perior structural life can be ordered at significant level of ca. need for improvement [1–3]. The enhancement of the fatigue resistance of welded joints is becoming increasingly significant in many areas such as the * Yupiter H. P. Manurung railway, aerospace and automotive industries. A recent meth- [email protected] od of enhancing the fatigue resistance of welded aluminium alloy (AA) structures is to use modern post-weld treatment 1 Faculty of Mechanical Engineering, Universiti Teknologi MARA processes. Improving the fatigue resistance of welded joints (UiTM), Shah Alam, Malaysia by conventional improvement methods such as grinding, shot 2 Universiti Kuala Lumpur Malaysia France Insitute (UNIKLMFI), peening, air hammer peening or tungsten inert gas (TIG) Kuala Lumpur, Malaysia dressing are well established. However, these techniques are 3 Technogerma Engineering & Consultancy, Mont Kiara, Kuala inconsistent, not always efficient, have limited application Lumpur, Malaysia areas and can cause other related problems. The relatively Author's personal copy
Int J Adv Manuf Technol new technique of high frequency mechanical impact (HFMI) most possible minimum value of process performance at the peening of weld toes as well as heat affected zones offers a optimum point of process parameters. Numerous researches favourable alternative for weld improvement. HFMI is termed involving the optimization of process parameters for FSW as a method in which a hardened steel pin impact on the surface well as other welding processes had been carried out previ- of the metal to be treated at a required frequency and pressure ously to obtain the optimal point of governing parameters by magnitude in accordance to specifications. employing the multi-objective Taguchi method (MTM) and There are a quite number of research conducted using con- response surface methodology (RSM). A mathematical model ventional and HFMI methods in the past decades. Rodopoulos was successfully developed for quality features of resistance et al. [3] investigated through experimental study the effects of spot welding [10]. A hybrid TM using the Taguchi quality loss ultrasonic impact treatment (UIT) on the fatigue resistance of function and RSM was employed for the multi-response opti- friction stir welded AA panels. The effects of laser and shot mization of a laser beam cutting process [11]. TM has been peening on the mechanical properties with iso-stress assump- successfully applied to determine the optimal FSW process tion were studied by O. Hatemleh [4] to calculate local stress– parameter combination that would maximize the tensile strain curves of friction stir welded 2195 AA joints. A signif- strength, notch tensile strength and the weld sub-surface hard- icant improvement in the fatigue resistance of friction stir ness of the AA6061 joints by Periyasamy et al. [12]. TM was welded AA7075 by applying ultrasonic peening (UP) was also effectively used to optimize the process parameters of reported out by Qiulin et al. [5] using a self-made device with FSWAA6061 in an attempt to minimize the heat affected zone a stress ratio of R = 0.5. The strengthened layer caused by the (HAZ) distance to the weld line [13]. The prediction of the plastic variation, surface hardening and consistency of tissue, optimum tensile strength by varying process parameters for as well as compressive transversal residual stress induced by joining of a butt joint dissimilar Al–Cu alloy AA2219 and UP were found to be the main reasons for the increased life AA5083 plates using TM technique was investigated by cycle. Microstructural and fatigue properties of FSW made of Koilraj et al. [14]. AA2043 with controlled shot peening was examined by Ali Based on past researches, it is obvious that fatigue strength et al. [6] and stated that the compressive residual stress intro- of material can be improved significantly with the help of duced by the peening process attributed to an increment in the various methods including HFMI and process parameter op- low cycle region. In an attempt to restore the degraded fatigue timization is an important criterion prior to the application of performance due to FSW, laser peening without coating welding and cutting process. However, no attempt has been (LPwC) was applied to FSW AA6061 joints by Sano et al. made yet to employ multi-objective parameter optimization [7] and obtained an increment of 30 MPa from an as-welded for FSW with subsequent post-weld treatment especially value of 90 MPa. It was pointed that a higher fatigue perfor- using the recently innovated pneumatic impact treatment mance can be expected if the processing parameters in LPwC (PIT) which also falls under the generic term of HFMI method were optimized. The effect of HFMI on butt welds, T-joints as mentioned in [15 ]. As first hypothesis in achieving opti- and longitudinal attachments for mild construction steel mum performance of HFMI/PIT is the dependency on the (S355) up to ultra-high strength steel (S960) on fatigue was operating parameters as well as on the possible correlation