Temperature and force response characteristics of friction stir welding ...

Temperature and force response characteristics of friction stir welding on Invar 36 alloy Y. Zhao*1, A. P. Wu2, J. L. Ren2, Y. S. Sato3, H. Kokawa3, M. Miyake3 and D. Y. Yan4 The temperature and force response during friction stir welding of Invar 36 alloy were investigated by experimental measurement and numerical simulation. The effect of welding parameters was studied. The temperature and force characteristics were roughly discussed. It is indicated that an elevation rotational speed results in increasing temperature and decreasing axial force, whereas rotational speed has no obvious influence on the longitudinal force. An elevation travelling speed produces increasing axial force and longitudinal force, and a decreased trend of temperature out of stir zone centre. Friction stir welding of Invar 36 alloy produced relatively high peak temperatures, high axial and longitudinal forces, and a narrow temperature distribution compared with those of the other reported high melting temperature materials. Keywords: Friction stir welding, Invar 36 alloy, Temperature, Force

Introduction Friction stir welding, as a solid state welding process, has generated considerable research interest due to its excellent weld properties and low environmental impact. It has been widely researched and successfully applied for joining of low melting temperature materials, such as aluminium alloys.1,2 In recent years, friction stir welding for high melting temperature materials, such as steel and titanium alloy,3,4 has been attracting increasingly research interests. Although it has been applied for some practical productions, the joining mechanism of friction stir welding is not quite clear enough at present. It is widely accepted that, during friction stir welding, the base material concurrently experiences high temperature and severe plastic deformation produced by a rotating welding tool, and a sound weld with equiaxed recrystallised microstructure is eventually acquired.1 Therefore, the welding temperature and forces during the welding process, which could reflect heat generation and material flow in some aspect, play an important role in revealing the friction stir welding processing. Previous studies have shown that the welding temperatures of aluminium alloys are generally around 400–500uC.1,3 For high melting temperature materials, 1

School of Aerospace, Tsinghua University, Beijing 100084, China Department of Mechanical Engineering, Tsinghua University, Key Laboratory for Advanced Materials Processing Technology, Ministry of Education, Beijing 10084, China 3 Department of Materials Processing, Graduate School of Engineering, Tohoku University, 6-6-02 Aramaki aza Aoba, Aoba ku, Sendai 980-8579, Japan 4 Beijing Institute of Astronautics System Engineering, Beijing 10076, China 2

*Corresponding author, email [email protected]

the peak welding temperatures are generally higher than those of aluminium alloys, as shown in Table 1.5–8 The process forces during friction stir welding mainly include axial force (down force/load) and longitudinal force (translational force) in the previous studies, as also shown in Table 1.5,6,9–14 Axial force is considered to have the most direct impact on the heat generation and the weld penetration. Generally, in order to acquire full penetration and defect free welds, higher axial forces were necessary for high melting temperature materials than those of aluminium alloys. By contrast, the longitudinal forces are significantly smaller, only about one-seventieth to one-eightieth of the axial force for aluminium alloys,14 indicated by the limited reports. In addition, the longitudinal force is implied to have some relations with the surface defect.14,15 Although several studies have been carried out for the temperature and forces during friction stir welding, the temperature and force researches on high melting temperature materials are relatively insufficient, and the influence of welding parameters on the temperature and forces needs to be further clarified. This paper focuses on the temperature and forces during friction stir welding of Invar 36 alloy. Invar 36 alloy (Fe–36 wt-%Ni) is a high melting temperature material, which is increasingly used as an important structural material for liquefied natural gas storage and transport.16,17 Friction stir welding of Invar 36 alloy18,19 has become a worthwhile topic since it has effectively solved the solidification cracking problem that is easily induced for conventional arc welding. Nonetheless, further research on these topics, such as the welding temperature and forces, is lacking. Moreover, Invar 36 alloy has weak capability of thermal conduction, since

ß 2013 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 6 April 2012; accepted 6 October 2012 DOI 10.1179/1362171812Y.0000000077

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1 a picture and b geometric dimension of PCBN tool

its heat conductivity and thermal diffusion coefficient are only 11 W m21 K21 and 2?6461026 m2 s21 respectively.20 It also has relatively high flow stress at high temperature and strain rate, since its flow stress reaches 130–231 MPa under the condition of temperature of 1100uC, strain rate of 10 s21 and strain of 0?1–0?3.21 These features should have some influence on the welding temperature and force response during the welding process. The aim of the present study is to clarify the temperatures and forces during friction stir welding of Invar 36 alloy. Welding temperature and forces during friction stir welding of Invar 36 alloy under different welding parameters were measured and a basic numerical modelling was also applied to aid the temperature analysis. The temperature and force characteristics were also roughly discussed.

