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Taguchi Analysis of the Effect of Process Parameters in Friction Stir Welding
R. Louro1,a, C. Leitão2,b, H. Gouveia1,c, A. Loureiro2,d and D. Rodrigues2,e 1 ISQ, Welding and Quality Institute, Avenida Cavaco Silva nº33, 2740-120 Porto Salvo, Portugal 2 CEMUC, Department of Mechanical Engineering, University of Coimbra, Rua Luís Reis Santos
Pinhal de Marrocos, 3030-788 Coimbra, Portugal [email protected], [email protected], [email protected], [email protected],
Keywords: Friction stir welding, Welding parameters, Weld quality.
Abstract. The task of obtaining suitable welding parameters for the friction stir welding process is often a difficult one, due to the lack of published data and the fact that the exact mechanism by which the process operates has not yet been fully determined. Therefore, suitable welding parameters often need to be obtained by using extensive, time consuming and expensive experimental methods. The work detailed in this paper pertains to the use of the Taguchi method as a mean to reduce the disadvantages of these experimental methods, more specifically, their cost. The Taguchi method accomplishes this task by substantially reducing the number of welding trials that are needed to obtain suitable welding parameters. This reduction leads to the parameters being obtained more rapidly and at a substantially smaller cost. In this paper a procedure for applying the Taguchi method to the friction stir welding process is presented as well as its application to the welding of a specific component. The method was applied to the welding of 4mm thick AA5083-H111 plates in a butt joint configuration, which constitutes one of the most common industrial welding scenarios. The purpose of the experimental tests was to maximize the welding speed whilst ensuring an acceptable welding quality. The quality of the welds was determined through visual inspection and tensile and bending tests. The application of the Taguchi method allowed, with a relatively small number of experimental welds, to provide some insight into the manner by which the parameters should be altered in order to optimize the process.
Introduction
Friction Stir Welding (FSW) is a relatively recent welding technology which consists in inserting a rotating welding tool into the plates to be joined and advancing it along their interface. The stirring motion of the tool carries out the weld. The process requires substantial forces to be applied during the operation, thus requiring the base material to be very tightly clamped. Since the process temperature is always below the fusion temperature of the base material, this process was classified as a solid state welding process, being particularly suited for the welding of lightweight alloys, especially aluminium alloys. Further information pertaining to the FSW process can be found in [1] and [2].
One of the main limitations associated with the FSW technology is related to the establishment of suitable process parameters. In fact, for each set of welding conditions (joint type, base material and joint thickness) a specific set of welding parameters must be used in order to ensure acceptable process behaviour. A set of process parameters is deemed suitable if their use will result in a weld of acceptable quality, as defined in [4], and if the welding speed is sufficiently high to offset the process costs, thus making more competitive when compared with the more established welding processes such as GTAW.
According to [3], the welding speed has a direct relationship with the process productivity, since the majority of the process costs are of a fixed nature. Therefore, if the welding speed is sufficiently high, the process has a good productivity. However, determining the welding quality is not an easy
Materials Science Forum Vols. 636-637 (2010) pp 1150-1156Online available since 2010/Jan/12 at www.scientific.net© (2010) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.636-637.1150
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task; in order to facilitate this task specific standards, such as [4], exist. According to them the quality of a weld can be ascertained by carrying out a number of destructive and non destructive tests. Generally speaking, the objective behind the selection of suitable welding parameters, in an industrial context, is to maximize the welding speed whilst ensuring acceptable welding quality.
The FSW process parameters are difficult to obtain due to the fact that the majority of the physical phenomena that occur during the welding operation are still poorly understood. In fact, despite a substantial amount of work is currently being carried out in this domain, only incipient results have thus far been obtained [1]. This generalized lack of information concerning the FSW process is a serious handicap for the industrial implementation of the process.
Due to the lack of widely available knowledge concerning the FSW process, the task of finding suitable welding parameters has to be conducted through experimental methods. These methods are often trial and error based and can be quite costly, time consuming, having no guarantee of success and requiring expert manpower, equipment occupation and weld quality tests. Therefore, any method that can significantly reduce the number of required experimental tests is of great importance.
