Research Article Volume 5 Issue 4

^{1}Centro de Investigación y Desarrollo en Mecánica, Instituto Nacional de Tecnología Industrial ^{2}Instituto de Tecnologías y Ciencias de la Ingeniería, Universidad de Buenos Aires-CONICET, Argentina^{3}Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina

**Correspondence:** Leonardo Nicolás Tufaro, Centro de Investigación y Desarrollo en Mecánica, Instituto Nacional de Tecnología Industrial. Av. Gral. Paz 5445, B1650KNA, San Martín, Buenos Aires, Argentina

Received: August 06, 2021 | Published: August 23, 2021

**Citation: **Tufaro LN, Buglioni L, Svoboda HG. Effect of welding parameters on heat generation mechanisms in friction stir welding of AA5083 aluminium alloy. *Material Sci & Eng*. 2021;5(4):124-132. DOI: 10.15406/mseij.2021.05.00167

Friction Stir Welding (FSW) is a solid-state welding process which has revolutionized several industries in the last thirty years. The heat needed to produce the joint is generated by friction at the tool-workpiece interface and plastic deformation of the material, being affected by parameters like tool rotational speed, travel speed, axial load and tool geometry, among others. The objective of this work was to analyse the energy transformations during FSW of AA5083 aluminium alloy in order to achieve a better comprehension of the whole process, and particularly the heat generation mechanisms and material flow, which will define the quality of the joint. Experimental measurements of process parameters and conceptual analytical and numerical models were implemented. Consumed electric current, axial load and thermal cycles were acquired during FSW of AA5083 plates, for different rotational and travel speeds. From these measurements, different magnitudes (heat and torque) associated to the energy transformations were calculated. The relationships between welding parameters, generated heat and torque, sticking and sliding components and contact condition were analysed and related with material flow. The sticking heat was always higher than the sliding one, even when contact condition was closer to pure sliding. For a giving heat input, a higher sticking component produced an increase in the material flow.

** ****Keywords:** axial load, thermal cycles, heat, torque, sticking, sliding

Friction Stir Welding (FSW) is a welding process in solid phase, which has become the most important research subject of welding science and technology in the last years. Particularly, it has found numerous applications in joining aluminium alloys for welded structures in industries such as aeronautical and aerospace, automotive and shipbuilding, among others. The heat needed to produce the required plastic flow is generated by the friction at the tool-work piece interface and also by the plastic deformation of the material, being affected by factors such as rotational and travel speeds, axial load and tool geometry, among others.^{1–3 }Colligan & Mishra^{4} proposed a conceptual model for process variables related to heat generation in FSW. As can be stated from this model, frictional stress and plastic flow stress are involved in torque and heat generation, responsible for the established thermal field, which also affect the first mentioned aspects. This interdependency shows the complexity of the heat generation study in this process. Schmidt et al*.*^{5} developed an analytical model for heat generation in FSW, being the most important difference with some pre-existent models the consideration of the contact condition at the interface through a dimensionless contact state parameter. This parameter is defined as the ratio between the stirred material velocity and the tool velocity at the interface, which can be called sticking rate (*d*).^{6} When contact condition is pure sticking *d* is 1, and when contact condition is pure sliding *d* is 0.^{3,5,6}

Finally, it is important to note that the heat generated due to plastic deformation is in essence a volumetric heat flux, which can be modelled in that way in fully coupled thermo-mechanical or computational fluid dynamics (CFD) models.^{3,10,11} In thermal models, this heat can be considered as a surface heat flux at the tool-workpiece interface like in the analytical model of Schmidt et al*.*^{5} used in this work.^{3,5,10} Recently, some authors have performed experimental measurements of different parameters such as torque, axial load and thermal cycles during FSW of aluminium alloys, which have been used with analytical and numerical models to analyse the generation and transfer of heat during this process.^{3,5,7,9,12–16} Despite the significant contribution of these investigations to this subject, it has not been reached yet a full understanding of the different energetic transformations during FSW. The aim of this work has been to perform a comprehensive analysis of the energy transformations during FSW of AA5083 aluminium alloy, through experimental measurements of electrical current, axial load and thermal cycles during welding and simple analytical and numerical models, to reach a large and better understanding of the effect of welding parameter on the different mechanisms of heat generation and material flow. In this way, from experimental measurements and simple analytical and numerical models it was proposed a methodology to quantify the magnitudes associated to these transformations, with the purpose to achieve a better understanding of the friction stir welding phenomena, as a previous instance to the development and analysis of more complex numerical models. In particular, although the sticking and sliding components of the generated heat and torque were taken into account in models of other works, in general the magnitude of each component and the effect of the variation of the contact condition on them were not analysed. In contrast, it was an objective of this work to quantify the sticking rate and each component of heat and torque, which could be useful in the optimization of the welding procedure as Arora et al. Proposed,^{8} through a better understanding of the effect of welding parameters on these aspects. These concepts are not discussed enough in the literature. Finally, it was aimed to correlate this sticking and sliding heat components with the material flow, considering its relationship with the quality of welded joints.