Experimental The base material applied in the present study is 3 mm thick Invar 36 alloy sheet. Its chemical composition is Fe–36?0Ni– 0?3Mn–0?19Si–0?03C–0?001S–0?002P (wt-%). A polycrystalline cubic boron nitride (PCBN) tool was used to perform the bead on plate friction stir welding. Geometric dimension of the PCBN tool is shown in Fig. 1; it has a convex shoulder with step spiral pattern having a diameter of 15?3 mm and a 2 mm length tapered pin. The pin tapered from 6?9 mm at the shoulder to 4?1 mm at the tip. The plunge depth was constantly controlled at y2?45 mm. The welding parameters were 1–5 mm s21 for travelling speed v and 100–1000 rev min21 for rotational speed v. Axial force and longitudinal force were measured during the welding process by force transducers equipped on the spindle.

Infrared temperature tests were conducted to measure the welding temperature during the whole welding process for all welding parameters by an infrared temperature camera. The infrared temperature test position was the front junction of the tool and the base material surface, as shown in Fig. 2. Thermocouple temperature test in the stir zone was also attempted for several welds in the position as shown in Fig. 2. Infrared temperature measurement is straightforward and efficient and can easily be used to record the real time temperature during the whole welding process. However, only the surface temperature could be detected by this method. Thermocouple temperature test method is able to detect the material temperature precisely at a certain point inside the weld but is extremely difficult to be successfully performed in the intense plastic deforming stir zone centre. In order to acquire the peak welding temperature and full scale temperature distribution for all welding parameters, the finite element analysis was also adopted in the present study. Finite element analysis was executed by the software of ABAQUS/Standard. A sheet with the geometric dimension of 150612063 mm (L6W6H) was used for the simulation, and the position of weld was along the length direction in the middle of wide direct of the sheet. The geometry models were all meshed by eightnode hexahedron elements in the modelling process. Meshes with the dimension of 1?2561?561 mm (L6W6H) were applied for the weld, while coarser meshes were used for other area. A sequentially coupled thermomechanical model was used for simulation. The heat source model was

Table 1 Reported peak temperatures and forces of different materials Material

Travelling Peak Axial Longitudinal Plate Shoulder Rotational Reference thickness/mm diameter/mm speed/rev min21 speed/mm s21 temperature/uC force/kN force/kN

304/304L stainless steel

3 3?2 3 Ti–6Al–4V 12* 5 Pure Ti 2 1018 mild steel 6?3 6056 Al alloy 3 7020 Al alloy 4?4 6061 Al alloy 6?35

16 19 19?05 25 y20 15 19 13 20 12?7

750–950 300–500 300 200–400 300 200 450–650 1850 1400 1500–4500

0?63–1 1?7 1?693 0?83–2?5 1?67 0?83–5 0?42 11?67 1?33 6?8–26?8

… … 900–1000 1004–1149 … 498–843 1100 … … …

9 31 …

… … … … 31 … … … 18?7 … 8 … 7?4–8?8 … … (0?15

12 10 8 6 11 7 5 9 13 14

*Actual weld depth y6 mm.


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2 Schematic position

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established based on the shear yield strength of the base material. The friction on the interface between welding tool and base material was assumed as sliding friction at the low temperature stage during welding. Since yield strength decrease with the increasing of welding temperature, when the welding temperature reaches a critical temperature T0, the acting force between the tool and base material should be equal to the yield stress. The friction was assumed as sticking friction after the welding temperature reaches T0. Thus, the acting force on the interface between the tool and base material could be expressed as (TƒT0 ) mFz Ff ~ (1) : t(T)~0 577s(T) (TwT0 ) where T is temperature, t is the shear yield strength, s is the normal yield stress of base material, Ff is the action force on the interface, Fz is the axial force of tool and m is the coefficient of sliding friction. In this case, the heat generation rate was expressed as Q~gW ~gMz v~ 0 ð B 0:577s(T)rdsz gv@ S(T§T0 )

ð

1 C mFz rdsA


(2)