accomplished by Leitner et al. [8]. Pagel et al. [9] studied the of objectives which can be analysed by using analysis of var- S/N curves for gas metal arc welded butt welds, T-joints and iance (ANOVA). Moreover, this research attempts to obtain overlap connections of a precipitation hardened AA EN-AW the optimized HFMI/PIT parameters for friction-stirred weld 6082—T6 with several types HFMI treatments and concluded of AA6061 using MTM to achieve the highest possible struc- that the fatigue life improvement depends on the shape of the tural life with nominal sub-surface hardness values. weld, the applicability of the methods under service condi- Furthermore, mathematical equations for calculating fatigue tions and on the loading conditions, if an improvement of life and sub-surface hardness are developed based on the weld seam geometry or a mechanical surface treatment second-order regression model using response surface model will give the better results. (RSM). The commonly used theories on ANOVA, MTM and A broad development in the usage of the design of exper- RSM can be referred to [16, 17]. iment (DoE) in diverse applications has been noted recently due to its capability of outlining the optimal settings of any process by determining the governing parameters associated 2 Experimental setup and details: FSW, HFMI/PIT, to the process to further improve the performance and capa- surface hardness and fatigue test bility. A well-established example among the many statistical techniques used to reduce the number of experiments required 2.1 FSW on AA6061 using conventional milling machine is the Taguchi method (TM) which enables safe identification of statistically essential parameters. Optimization in common The HFMI/PIT-treated friction stir welded AA6061 was in- is known as a process that permits the approximation of the vestigated in this research. Each butt welds were performed on Author's personal copy
Int J Adv Manuf Technol
Table 1 Friction stir welding tool dimensions and parameters [19] Cylindrical tool dimension Process parameter
Shoulder diameter ∅ Pin diameter ∅ Pin length Rotational speed Transverse speed (mm) (mm) (mm) (rpm) (mm/s)
18 6 5.5 950 4.55 rolled plates with 6 mm of thickness perpendicular to the the fatigue resistance of welded joints. The mechanical pulses rolling direction with square weld preparation. The chemical produced by a unique fluidic muscle are conveyed to the sur- composition of the workpiece contains 0.44% Si, 0.22% Fe, face to be treated through hardened pins, which are adapted to 0.034% Cu, 1.03% Mn, 0.054% Mg, 0.007% Cr, 0.44% Ni the geometry of the respective application. In this pneumatic and Al. drive system, air filled into the fluid muscle will cause its Plates of 250 mm of length and 100 mm of width were cut diameter to increase while contracting the length. This allows out using a milling machine and welded along their long edge. a flowing, elastic movement that can be very precisely con- The friction stir welding was done according to the tool di- trolled with regards to kinematics, speed and force. mension and parameters as shown in Table. 1. After welding, The hand-held and programmable logic controller (PLC) specimens were produced by milling for fatigue tests in accor- control unit used in the HFMI/PITsystem is depicted in Fig. 2. dance to the specifications in ISO/TR 14345:2012(E) [18]. In the HFMI/PIT technology, both the frequency and the force The FSW was accomplished on the vertical head milling of impact can be regulated independently of one another. This machine with the position of the tool fixed relative to the enables to meet the varying parameters for different materials; surface of the plate. The workpiece was firmly clamped to hence, each type of material should be treated with the suitable the bed and a cylindrical tool was plunged into the selected process parameters accordingly to achieve the best possible area of the material for sufficient time in order to plasticize results. around the pin as shown in Fig. 1. The vibrations of the HFMI/PIT process is kept as slight as possible for the operator, with a system that works against a 2.2 HFMI/PIT: system technology, function, preparation further springing mechanism, completely uncoupling the and operation hand-held unit from the force of impact. The springing mech- anism also ensures that the system’s applied intensity is al- The HFMI/PIT technology is a high frequency impact ways similar with good reproducibility although used by mul- peening process that has been developed mainly to improve tiple operators. The impact frequency can be set at the control unit to four (4) stages within the range of 80–120 Hz. The parallel regu- lation of the air pressure within the range of 4–6barforthe selected frequency allows the force of impact to be infinitely adjusted. A separate control unit with PLC controls permits entry of the treatment parameters for the various materials and different types of weld joints at a touch screen. This makes it
Compressed Control Air Inlet Box HFMI/PIT Handheld Unit