**Problem description**

In Figure 1 is shown a graphical representation of the energetic balance during FSW process, in which the magnitudes can be considered values of energy per unit length (J/mm). In this scheme, it can be appreciated the energy transformations and losses which take place in FSW, from the consumed electrical energy by the machine to the net heat input which produces the thermal field in the welded sample (plates). This energy transformations that take place in FSW can be divided in three stages. The first one is associated to the machine energetic balance, in which the difference between consumed electrical energy and total mechanical work (*W _{Total}*) generated at the tool-workpiece interface (total torque ´ pitch) is associated to electrical and mechanical losses at the machine itself, i.e. the machine efficiency (

${Q}_{Total}=\frac{2}{3}\pi .\omega \left[\delta .\frac{{\sigma}_{y}}{\sqrt{3}}+\left(1-\delta \right)\mu .p\right]\left[\left({R}_{s}{}^{3}-{R}_{p}{}^{3}\right)\left(1+\mathrm{tan}\alpha \right)+{R}_{p}{}^{3}+3{R}_{p}{}^{2}{H}_{p}\right]$ (1)

This model is a function of rotational speed (*w*), shoulder radius (*R _{s}*), pin radius (

**Welding and data acquisition **

AA5083 plates of 150´75´3mm were butt welded with FSW process with different operating conditions, using an adapted milling machine. In Table 1 are shown the welding parameters used for the six different analysed conditions, as well as the resulting pitch number. The *w* and *U* were varied, whereas tool tilt angle was kept constant in 1.5° The employed tool was built with H13 tool steel with a smooth tapered pin and a concave shoulder. The pin length was 2.8mm, the shoulder diameter was 12mm and major and minor diameters were 4 mm and 3mm, respectively. During FSW, different process variables were acquired using a data acquisition card of 4 differential channels. Firstly, thermal cycles were measured with two K type thermocouples (TC1 and TC2), which were placed at mid length of the sample, at the advancing side. These were located at different distances of weld centreline, TC1 between 7 and 8mm and TC2 between 11 and 12mm, inside holes with 1mm diameter and 2mm depth. In Figure 2A is presented the FSW a detail of the thermocouples (TCs) location respect to weld centreline, whereas in Figure 2B is shown a welded sample with the experimental set up. Secondly, the line current (*I _{L}*) of the asynchronous three-phase motor of the FSW machine was obtained by using a current transducer with a 10mV/A sensitivity and calculating the signal root mean square (rms) value. Finally, the axial loads (