S(TvT0 )

where Q is the heat generation rate, W is the tool mechanical power, g is the efficiency of mechanical work transfer to heat, Mz is the tool torque, v is the rotational speed, r is the distance from a point on the interface to the tool rotation axis and S(T>T0) and S(T,T0) are the area of the regions on the interface in which the temperature is equal to or higher than T0 and lower than T0 respectively. This heat source model was also successfully applied on aluminium alloys.22,23 Yield strength of Invar 36 alloy was measured by Gleeble-1500D at different temperatures and strain rates. According to these measurement results, the yield stress applied to numerical simulation was set as three stages: linear decreases from 320 to 176 MPa in the temperature range from 20 to 200uC, from 176 to 16 MPa in the temperature range from 200 to 1200uC and finally from 16 to y0 MPa in the temperature range from 1200 to 1450uC. g was set as 0?95. Density, specific heat, thermal conductivity and melting temperature of Invar 36 alloy were set as 8100 kg m23, 515 J kg21 uC21, 11 W m21 uC21 and 1450uC respectively.20 Temperature dependences of these properties were not considered in order to simplify the present simulation. The heat transfer coefficient in the top and side surface was varied from 15 to

3 Typical temperature and (200 rev min21, 1 mm s21)

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160 W m22 uC21 in the temperature range of 20–800uC and that of the lower surface was varied from 20 to 15 000 W m22 uC21 in the temperature range of 20– 350uC.

Results and discussion Typical infrared temperature and force response Typical profiles of infrared temperature, axial force and longitudinal force versus time in the same welding condition are shown in Fig. 3. Significant increase and then a fluctuation for the infrared temperature and axial force were observed during the plunge process at the starting point of weld. Fluctuation of axial force, temperature as well as longitudinal force also occurred when the tool began moving along the welding direction after the plunge process. Then, a long term and relatively stable stage was observed after the fluctuation stage mentioned above. The axial force, longitudinal force and infrared temperature kept a fairly stable value until the tool reached the end of the weld. The stable stage of axial force, longitudinal force and infrared temperature appeared in the same period of time during welding; this period was defined as the stable welding stage in this paper. In addition, the longitudinal force is lower than the axial force, which is consistent with the previous reports.14 The average infrared temperature, axial force and longitudinal force in the following parts were defined as the average values of those at the stable welding stage respectively.

Temperature at various welding parameters Temperature measurement

The average infrared temperatures versus rotational speed and travelling speed were shown in Fig. 4. The average infrared temperatures vary from 658 to 1160uC under the welding parameters of rotational speed of 100– 1000 rev min21 and travelling speed of 1–5 mm s21. Average infrared temperatures obviously increase with an increase in rotational speed and have a decrease trend with an increase in travelling speed. When the rotational speed varies from 600 to 1000 rev min21 and the travelling speed is >3 mm s21, the changes of average infrared temperature were relatively not obvious, which were varied from 870 to 957uC. Several thermocouple temperature tests were executed in the position of interior stir zone; however, since the


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4 Effect of a rotational speed and b travelling speed on average infrared temperature

extremely severe plastic deformation was in the stir zone, most of the thermocouples were damaged during the testing procedures. Just one valid thermocouple temperature test result was acquired. The result reveals that the peak temperature in stir zone reached 1200uC of the ‘1000 rev min21, 3 mm s21’ weld. Simulated temperature estimation

In order to estimate the temperature distribution of the joint, the temperature field computation of friction stir welding of Invar 36 alloy was also applied to aid the temperature analysis. The validity of computational results is verified by comparing the simulated results with the experimental temperature test results at the same point of the joint. As shown in Fig. 5a, the variation tendency of the computational temperatures at different welding parameters is similar to that of the infrared temperatures, i.e. the temperature increases with increasing rotational speed. The computational temperature is close to the thermocouple tested temperature in the same position but is generally y100uC higher than the infrared temperature, because the temperature reduction is very fast out of the ideal infrared temperature test position, as shown in Fig. 5b. There should be more or less some positional deviation during actual infrared temperature test process. These characteristics indicate that the numerical simulation method acquired reasonable temperature results of friction stir welding of Invar 36 alloy.