Indenter
Indenter Indenter Holder Assortment
Fig. 1 Setup of FSW using conventional horizontal milling machine Fig. 2 A HFMI/PIT hand-held device and controller with the available (brand : Richmond) indenter pins Author's personal copy
Int J Adv Manuf Technol
Table 2 Control factors and their levels used in OA design matrix
Symbol Factors Unit Level 1 Level 2 Level 3
A Air pressure Bar 4 5 6 B Impact frequency Hz 80 100 120 C Indenter diameter mm 1.5 2 4
AB C Upon fixing of the test strips into the holder, the curvature was measured and screwed lightly and evenly to prevent the holder from tilting. A frequency of 90 Hz (level 2) and pres- sure 6.0 bar (level 3) were used for the impact process for 45 s. Even 4–5 overlapping impact tracks were performed with Fig. 3 The HFMI/PIT indenter with varied pin diameters: A =∅1.5 mm, changes to the side which lead the bolt as well as the angle ∅ ∅ B = 2.0 mm and C = 4.0 mm along the respective edge to fill out the entire surface of the test strip. The curvature difference before and after the treat- possible to record the treatment data over a prolonged period. ment was recorded to ensure an achievement of a mean value The hammer pins are hardened steel pins with varying pin above the threshold value of 0.8 mm. The equipment and diameters from 1.5 to 4 mm, depending on the treatment sit- impact of test strips is shown in Fig. 4. uation, as shown in Fig. 3. The typical treatment speed for Three HFMI/PIT parameters namely the air pressure, im- application on aluminium material to be set is in the range of pact frequency and indenter pin diameter were selected for approximately 2 to 4 mm/s. experimentation with three levels of each factor. The values To achieve outstanding quality result, HFMI/PIT’squality of the HFMI/PIT process parameters at the different levels are monitoring guidelines with reference to the recent proposed tabulated in Table 2. Experimental process was conducted procedures and quality assurance guidelines for HFMI-treated using L9 orthogonal array in Taguchi Method which has nine joints are established by Gerster et al. [20]. Compared to con- rows corresponding to the number of experiments as shown in ventional quality assurance method using depth gauge or cal- Table 3. Three HFMI/PIT parameters namely the air pressure, liper to measure the groove depth (normally max 2 mm), the impact frequency and indenter pin diameter were selected for quality monitoring technique for HFMI/PIT is carried out experimentation with three levels of each factor. The speci- through visual inspection using digital or manual magnifier mens were profiled into fatigue specimen size mentioned in to ensure that no rest notch is available on treated surface. The ISO/TR 14345:2012(E) [18] and then HFMI/PIT treated to decision on “no rest notch” surface inspection instead of just avoid any residual stresses induced by the milling process to measurement of groove depth is very crucial due to following influence the results of the fatigue tests. The specimens for reasons: (i) no single groove dimension is optimal in all situ- hardness measurement were HFMI/PIT treated first and then ations, (ii) material hardness at the weld toe may vary and cut and polished for hardness measurement. hence treatment needs to be systematically adapted, (iii) The post-weld treatment of the FSW joints using deeper undercut is allowed based on quality standard such as HFMI/PIT technology was carried out on the finished fatigue ISO EN 5817 [21] and (iv) strong weld reinforcement pos- test specimens using a HFMI/PIT hand-held device. The treat- sesses deep transition groove. ment was always carried out at fatigue prone areas covering The Almen test was executed prior to the HFMI/PIT as a the stir zone, thermomechanical zone and heat affected zone, calibration method to verify the intensity and performance of covering a total length of 60 mm. Figure 5 shows the the system. Test specimens of material S355J2G3 with dimen- HFMI/PIT-treated surface on the FSW butt joints AA6061 sions of 200 mm × 20 mm × 4 mm was used to examine for with varying HFMI/PIT process parameters. The quality of curvature on a flat, even surface with a simple finger test. the post-weld treatment was inspected visually on the basis
Fig. 4 The Almen test strip during impact test (left), and Almen test equipment (right) Author's personal copy
Int J Adv Manuf Technol
Table 3 Experimental layout using L9 Experiment Levels of factors orthogonal array number ABC
1111 3mm 2122 3133 4212Fig. 6 Macrostructure of HFMI/PIT-treated specimen showing sub- surface hardness measurement points 5223 6231 7313crack or a total fracture occurred. The number of load 8321cycles from the crack initiation to a total fracture was 9332observedtobenegligiblysmallinrelationtothetotal number of cycles. The as-welded and post-weld treated specimens are presented in Fig. 7. The overlapping of of the contour of the treatment track to ensure the nonexis- the separate pin impressions to form an almost regular tence of any remaining notch. track can be clearly seen.