**Figure 2 **A) TCs locations for thermal cycles acquisition during FSW, B) Welded sample and FSW experimental set up.

**Finite element modelling of heat transfer**

To obtain net values of power (*Q*) and heat input (*H*) from acquired thermal cycles, the heat transfer during welding process was modelled by finite element method (*FEM*). To achieve this, three-dimensional heat transfer was considered only by conduction in the plates, solving the model in a transient way.^{17} The thermal properties of the material were considered constants, whose values were a density of 2660kg/m^{3}, a thermal conductivity of 117W/(m.K) and a specific heat of 900J/(kg.K).^{18 }Heat density generated by the friction between tool shoulder and plates increases with tangential velocity, which is indeed the product between rotational speed and distance to the heat source centre.^{6,10,12} For this reason, in the model it was used a spatial distribution of heat source, linear with distance to tool centre. The heat losses to the ambient by conduction to the backing plate and by convection in the remaining boundary surfaces were considered. Dissipated heat by convection depends on convection coefficient between plate boundary and ambient, which has been considered of 20W/(m^{2}.K).^{19} It was supposed also that heat transfer towards backing plate is with a large enough solid to not modify its temperature (20°C).^{12} This dissipated heat depends on conduction coefficient, which was determined experimentally when finite element model was calibrated. The conduction coefficient resulted to be 200 W/(m^{2}.K), which is similar to the value reported by other authors.^{12} Net power (*Q*) was obtained matching the thermal cycles with the numerically obtained ones corresponding to the location of each thermocouple. Then, the neat heat input (*H*) was calculated as the ratio between net power (*Q*) and travel speed (*U*).^{17}

**Macrographic analysis of welded joints**

Cross section samples were extracted from the middle length of different welded joints and were prepared for metallographic observation. By means of Light Microscopy (LM) different characteristics of the welding nugget (*WN*) were analysed. Particularly, the welding nugget area (*WNA*) for each welding condition was calculated using an image analysis software. Other aspects related to material flow were also assessed, as penetration or defects presence.

**Thermal cycles **

Acquired thermal cycles for the studied welding conditions for extreme values of pitch number (4.7 and 12.4) are shown in Figure 3. It can be seen significant variations in peak temperatures, cooling rates and duration of the thermal cycles between the maximum pitch value and the minimum one, showing the influence of welding conditions on the generated thermal fields. In Table 2 the characteristic values obtained from the acquired thermal cycles for the different welding conditions are shown. The effective positions referred to weld centreline (*y _{TC1}* and

**Figure 3** Acquired thermal cycles for extreme pitch values: A) Pitch 4.7 (680-146), B) Pitch 12.4 (903-73).

**Consumed electrical power and axial load**

Consumed electrical power were calculated from the current measured values using the expression which corresponds to an asynchronous three-phase motor, which is shown in Equation 2, where line tension (*V _{L}*) was 380 V, line current (

$P=\sqrt{3}.{V}_{L}.{I}_{L}.\mathrm{cos}\phi $ (2)

In Figure 4 are shown the values of consumed electrical power (*P*) and axial load (*F0*) against time for the extreme conditions of pitch number (4.7 and 12.4). To ease the comparison between both process variables, consumed power does not include the base power (*P _{0}*) associated to unloaded condition. At the beginning of the welding, during the tool insertion, it was observed that

${Q}_{Sliding}=\left(1-\delta \right)\frac{2}{3}\pi .\omega .\mu .\left(\frac{F}{\pi .{R}_{s}{}^{2}}\right)\left[\left({R}_{s}{}^{3}-{R}_{p}{}^{3}\right)\left(1+\mathrm{tan}\alpha \right)+{R}_{p}{}^{3}+3{R}_{p}{}^{2}{H}_{p}\right]$ (3)

**Figure 4 **Consumed electrical power and axial load records for extreme pitch values: A) Pitch 4.7 (680-146), B) Pitch 12.4 (903-73).

On this expression, mean contact pressure (*p*) is calculated as the ratio between axial load (*F*) and the projected tool shoulder area.^{3,5} Concerning friction coefficient (*m*), in bibliography can be found a wide range of values used in FSW heat generation models, depending on relative displacement velocity, pressure and temperature.^{3,25} In this work, it was used a coefficient of 0.1. Finally, the mean power values *̄P̅* and *Q̅ _{Sliding}*

**Heat generation mechanisms**

Figure 5 shows the variation of consumed electrical energy *(E)* and net heat input (*H _{Net}*) with the pitch number. It can be appreciated that

**Figure 5** Consumed electrical energy (*E*) and net heat input (*H _{Net}*) vs. pitch number. Note: Full markers corresponds to

As mentioned before, thermal efficiency (*h** _{T}*) is found commonly between 0.7 and 0.9.