The numerical computational peak temperatures in stir zones are shown in Fig. 6. Peak temperatures vary from 1010 to 1309uC and increase with increasing rotational speed. Travelling speed has no obvious influence on the calculated peak temperatures. The peak temperature increase tendency became weaker when the rotational speed was .600 rev min21. Calculated temperature field of friction stir welding of Invar 36 alloy under the welding parameters of rotational speed of 1000 rev min21 and travelling speed of 3 mm s21 was shown in Fig. 7a and b. In the top surface, the temperature field became wider following the tool, and the temperature in the rear part under the shoulder is slightly higher than that of the front part. In the cross-section, the highest temperature appeared next to the shoulder, and the temperature field in the advancing side and retreating side is symmetric. As can be predicted, Invar 36 alloy has quite narrow temperature distribution, i.e. the narrow high temperature range and big temperature gradient, compared with the report of that of the aluminium,1,3,22 which was attributed to its low thermal diffusion coefficient. In the present welding condition, the narrow temperature distribution probably leads to a relative high cavity defect susceptibility of friction stir welding of Invar 36 alloy, since the volume of softening material that was used to material flow during friction stir welding is closely related to the temperature distribution. The narrow high temperature range and big

5 a comparison of experimental measured temperatures and numerically simulated temperatures and b simulated temperature distribution of longitudinal section in position of top surface along welding line


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6 Peak welding temperatures

temperature gradient probably results in insufficient softening material, where flowing material cannot fully fill the area behind the welding tool, and finally produces cavity defects in the stir zone. Cavity defects were observed in the ‘1000 rev min21, 3 mm s21’ weld as shown in Fig. 7c.

Axial force at various welding parameters The average axial forces versus rotational speed and travelling speed were shown in Fig. 8. The average axial forces vary from 10 to 30 kN, and decrease with an increase in rotational speed and increase with an increase in travelling speed. It is worth to note that when the rotational speed varies from 600 to 1000 rev min21 and travelling speed is >3 mm s21, the changes of average axial forces were not quite obvious, which were varied from 17 to 20 kN. This is corresponding with the variation tendency of infrared temperature (as shown in the section on ‘Temperature at various welding parameters’), which indicates the relation between welding temperature and axial force. The relationship between average axial force and infrared temperature at different welding parameters is shown in Fig. 9. Axial force could be considered as a reflection of material flow stress in some aspect. Therefore, the axial forces decrease with an increase in temperature is consistent with the relationship between material flow stress and temperature. When the base material and welding tool are the same, the main factors to influence axial force are


7 Temperature field in a top surface and b cross-section and c cavity defects in weld acquired by same welding parameter (1000 rev min21, 3 mm s21)

welding parameters. Different welding parameters (such as rotational speed, travelling speed and plunge depth) through influencing physical effects (such as temperature and strain rate) finally produce different axial force responses. The plunge depth was fixed in the present study; hence, increase in rotational speed and reduced travelling speed lead to an elevated temperature as well as an elevated strain rate (the effect of travelling speed on strain rate was ignored here since the strain rate produced by travelling speed is much smaller than that produced by rotational speed). It needs to be noticed that the elevated temperature and elevated strain rate have opposite effects on the material flow stress, which lead to the decrease and increase in material flow stress respectively. One of these two factors should be the dominant factor in the axial force. As shown in Fig. 8a, an elevated rotational speed produces a decreased axial force during friction stir welding of Invar 36 alloy, which indicates the temperature change plays the dominant role on axial force. This also explains the same variation tendency of temperature and axial force when the rotational speed is in the range of 600–1000 rev min21 and the travelling speed is >3 mm s21.

Longitudinal force at various welding parameters The average longitudinal forces versus rotational speed and travelling speed are shown in Fig. 10. The average

8 Effect of a rotational speed and b travelling speed on average axial force


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9 Relationship between axial force and welding temperature

longitudinal forces vary from 0?6 to 3?5 kN, and increase with the increase in travelling speed, but are not obviously affected by the rotational speed. The longitudinal force is not only related to the material flow stress in stir zone but also significantly affected by the material flow stress in advance of the tool. The material flow stress in advance of the tool has close relation to the temperature distribution in that area. The elevation in the travelling speed will result in the reduction of high temperature range, an increase in the temperature gradient, as well as the faster tool pass through its advance area. Therefore, an increase in travelling speed produces an elevated longitudinal force as shown in Fig. 10b. On the other hand, theoretically, increased rotational speed will widen the high temperature range and produces a decreased longitudinal speed. However, the high temperature range is very narrow in this study because Invar 36 alloy has an extremely low thermal diffusion coefficient, as shown in the section on ‘Simulated temperature estimation’. The low thermal diffusion capability of the base material weakens the positive interaction of rotational speed on the temperature distribution in advance of the welding tool. This is a possible reason why the rotational speed has no obvious influence on the longitudinal force.