2.3 Sub-surface hardness measurement and fatigue test procedure 3 Result and discussion Before hardness tests were performed, samples for macro profiles were prepared by the usual metallurgical 3.1 Sub-surface hardness values and fatigue test results polishing methods and etched with Keller’s reagent. The hardness field was established on the cross section It is found that the hardness of base material varies be- of the weld seam according to the ISO 6507-2 [22]stan- tween 105 and 110 HV. Compared to the parent material, dard with three measured points in the sub-surface with dynamic recrystallization in FSW joints plays a major 1 kgf using a Struers Duramin Micro-Vickers Hardness role in the elimination of strain hardening which signif- test machine. The hardness of the sub-surface was mea- icantly softens the weld zone. This in turn causes a dec- sured in centre as well as in both retreating and advanc- rement of the hardness values in the vicinity of the weld ing sides at a depth of 0.5 mm from the surface as stir zone. The mean hardness value of the sub-surface in depicted in Fig. 6. the as-welded condition for FSW AA6061is recorded at The fatigue tests were carried out to quantify the in- 72 HV [19] compared to the average value of 95 HV fluence of the varied HFMI/PIT process parameters on obtained for the HFMI/PIT-treated FSW AA6061. From the fatigue resistance of FSW AA6061 butt joints. The total number of experiments, 50% of the hardness values fatigue resistance was ascertained in conventional con- attained by post-treatment were comparable to the base stant amplitude fatigue tests with a constant stress ratio material hardness value. It is noted that a higher value of of R = 0.1 and a frequency of 25 Hz with maximum load air pressure resulted in an increment between 35 and of 120 MPa equivalent to 70% of the ultimate tensile 40% from the as-welded stir zone hardness value. The strength of the FSW AA6061 butt joint. The tests were impact frequency of 120 Hz recorded a lower value of carried out on an Instron all-purpose servo-hydraulic ma- hardness compared to the other frequencies while the chine with a maximum test force of 250 kN. The tests indenter pin did not show any clear configuration of were run without intermission until a through-going decrement or increment.
Fig. 5 Treated surface of different fatigue test specimens with varied HFMI/PIT parameters
3mm Author's personal copy
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Table 5 Quality loss values for fatigue life cycle and sub-surface hardness A Experiment Quality loss values (dB) number Cycles to failure Sub-surface hardness B 1 5.97017E-12 1.603 10mm 2 9.977E-12 34.09 3 8.66491E-12 1.843 4 5.21889E-11 4.333 Fig. 7 Fatigue test specimens (A): as-welded and (B): HFMI/PIT treated and the length of treatment 5 4.812E-11 26.56 6 1.83838E-11 19.94 7 1.83959E-11 4.57 Fatigue test conducted with a stress value of 120 MPa on 8 3.1907E-11 9.963 the HFMI/PIT-treated FSW AA6061 generated overall mean 9 1.87396E-11 13.56 fatigue strength of 272,198 cycles in comparison to the as- welded FSWAA 6061 of approximately 60,000 cycles which is comparable to the result of ca. 50,000 cycles based on The values of the observed data for the three fatigue speci- investigation stated in [16]. The highest obtained mean fatigue mens and the average cycles to failure and Vickers hardness life cycle of 519,327 cycles is almost nine times the fatigue values are shown in Table 4. resistance of the as-welded condition while the lowest record- ed mean fatigue life cycle of 140,444 does not depict signifi- 3.2 Multi-objective optimized parameters cant improvement. The single highest recorded life cycle is 722,843 which are 12 times higher than the as-welded condi- From Tables 5 and 6, quality loss values for the quality char- tion. Results acquired from the experiments indicated that acteristics of “nominal-is-better” and “higher-is-better” in nearly 60% of the samples recorded a fatigue resistance im- each experimental run are calculated using equations stated provement below the mean value between 140,000 to in [19]. These quality loss values are depicted in Table 5. 