The Taguchi design of experiments are a set a statistical methods that can be used for optimizing a system of unknown behaviour. They are widely applied in the field of industrial and quality engineering and are now becoming accepted in various scientific fields. Their main advantage is the fact that they allow a significant reduction of the number of experimental tests needed for conducting an optimization operation. By reducing the number of experimental tests, the cost and time requirements for obtaining suitable welding parameters are greatly reduced. Furthermore, it also outlines a manner to easily analyze the experimental results. In the present work, the Taguchi method was applied to plan the welding tests of 4mm thick AA5083-H111 plates in a butt joint configuration.
Application of the Taguchi method
As previously mentioned the Taguchi method, which is fully described in [5], is a statistical method for designing experiments and optimizing process parameters. This method has the advantage of being applicable to even the most uncertain of scenarios. Nevertheless, they also have the disadvantage that the conclusions that are drawn by applying it are only valid in the parameter range used for carrying out the experimental tests.
The application of the Taguchi method requires several steps. The first step consists in fully defining the factors that have an influence in the process. In the specific case of the FSW process these factors are the welding parameters, depicted in Figs. 1 and 2, namely: welding speed (V), rotating speed (W), vertical force (F), pitch angle (α), pin diameter (φp) and the shoulder to pin diameter ratio (φe/ φp). It should be mentioned that the latter two parameters are not process parameters, but rather welding tool parameters. Nevertheless, the welding tool is of such importance to the process, and its influence so poorly understood, that these parameters were included in the work presented in this paper. However, since φe is always greater than φp and significant bibliographic references, such as [1] and [2], mention that φe /φp should be taken into account, this ratio was considered in the Taguchi analysis, taking into account the influence of φe, albeit in an indirect manner.
The pin length cannot be seen as a welding parameter. In fact according to [6] if this parameter is set so as to guarantee that, during the welding operation, the lowermost surface of the pin does not come in contact with the backing bar and that it remains at 0.1mm or less from the backing bar, the weld quality is not affected by the pin length. The value of the joint thickness and the pitch angle (α) must be taken into account while computing the appropriate pin length in order to ensure that this parameter will not have an adverse effect in the weld quality. During the execution of the experimental welds the pin length was appropriately set so as to not have any influence in the welding quality. As such this parameter was not considered in the Taguchi analysis.
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The next step in the Taguchi method is to select the levels at which these parameters will be tested. This step comprises the selection of the number of different parameter levels, for each factor, as well as the numerical value for these factors.
Fig. 1. FSW process parameters. Fig. 2. FSW tool parameters.
The difficulties felt by the overall scientific community in modelling the FSW process ([6]) lead
to the assumption that the FSW process is highly non linear in nature ([6]). Therefore, it becomes necessary to have at least three different levels for each factor, enabling second order relationships to be taken into account, in order for the model to have some capabilities for dealing with non linear relationships. However, due to physical limitations, it was only possible to use two different levels for the FSW tool parameters. The levels were selected based on past experience, bibliographic references, more specifically [3], and on the capability of the available equipment. They are depicted in Table 1.
Table 1. Welding parameters levels.
V [mm/min] W [RPM] F [k"] α [º] φp [mm] φe / φp 300 400 500 400 500 600 7 11 15 1 2 3 5 6 2.5 3
Once the number of levels have been selected it is necessary to determine the minimum number
of experimental welds that have to be conducted. According to [5], there are two conditions that have to be met: the number of tests must be greater than the degrees of freedom of the statistical model and the number of tests must be sufficient to ensure the orthogonal condition. The first step in ensuring the first condition is to define the statistical model that we want to consider. This task has been started when the parameters and levels were selected; the only information remaining is to determine if any interactions should be taken into account. An interaction between parameters, as defined in [5], occurs when the level of one parameter influences the effect that the level of another parameter will have. In the case of the FSW process, it is assumed that the majority of the existing interactions will be related to the welding tool, and the following interactions were taken into account: the interaction between the φe/ φp ratio and φp and the interaction between V and φp. According to these assumptions, the number of degrees of freedom for this statistical model, and therefore, the minimum number of tests is 14.
