WO2023119329A1

WO2023119329A1 - Forced cooling friction stir welding tool and method thereof

Info

Publication number: WO2023119329A1
Application number: PCT/IN2022/051114
Authority: WO
Inventors: Muthukumaran SHANMUGAM; Dinesh Kumar Rajendran
Original assignee: National Institute of Technology Tiruchirappalli
Priority date: 2021-12-22
Filing date: 2022-12-21
Publication date: 2023-06-29

Abstract

The present invention relates to the field of welding. More particularly, it relates to friction stir welding tool having a means for cooling. It discloses a friction stir welding tool comprising a primary tool holder [101], a rotatable secondary tool holder [102], a shoulder [103] at a distal end of welding tool body [109], a pin [104], and a cooling element or cooling unit [105, 106]. Advantageously, the present invention gives a method for carrying out a welding process under normal cooled, forced air cooled and water cooled conditions.

Description

FORCED COOLING FRICTION STIR WELDING TOOL AND METHOD THEREOF

FIELD OF THE INVENTION

The present invention relates to the field of welding. More particularly, it relates to friction stir welding apparatus having a means for cooling. Advantageously, the present invention offers a method for carrying out a welding process under normal cooled, forced air cooled and water cooled conditions.

BACKGROUND OF THE INVENTION

Joining of thin sheets is gaining a huge momentum due to its vast application and usage. AA6061 being heat treatable aluminium alloy, it is very difficult to be joined by the fusion welding process; and even if welded, they are subjected to defects like porosity, slag inclusion, solidification cracking and distortion. Friction Stir Welding (FSW) was an accidental invention by The Welding Institute (TWI) of UK in 1991 which is now being an ideal solution for most of the conventionally non-weldable aluminium alloys. Aluminium alloys are efficiently utilized for FSW process which has better - joining strength compared to other fusion welding process. At higher rotational speed placing AA6061 on advancing side the tensile strength increases. The fracture surface reveals that the low rotating speed leads to poor mixing resulting in the failure of weld nugget. The tool pin with taper thread is found to have better mixing and strength. The microstructural analysis exhibits high-density precipitates of zinc and magnesium causing hardening of processing zone and high level of mechanical strength. Many researchers reported the effect of tool pin profile on mechanical and metallurgical properties. However, there is limited work on the study of cooling effect on the mechanical and metallurgical property of weld join. This study mainly focuses on the effect of different cooling condition such as normal cooled, water cooled and forced air cooled conditions on the mechanical property of AA6061 sheets. JPW02012101786(A1) discloses “Rotate tool for friction stir welding, and friction stir welding method” the purpose of the invention is to prevent the excess increase in temperature of a tool main body in the friction stir welding of a high-melting-point material such as a steel, thereby prolonging the service life of the tool main body. A heat removing body, which is a copper ring, is fixed between metal plates so as to form a void space between the metal plates in such a manner that the heat removing body can be attached to or detached from an outer peripheral surface of a tool main body and does not contact with the metal plates. The cited patent deals with cooling of tool by placing a copper ring to improve the tool life.

US2018/0281104A1 discloses “Friction stir welding apparatus, friction stir welding control device and friction stir welding method” discloses a friction stir welding apparatus includes a FSW tool that is held by a housing and welds to be welded members to each other by friction stir and gradually cooling device is a contactless heat source that heats the weld site without coming in contact with the weld site. The cited patent deals with contactless heat source that heats the weld without coming in contact.

US6516992B1 discloses “Friction stir welding with simultaneous cooling” A method and apparatus for friction stir welding that produces a weld of significantly reduced surface roughness at significantly higher welding rates in materials that are difficult to weld such as non extrudable aluminium alloys. The method includes cooling the stir welding tool during the welding process; thereby reduce the tendency of soft metal to adhere. The cited patent deals with cooling of tool by passing coolant inside the tool. US7845544B2 discloses “Friction stir welding method and friction stir welding tool” discloses a friction stir welding method for friction stirring and welding a materials by using rotating tool having a shoulder and a pin at a front portion of a shaft portion and ejecting a cooling agent from a cooling nozzle to the tool to be cooled. The cited patent deals with cooling of tool by passing coolant on the tool surface. However, the in our case the invention deals with forced cooling method and it focus on cooling of the workpiece.

US6772935B2 discloses “Method and device for friction stir welding with simultaneous cooling” A method for friction stir welding using liquid cooling. The method includes the steps of moving a pin tool across a welding location, spraying a cooling liquid in a localized manner from a cooling ring moving with the pin tool on the trailing region onto lateral regions of the welding location adjacent to the pin tool, and blowing cooling gas from a gas jet moving with the pin tool from a front of pin tool against the cooling liquid emerging from the cooling ring. The cited patent deals with cooling of trailing plate by passing air or gas through the nozzle to the ring around the tool.

RU2686494C1 discloses “Method of friction welding with mixing of joints of aluminium alloys” This invention can be used for joining parts from aluminium alloys having low weldability. Rotating tool consists of a tip in the form of a body of revolution and a collar is immersed in the joint of the welded parts until the collar contacts their surface and is maintained. Movement of the tool along the butt is switched on so that the tool movement speed along the joint is maintained within range of 0.08-0.48 mm/rev. Directly behind the tool there performed is forced cooling of seam and the zone of thermal influence at the rate of not less than 0.5 Deg. C/s. Cooling is performed by supplying jet of cooling liquid or its mixture with air from sprayer directly behind tool or by immersion of welded parts and working tool into bath with circulating liquid.