272,000 cycles. Notably, a lower air pressure of 4 bar resulted The normalized quality loss values for both quality charac- in a significant increment while the air pressure of 5 and 6 bar teristics in each experimental run have been calculated using recorded reasonably equivalent increments. Although the im- Eq. 6 as used in [19] and the obtained values are shown in pact frequency of 80 Hz produced the highest single fatigue Table 6. The total normalized quality loss values (TNQL) and life cycle increment, the impact frequency of 120 Hz produced MSNR for multiple quality characteristics for fatigue life cy- a more constant and substantial increment while the impact cle and weld sub-surface hardness has been calculated using frequency of 100 Hz recorded below mean value improve- equations stated in [19]. These results are presented in Table 7. ments. The indenter diameter of 1.5 mm generated significant In calculating the total normalized quality loss values, two enhancements to the life cycle whereas 2.0 and 4.0 mm gen- unequal weights of w1 and w2 were assigned namely w1 erated similar fatigue strength with average improvements. being 0.8 for number of fatigue life cycles to failure and w2
Table 4 Fatigue experimental results for fatigue life and sub- Experiment Fatigue life Sub-surface hardness surface hardness number Mean (μc) (cycle) Std. dev. (σc) (cycle) Mean (μs)(HV) Std.dev.(σs)(HV)
1 517,972 203,530 97 1.27 2 321,642 42,008 88 5.84 3 360,297 52,795 86 1.36 4 140,444 12,720 100 2.08 5 183,164 72,400 106 10.09 6 259,771 83,438 92 5.46 7 238,524 35,639 101 2.14 8 195,847 66,928 101 3.16 9 232,126 14,554 90 3.68 Author's personal copy
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Table 6 Normalized quality loss values Table 8 Multiple S/N response (average factor effect at different level)
Experiment Normalized quality loss values (dB) Symbol Factors Mean of multiple S/N ratio (dB) number Cycles to failure Sub-surface Level 1 Level 2 Level 3 hardness AAirpressure 17.196a 1.410 11.763 1 0.114395 0.047028 B Impact frequency 10.846 5.084 14.440a 20.1911711 C Indenter pin diameter 13.447a 8.102 8.821 3 0.16603 0.054067 a 4 1 0.127102 Optimum level 5 0.922036 0.779136 6 0.352255 0.584963 a factor with a high percentage contribution, a small variation 7 0.352487 0.134044 will have a great influence on the performance. 8 0.611375 0.292237 The percentage contribution of different control factors on 9 0.359072 0.397829 multiple quality characteristics, namely fatigue life cycles to failure and weld sub-surface hardness, shows that air pressure was the major factor (66.57%), followed by impact frequency (21.5%) and indenter pin diameter (10.24%). Using being apportioned at a value of 0.2 for weld sub-surface hard- HFMI/PIT, air pressure and impact frequency have the ness. Higher weighting factor has been assigned to the number greatest effect on the fatigue resistance and hardness profile. of fatigue life cycles to failure rather than the weld sub-surface hardness as it is more important to achieve a favourable fa- tigue resistance with post-weld treatment in FSW process. 3.3 Second-order response surface model for objectives The effect of different control factors on MSNR is shown in Table 8. The optimum levels of different control factors for The second-order response surface model for fatigue life cy- fatigue life cycles to failure and weld sub-surface hardness cles and sub-surface hardness value has been developed from obtained are air pressure at level 1 (4 bar), impact frequency the experimental response values obtained using OA experi- at level 3 (120 Hz) and indenter pin diameter at level 1 mental matrix. These equations were developed using RSM in (1.5 mm). MINITAB software. ANOVA technique was further employed to detect signif- Fatigue life ¼ 248383−102755A−32715B−46563C icant factors in multi-objective optimization for fatigue life cycles to failure and weld sub-surface hardness. The result þ 29152A2 þ 14493B2 þ 5022C2 ð1Þ of ANOVA for the HFMI/PIT-treated outputs is presented in Sub−surface hardness ¼ −42:61 þ 43:95A þ 1:23B ð2Þ Table 9. The analysis conducted indicates that air pressure was 2 2 2 statistically significant since its p value is less than 0.05. −21:0367C−21:04A −0:075B þ 