The orthogonal condition ([5]) is satisfied if the number of welding tests is a multiple of the smallest common multiple of the number of levels of all different actions ([5]). In this case, the smallest common multiple is 3x3x2=18. Therefore the number of welding tests has to be a multiple of 18 and has to be greater than 14.
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The next step in the method comprises the selection of one of the many published Taguchi tables, see [5], that complies with all of the previously mentioned conditions. The table that best complies with the requirements is the L36(23×313) table. The use of this table implies carrying out 36 tests, which is a significant improvement when compared with the 34×22=324 tests required by a factorial experimental plan. By applying the Taguchi method it is possible to reduce the necessary welding tests to a mere 11% of the original number. The experimental welding plan is depicted in Table 2. This plan was constructed using the 1st, 2nd, 5th, 4th, 6th and 7th columns of the L36(23×313) table. Another advantage of the selected Taguchi table is that it also allows for the interaction between V with the φe / φp ratio to be taken into account. At this point a well defined experimental plan was set.
Experimental results
The welding tests were conducted according to the test plan of Table 2. The quality of the welds was established based on the method depicted in [4] and was assessed through the use of visual inspection and bending and tensile tests. These tests were carried out sequentially, which means that, if the weld failed the visual inspection, it would not be subjected to the bending and tensile tests. In order to enable the statistical computations, the test results need to be merged into a single numerical value, which was accomplished through the use of the conditions detailed in Eq. 1. This equation, which was derived for this specific application, provides a linear scale that could unify the results of all the quality assessment tests. As such if IV=2 then Q=1, if IV=1 then Q=0.75, if ET=1 then Q=0.5, if ED=1 then Q=0.25 and if all of the tests had an acceptable result then Q=0. This equation is essential for analyzing the test results in a unified manner.
=
>=
−>
=
EDETif
ETETifIVif
IVIVIVifQ
25.0,0
50.0,0,0
25.0,0 2
(1)
where: Q – Quality value (Q<0.25 is indicative of an acceptable welding quality) IV – Visual inspection test result (IV=2 refers to a pin fracture, IV=1 refers to a visible weld defect ([7]), IV=0 refers to an acceptable result) ET – Stress test result (ET=1 refers to a non acceptable result, which occurs if the ultimate tensile stress is lesser than 300MPa, see[2], ET=0 refers to an acceptable test result) ED – Bending test result (ED=1 refers to a non acceptable test result, which occurs if no cracks occur in a 180º bending angle, see[2], ED=0 refers to an acceptable welding result)
The test plan and results are shown in Table 3. A method for relating the levels of the factors
with the test results is presented in [5]. This method comprises the computation of a number dubbed effect, defined according to Eq. 2.
RRE pl
pl −= (2)
where:
plE - is the effect of parameter p at level l. p
lR - is the average of the results where parameter p at was set at level l.
R - is the overall result average.
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The effect can be seen as a measurement of the manner and strength by which the parameter levels affect the overall average of the results. The values for the effects of each factor are presented in Table 3.
Table 2. Experimental welding plan and results.