${Q}_{Sticking}=\delta \frac{2}{3}\pi .\omega .\frac{{\sigma}_{y}}{\sqrt{3}}\left[\left({R}_{s}{}^{3}-{R}_{p}{}^{3}\right)\left(1+\mathrm{tan}\alpha \right)+{R}_{p}{}^{3}+3{R}_{p}{}^{2}{H}_{p}\right]$ (4)

**Figure 6** Total generated heat (*H _{Total}*), generated heat for pure sliding (

Finally, generated heat for pure sticking (*H _{Stiking}*

${H}_{Total}={H}_{Sticking}+{H}_{Sliding}=\delta .{H}_{Sticking}^{\delta =1}+\left(\delta -1\right).{H}_{Sliding}^{\delta =0}$ (5)

This equation also shows that limit values for *H _{Total}* are the

**Figure 7** Total generated heat (*H _{Total}*), sliding generated heat (

It can be observed that for both cases *H _{Sticking}* was greater than

Finally, it was possible to obtain the exclusive FSW process efficiency (*h** _{FSW}*), calculating the ratio between

**Material flow**

Cross sections macrographs of the welding joints are shown in Figure 8, indicating the calculated *WNA* for each case. Defect free and full penetration joints were obtained for all welding conditions. A defect free *WN* indicates that an adequate material flow is achieved during the welding process. Changes in the shape and size of the *WN* with the *w* and *U* can be appreciated. The material flow in the WN is composed by two flows, one related with the tool shoulder action (upper zone and wider) and other with the tool pin one (lower central zone and narrower).^{29} These variations in the *WN* were observed in both zones, thus the components of sticking and sliding would affect both types of material flow. For the lower *U*, the shape is smoother, and the size is bigger, which are related with a higher volume of stirred material. An increase in *U*, generates a sharper and smaller *WN, *especially for 903rpm. Figure 9A shows the welding nugget area (*WNA*) vs. pitch. The *WNA* rises linearly with the pitch number for the same *w* (or *d*), similarly to the observed for the *H _{Total}* vs pitch (Figure 6). This behaviour is related to the fact that a decrease in

- From experimental measurements of electrical current, axial load and thermal cycles during FSW of AA5083 plates for different rotational and travel speeds, and simple analytical and numerical models it was proposed an original methodology to quantify the magnitudes associated to the energy transformations which take place during FSW in order to perform an analysis of the heat generation mechanisms and its effects in material flow.
- Consumed electrical energy and the net heat input increased linearly with pitch number. There were obtained the experimental expressions with good correlation factor and the total process efficiency.
- Variations in contact condition at the interface tool-work piece was evaluated. Sticking rate is defined by tool rotational speed, decreasing with its increase. The obtained values for 680 and 903rpm were 0.55 and 0.31, respectively.
- Sticking and sliding generated heat components were estimated, as well as its variation with pitch number. As the sticking rate decreases, the total and sticking generated heat decreases and the sliding generated heat increases. Despite of that, sticking heat was greater than sliding heat for the analysed conditions. Contact condition closer to sliding do not imply that the sliding heat is greater than sticking heat, because the generated heat for pure sliding (
*d*=0) is the lower limit for total generated heat. This fact indicates that the main heat generation mechanism in FSW would be in general sticking, associated to the plastic deformation of the material. - The slope of the linear equations obtained were related to the torque. Both components of torque were calculated, showing a similar behaviour to the generated heat. Total torque resulted to be 6.6 and 5.0 Nm for 680 and 903rpm, respectively, which means that total torque decreases with rotational speed. Despite the effect of temperature on material flow stress, this behaviour is mainly due the sticking rate decrease.
- The weld nugget area was measured for the different welding conditions, decreasing with rotational and travel speed. There was shown that the sticking generated heat controls the weld nugget area, directly affecting the material flow.
- The obtained results agree with the tendencies previously observed for other authors, validating the proposed methodology. It can be extended to other materials and welding conditions, being a useful tool to reach a deeper understanding of the FSW process and to optimize the welding procedure.

The authors acknowledge to Universidad de Buenos Aires, Agencia Nacional de Promoción Científica y Tecnológica (ANPCYT) and Instituto Nacional de Tecnología Industrial for the funding and the facilities in order to develop this work and also to the personnel of Instituto de Tecnologías y Ciencias de la Ingeniería and Centro de Investigación y Desarrollo en Mecánica for its collaboration.

The authors declare that there is no conflict of interest.

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