3:8C Furthermore, it also shows the percentage contribution which Where A is the air pressure, B is the impact frequency and C indicates the relative power of a factor to reduce variation. For is the indenter pin diameter, respectively. In conformance of a model with well-fitted data, observa- tions of the standard errors of samples (S) and coefficient of Table 7 Total normalized quality loss values (TNQL) and Multiple S/N 2 ratios (MSNR) determination (R ) are essential. Normally, a greater value of R2 and a smaller value of S will determine the appropriateness Experiment number TNQL MSNR (dB) of a regression model. The calculated values from the devel- oped models for the S value of the regression analysis on 1 0.100922 9.960147 fatigue life cycle is 1.011, while sub-surface hardness is 2 0.352937 4.523032 0.3049, whereas the obtained R2 values are reasonably high 3 0.143637 8.427329 for fatigue life cycle and sub-surface hardness with 95.6 and 4 0.82542 0.833248 90%, respectively. 5 0.893456 0.48927 6 0.398796 3.99249 7 0.308798 5.103254 3.4 Confirmation tests on parameters and responses 8 0.547548 2.615782 9 0.366824 4.355428 The ultimate step is the validation of the optimum pa- Mean of MSNR of all experimental runs 4.4778 rameter settings suggested by the matrix through experi- mental verification to determine whether these conditions Author's personal copy
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Table 9 ANOVA result Factors Air pressure Impact frequency Indenter diameter Error Total
DoF 2 2 2 2 8 Sum of square 52.52 16.98 8.08 1.45 78.9 Mean of square 26.26 8.49 4.0 0.72 F 36.32 11.74 5.54 P 0.027 0.079 0.153 Contribution % 66.57 21.5 10.24
produce the projected improvements. Hence, a specific times increment of fatigue strength improvement from the combination of the factors and levels previously evaluat- untreated specimens. The sub-surface hardness values show ed will be used in the confirmation experimental test. reasonable values. Overall, a good agreement is seen in the Subsequent to defining the optimal conditions, a new predicted and experimental results obtained for both fatigue experiment was conducted using the determined optimum life cycle and sub-surface hardness values. levels of governing parameters A1B3C1. Then, the pre- dicted value of MSNR (ηopt) at the optimum parameter levels was calculated by using the following equation: 4 Conclusion X p η ¼ η þ ðÞη −η ð3Þ opt m i¼t mi m A multi-objective optimization has been applied with simul- taneous consideration of multiple response (fatigue life cycle
Where ηm is the mean MSNR of all experimental runs, p is and hardness profile) using Taguchi method to optimize the the number of main welding parameters that significantly af- multiple quality characteristics in high frequency hammer fect the performance and ηmi is the average MSNR at the peening process. Based on the optimization and modelling optimal level. results, the following conclusions can be drawn: The predicted value of MSNR and that confirmation exper- iment is shown in Table 10. This verification depicts an im- (1) The multiple characteristic such as fatigue life cycle and provement in multiple S/N ratio of 3.0796 dB upon the alter- hardness profile can be simultaneously considered using ation of the initial governing parameter setting of A2B2C3 to multi-objective Taguchi method. the optimal setting of A1B3C1. Since this was the inaugural (2) The mean hardness value of the sub-surface in the as- attempt to apply the HFMI/PIT on FSW AA6061 butt joints, welded condition for FSWAA6061 is recorded at 72 HV the initial parameters was chosen based on a trial mode to use compared to the average value of 95 HVobtained for the a moderate air pressure and impact frequency combined with a HFMI/PIT-treated FSWAA6061which is very much ide- large indenter pin diameter to obtain the required fatigue life al to the base material hardness value of 105 HV. enhancement. This attempt showed reasonable improvement (3) The role of different control factors is air pressure in both responses, namely the fatigue life cycle and the sub- (66.57%), impact frequency (21.5%) and indenter pin surface hardness values with the multi-response optimization diameter (10.24%). The air pressure plays a major role used as compared to the initial values of the fatigue life cycles in determining reasonable surface hardening and superi- and sub-surface hardness values obtained. or fatigue life cycle in FSW joint. The fatigue resistance shows significant changes with im- (4) The optimum parameters for a higher fatigue life cycle provement from the initial enhancement of more than three and hardness are as follows: air pressure at level 1