Test Number
Pin Diameter (φp) [mm]
φe/ φp Pitch
Angle (α) [º]
Welding Speed (V) [mm/min]
Rotating Speed (W)
[RPM]
Vertical Force (F)
[kN]
Visual Inspection
Ultimate Tensile
Stress [Mpa]Bending Test
Quality index (Q)
1 5 2.5 1 300 400 7 2 12 5 2.5 2 300 500 11 0 292.41 Acceptable 0.53 5 2.5 3 300 600 15 0 304.85 Acceptable 04 5 3 1 300 400 7 1 0.755 5 3 2 300 500 11 0 302.78 Acceptable 06 5 3 3 300 600 15 0 305.93 Acceptable 07 6 2.5 1 300 400 11 1 0.758 6 2.5 2 300 500 15 0 303.27 Acceptable 09 6 2.5 3 300 600 7 1 0.75
10 6 3 1 300 400 15 0 294.90 Acceptable 0.511 6 3 2 300 500 7 1 0.7512 6 3 3 300 600 11 1 0.7513 5 2.5 1 400 500 15 0 277.21 Not Acceptable 0.514 5 2.5 2 400 600 7 1 0.7515 5 2.5 3 400 400 11 2 116 5 3 1 400 500 15 0 275.44 Acceptable 0.517 5 3 2 400 600 7 2 118 5 3 3 400 400 11 0 262.81 Not Acceptable 0.519 6 2.5 1 400 500 7 1 0.7520 6 2.5 2 400 600 11 1 0.7521 6 2.5 3 400 400 15 0 307.68 Acceptable 022 6 3 1 400 500 11 0 174.35 Not Acceptable 0.523 6 3 2 400 600 15 0 307.96 Acceptable 024 6 3 3 400 400 7 2 125 5 2.6 1 500 600 11 0 228.21 Not Acceptable 0.526 5 2.5 2 500 400 15 2 127 5 2.5 3 500 500 7 2 128 5 2.5 1 500 600 11 0 206.75 Not Acceptable 0.529 5 3 2 500 400 15 0 240.04 Not Acceptable 0.530 5 3 3 500 500 7 0 206.96 Not Acceptable 0.531 6 2.5 1 500 600 15 0 306.78 Acceptable 032 6 2.5 2 500 400 7 2 133 6 2.5 3 500 500 11 0 246.37 Not Acceptable 0.534 6 3 1 500 600 7 1 0.7535 6 3 2 500 400 11 2 136 6 3 3 500 500 15 2 1
Average 0.59
Table 3. Effects of the welding parameters on quality index.
Level [mm] Effect Level Effect Level [º] Effect Level [mm/min] Effect Level [RPM] Effect Level [kN] Effect5 -0.007 2.5 0.007 1 -0.007 300 -0.111 400 0.160 7 0.2436 0.007 3.0 -0.007 2 0.014 400 0.014 500 -0.049 11 0.014
3 -0.007 500 0.097 600 -0.111 15 -0.257
Welding Speed (V) Rotating Speed (W) Vertical Force (F)Pin (φp)Pin to Shoulder diameter
ratio (φe/ φp)Pitch Angle (α)
Finally, the interactions between the previously selected variables are computed through Eq. 3,
see [4]. REERI p
lp
llplplplp −−−= 22
1122112211 (3)
where, 2211 lplpI - is the interaction between parameters p1 at level l1and p2 at level l2.
plE - is the effect of parameter p at level l.
2211 lplpR - is the average of the results where parameter p1 was set at level l1and p2 was set at level
l2. R - is the overall result average.
The number that is computed from Eq. 3 can be seen as the manner by which a parameter being set at one level would enable another parameter to affect the end result. The values for the interactions that were taken into account are given in Table 4, Table 5 and Table 6.
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Table 4. Interaction between V and φp.
Pin diameter (φp) [mm]
300 400 500
5 -0.097 0.111 -0.0146 0.097 -0.111 0.014
Welding Speed (V) [mm/min]
Table 5. Interaction between φp and the φe/ φp ratio.
Pin diameter (φp) [mm]
2.5 3
5 0.104 -0.1046 -0.104 0.104
Pin to Shoulder Ratio (φe/ φp)
Table 6. Interaction between the φe/ φp ratio and V.
Pin to Shoulder ratio (φe/ φp)
300 400 500
2.5 0.014 0.014 -0.0283 -0.014 -0.014 0.028
Welding Speed (V) [mm/min]
Discussion and Conclusions
By analyzing the data in Table 2 it can be concluded that the tool parameters, φp and φe, as well as the pitch angle α do not possess a strong direct influence in the welding quality. It should be noted that in the notation used an acceptable welding quality pertains to a Q value smaller than 0.25. On the other hand V and W have a strong influence in the welding quality; however the parameter with the greatest direct influence is F. It is also evident that F and W should be increased in order to improve the welding quality and that V should be decreased in order to achieve the same result. However, decreasing V is undesirable since it would lead to a decrease in the process productivity.