Characteristics of welding temperature and force response It is necessary to note that to analyse the characteristics of welding temperature and force response of Invar 36


11 Peak temperatures of Invar 36 alloy and other high melting temperature alloys

alloy, the above mentioned results should be compared with those of other materials. However, as shown in Table 1, it is difficult to find temperature and force reports of different materials that are friction stir welded in the same welding condition from the existing references, especially that the welding tool material and geometry are various. In this case, the following analysis and discussion is generally a rough comparison based on the existing research condition; more strict study is required in the future. A pseudo-heat index w[w5v/(n6104)] was introduced here to normalise the influence of rotational speed and travelling speed according to Arbegast and Hartley’s study.24 Peak temperatures during friction stir welding of Invar 36 alloy and the other reported high melting temperature materials versus the pseudo-heat index w are shown in Fig. 11. The figure reveals that the peak temperature of Invar 36 alloy increases with the increasing of w. In the same w, the peak temperatures of Invar 36 alloy were generally higher than those of the pure titanium and 1018 mild steel, and close to that of the 304L stainless steel and Ti–6Al–4V alloy. This characteristic is probably correlated with the following aspects. First, the thermal diffusion coefficient of Invar 36 alloy (2?6461026 m2 s21) is relatively lower than other materials,20,25 which leads to relatively less heat dissipation, and results in the higher temperature in stir zone centre. Second, it is also necessary to consider the tool material and geometry for different studies.

10 Effect of a rotational speed and b travelling speed on average longitudinal force


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12 Axial force in unit shoulder area of Invar 36 alloy and other alloys

Besides Invar 36 alloy, the peak temperature data reported in Fig. 11 are all acquired by tungsten base tools with shoulder diameter from 15 to 25 mm, which have lower thermal conductivity of tool material and generally bigger shoulder diameter than the PCBN tool applied for Invar 36 alloy in the present study. These features of tools should lead to higher peak temperature for other high melting temperature materials. However, as shown in Fig. 11, the results are basically opposite. This means the tool material and shoulder diameter are probably not dominant factor on the peak welding temperature for the existing researches, while the thermal conductivity of base materials is probably the key factor on the peak welding temperature. The welding axial forces for different high melting temperature materials are shown in Fig. 12. Besides pseudo-heat index w, the axial force divided by tool shoulder area (Fz/Ashoulder) was also applied to eliminate the influence of tool geometry on the axial force in different researches as far as possible. It reveals that for the same w, Invar 36 alloy generally has relatively high values of Fz/Ashoulder. The higher flowing stress of Invar 36 alloy in the welding condition is probably a reason for this characteristic. However, since the tool materials are various in Fig. 12, and the influence of tool material on the axial force is not very clear yet, the dominant reason for the axial force characteristic is difficult to discuss.

Conclusions Stable stages of temperature, axial force and longitudinal force were examined during friction stir welding process of Invar 36 alloy. The welding temperature increased with an increase in rotational speed and tended to decrease with an increase in travelling speed out of the stir zone centre. The peak temperature in the stir zone reached 1010–1309uC at different welding parameters. The temperature distribution during friction stir welding of Invar 36 alloy was very narrow due to its low thermal diffusion coefficient. Average axial force varied from 10 to 30 kN during friction stir welding of Invar 36 alloy at different welding parameters. The axial forces decreased with the increase in rotational speed and increased with the increase in travelling speed. The variation range shrunk when the


rotational speed is .600 rev min21 and travelling speed is .3 mm s21. The average axial forces decreased with the increase in welding temperature. The longitudinal force varied from 0?6 to 3?5 kN for friction stir welding of Invar 36 alloy. The longitudinal forces increased with the increase in travelling speed; however, they had no obvious influence by the rotational speed. This is probably related to the narrow temperature distribution that was attributed to the low thermal diffusion coefficient. Peak temperature and axial force response during friction stir welding of Invar 36 alloy are relatively higher than those reports of other high melting temperature materials. It is probably attributed to the lower thermal diffusion coefficient and higher flow stress under the welding condition respectively. However, the influence of tool geometry and material should not be neglected; therefore, it is necessary to make further research about the influence of base material properties and welding tool on welding temperature and force response in the future.

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