Table 10 Result of the confirmation experiment Initial parameter setting Optimal process parameters
Prediction Experiment Error (%) Level A2B2C3 A1B3C1 A1B3C1
Fatigue life cycle (N) 183,164 652,843 688,626 5.2 Sub-surface hardness (HV) 102.6 104.5 105 0.004 Multiple S/N ratio (dB) 0.605818 3.5843 3.68543 Improvement in multiple S/N ratio = 3.0796 dB Author's personal copy
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(4 bar), frequency at level 3 (120 Hz) and indenter diam- 8. Sano Y, Masaki K, Gushi T, Sano T (2012) Improvement in fatigue performance of friction stir welded A6061-T6 aluminium alloy by eter at level 1 (1.5 mm). – (5) laser peening without coating. Mater Des 36:809 814 The HFMI/PIT process parameter optimization is signif- 9. Leitner M, Stoschka M, Eichlseder W (2012) Contribution to the icant due to the fact that each variation instigates an im- fatigue enhancement of thin-walled, high-strength steel joints by provement between three to 12 times, hence the wrong high frequency mechanical impact treatment. IIW Document process parameter may deteriorate the maximum possi- XIII-2416-12 ble fatigue enhancement. 10. Nitschke-Pagel T, Eslami H, Dilger K (2013) Influence the deformation intensity on the fatigue strength of aluminium (6) The quadratic response surface model established for the welds with different mechanical surface treatments, IIW-Doc. prediction of fatigue life cycle and sub-surface hardness XIII-2483-13 has been found to be well fitted. 11. Muhammad N, Manurung YHP, Hafidzi M, Abas SK, Tham G, (7) HFMI/PIT is a post-weld mechanical treatment that can Haruman E (2012) Optimization and modeling of spot welding parameters with simultaneous multiple response consideration be used to significantly enhance the fatigue resistance using multi-objective Taguchi method and RSM. J Mech Sci level of FSW AA6061. Technol 26(8):2365–2370 12. Kumar Dubey A, Yadava V (2008) Multi-objective optimi- Acknowledgement The authors would like to acknowledge the sation of laser beam cutting process. Optics & Laser Advanced Manufacturing Excellence Center (AMTEx) and Laboratory Technology 40(3):562–570 of Welding at Faculty of Mechanical Engineering, Universiti Teknologi 13. Periyasamy P, Mohan B, Balasubramanian V, Rajakumar S, MARA (UiTM) as well as staff members of the Welding Department Venugopal S (2013) Multi-objective optimization of friction stir and Laboratory, Universiti Kuala Lumpur Malaysia France Institute welding parameters using desirability approach to join Al/SiCp (UNIKLMFI) for providing the experimental facilities and expertise. A metal matrix composites. Trans Nonferrous Metals Soc China special gratitude is also addressed to TECHNOGERMA Engineering & 23(4):942–955 Consultancy (TEC) in Mont Kiara, Kuala Lumpur, MALAYSIA and 14. Nourani M (2011) Taguchi optimization of process parameters in PITEC GmbH in Duermentingen, GERMANY for granting the research friction stir welding of 6061 aluminium alloy: a review and case with financial support from the TEC-PITEC-AMTEx-Research study. Engineering 03(02):144–155 Collaboration program (Project Nr.: TEC-PITEC-AMTEx-RC-011-2016). 15. Koilraj M, Sundareswaran V, Vijayan S, Koteswara Rao SR (2012) Friction stir welding of dissimilar aluminium alloys AA2219 to AA5083—optimization of process parameters using Taguchi tech- – References nique. Mater Des 42:1 7 16. A. S. Ribeiro, A. M. P. De Jesus, and I. Feup (2009) Fatigue behav- iour of welded joints made of 6061-T651 aluminium alloy. 1. Almanar IP, Hussein Z (2011) Basic consideration for weldment Aluminium Alloy, Theor Appl ISBN 978–953–307-244-9 formation in friction stir welding. Nova Science Publishers, New 17. 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