The indirect influence of the parameters can be inferred from the values of the interactions that are present in Table 4, Table 5 and Table 6. It is evident that the interaction between the φe/ φp ratio and V is negligible. Nevertheless the interactions between V and φp and between φp and the φe/ φp ratio are robust. This means that when selecting a value for V or φp these interactions should be used for determining the values from the remaining parameters.
The initial objective behind present analysis was to optimize the FSW parameters for joining 4mm thick AA5083-H111 plates in a butt joint configuration. For optimizing productivity, V must be the maximum test value (500 mm/min). According to Table 4, for this value of V, φp can be either 5mm or 6mm. The latter value was selected because its greater diameter decreases the likelihood of a pin fracture. The value for φe is selected by analyzing the data in Table 5, which means it should be 15mm. The remaining values are selected from Table 3, W is 600 rpm and F is 15kN. If α is set at 1º, then this parameter combination equals experimental test number 31, which is depicted in Table 2. Since this test has led to an acceptable welding quality the validity of the analysis is reinforced. Nevertheless this is a statistical method and therefore for these conclusions to be fully validated a substantially larger number of tests are required.
It should be pointed out that according to several sources, [1], [2] and [6], the pitch angle α is of great importance for obtaining welds with an acceptable quality. Since the direct effect of this parameter is negligible in all likelihood a strong interaction between this parameter and one of the remaining welding parameters exists.
The work detailed in this paper has enabled a demonstration of the use of the Taguchi method for obtaining suitable welding parameters. The same method could be used for optimizing the welding speed. In this version the purpose of the method is to provide sets of welding parameters that will enable an acceptable welding quality to be reached while maximizing the welding speed. The maximization of the welding speed has the purpose of increasing productivity. In this scenario it would be necessary to conduct several iterations of the method, of which only one was depicted in this paper.
Acknowledgements
The work present in this paper was partly funded by the project PDTC/EME-TME/69999/2006 from the Portuguese Fundação para a Ciência e Tecnologia and by project 13-05-04-FDR-00046 from the Portuguese Agência de Inovação.
Thanks are also due to Arsenal do Alfeite, especially Mr. João Freire, for all the support in the execution of the experimental welding plan.
Materials Science Forum Vols. 636-637 1155
The authors would also like to thank Thyssen Krupp Portugal, more specifically Mr. Domingos Granja, for all the cooperation regarding the thermal and superficial treatments of the FSW tools.
References
[1] R.S. Mishra, Z.Y. Ma: Friction stir welding and processing, Materials Science and Engineering Vol. R50 (2005), p. 1 – 78
[2] W.M. Thomas, E.D. Nicholas and J.C. Needham: European Patent Specification 0615480, 1992
[3] J. Mononen, M. Sirén, H. Aniñen: Welding in the World, Vol. 47 nº11/12 (2003), p.32-35
[4] European Standard E! 15614-2: Specification and qualification of welding procedures for metallic materials – Welding procedure test – Part 2: Arc Welding of aluminium and its alloys, European Committee for Standardization, 2005
[5] M. Pillet: Introduction aux plans d’experiences par la méthode Taguchi (Les Editions D’ Organization 1994)
[6] Mishra, R.S.; Mahoney, M.W. “Friction Stir Welding and Processing” (ASM International, 2007)
[7] European Standard E! 17637:!on-destructive testing of welds – Visual Testing of fusion-welded joints , European Committee for Standardization, 2003
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Advanced Materials Forum V 10.4028/www.scientific.net/MSF.636-637 Taguchi Analysis of the Effect of Process Parameters in Friction Stir Welding 10.4028/www.scientific.net/MSF.636-637.1150
DOI References
[1] R.S. Mishra, Z.Y. Ma: Friction stir welding and processing, Materials Science and Engineering Vol. R50
(2005), p. 1 – 78
doi:10.1016/j.mser.2005.07.001 [1] R.S. Mishra, Z.Y. Ma: Friction stir welding and processing, Materials Science and Engineering ol. R50
(2005), p. 1 – 78
doi:10.1016/j.mser.2005.07.001