Metals 2019, 9, 304 article titled “Effect of Forced Air Cooling on the Microstructures, Tensile Strength, and Hardness Distribution of Dissimilar Friction Stir Welded AA5A06-AA6061 Joints” by Guangjian Peng et al., Both natural cooling (NC) and forced air cooling (FAC) conditions were considered in the FSW process. For FAC, forced air was blown on the welded area through a nozzle. The rectangular nozzle with a size of 10 mm, 2 mm was placed 20 mm behind the tool and about 20 mm above the surface of the materials. The pressure of the forced air was 0.5 MPa, and the blowing direction was along the welding direction and had an intersection angle of 30 with the surface of the materials. The tapered left-hand threaded cylindrical tool used for FSW was made of “H13 steel”, and the dimensions are shown in Figure lb. During the welding process, the tool was plunged into the butt surface of the two base materials, with a tilt angle of 2.8 and a depth of 4.96 mm. Three different tool rotational speeds (RS) — i.e., 600, 900, and 1200 rpm — and two welding transverse speeds (TS) — i.e., 100 and 200 mm/min — were applied. A clockwise rotation direction of the tool was used in the experiments. The cited patent deals with cooling behind the tool by forced cooling of seam by a jet of air passing through a nozzle.

J. of MateriEng and Perform (2012) 21 :1182-1187, article titled “Effect of Water Cooling on the Performances of Friction Stir Welding Heat- Affected Zone” by Huijie Liu et al., In order to improve the mechanical properties of the HAZ by controlling the temperature level, underwater friction stir welding (FSW) of an Al-Cu aluminum alloy was conducted in the present study. The results indicate that the hardness of the HAZ can be improved through underwater FSW. Microstructural analysis reveals that the hardness improvement is attributed to the lowering of precipitate coarsening level and the narrowing of precipitate free zone, which are essentially induced by the variations of welding thermal cycles under the cooling effect of water. The cited article deals with under water friction stir welding to improve the properties.

IOP Conf. Series: Materials Science and Engineering 307 (2018) 012072, article titled “Effect of water-cooling treatment times on properties of friction stir welded joints of 7N01-T4 aluminum alloy” by T H Zhang et al., The weld joints were heated to 350°C by oxy propane flame after welding, and then the water cooling treatment was applied in different times. Specimens with once, twice and thrice water-cooling treatment and specimens with no water-cooling treatment were made for corresponding testing. The microstructures of the joint area were observed by VHX-500FE ultra-high resolution digital microscopy system, and the micro-hardness of the joint area was measured. The cited article deals with the post weld heating and cooling to change the metallurgical and mechanical properties.

International Journal on Theoretical and Applied Research in Mechanical Engineering (IJTARME); ISSN (Print): 2319-3182, Volume -3, Issue-4, 2014, article titled “Influence of Cooling Media on Mechanical Properties of Friction Stir Welded 1060 Aluminium Alloy” by Sunil Pandey et al., welding was carried out at the determined parameters in water as cooling media. Running flow of water at room temperature was used during welding. Specimens for tensile and impact test were taken from the weldament. The cited article deals with carrying out the welding by running flow of water at room temperatures.

Advanced Materials Research Vol. 445 (2012) pp 789-794, article titled “Influences of Tool Pin Profile and Tool Shoulder Curvature on The Formation of Friction Stir Welding Zone in AA6061 Aluminium Alloy” by Vahid Moosabeiki et al., six different tool pin geometries (threadless triangular pin with/without conical shoulder, threaded triangular pin with conical shoulder, threadless square pin with/without conical shoulder, threaded square pin with conical shoulder) are used to fabricate the joints. The formation of FSP zones are analyzed macroscopically. Tensile properties of the joints are evaluated and correlated with the FSP zone formation. Consequently, it is obtained that welding creates a higher quality compared to other tool pin profiles using the square tool with curved shoulder and having threaded pin. The cited article deals with the modification of tool profile to change the heat generation rate during welding.

Materials & design 40:7-16, article titled “Effect of tool rotational speed and pin profile on microstructure and tensile strength of dissimilar friction stir welded AA5083-H111 and AA6351-T6 aluminum alloys” by R. Palanivel et al., the effect of tool rotational speed and pin profile on the microstructure and tensile strength of the joints were studied. Dissimilar joints were made using three different tool rotational speeds of 600 rpm, 950 rpm and 1300 rpm and five different tool pin profiles of straight square (SS), straight hexagon (SH), straight octagon (SO), tapered square (TS), and tapered octagon (TO). Three different regions namely unmixed region, mechanically mixed region and mixed flow region were observed in the weld zone. The tool rotational speed and pin profile considerably influenced the microstructure and tensile strength of the joints. The joint which was fabricated using tool rotational speed of 950 rpm and straight square pin profile yielded highest tensile strength of 273 MPa. The two process parameters affected the joint strength due to variations in material flow behavior, loss of cold work in the HAZ of AA5083 side, dissolution and over aging of precipitates of AA6351 side and formation of macroscopic defects in the weld zone. The cited article deals with the modification of tool profile to change the heat generation rate during welding.

The present invention deals with forced cooling method by a blower/fan attached to the tool body and it focus on cooling of the work piece by blower/fan with or without sprinkling of water. The present invention is different and deals with modification of the metallurgical and mechanical properties by increasing the cooling rate of the weldment during welding by means of forced cooling method.

OBJECT OF THE INVENTION It is the main object of the present invention to provide a Forced cooling based Friction Stir Welding Tool.

It is another object of the present invention to provide a welding apparatus and method for the study on effect of different cooling condition such as normal cooled, water cooled and forced air cooled conditions on the mechanical properties of alloys.

It is another object of the present invention to provide method for friction stir welding of joints under varying welding parameters subjected to cooling conditions.

SUMMARY OF THE INVENTION

One or more of the problems of the conventional prior arts may be overcome by various embodiments of the present invention.

It is the primary aspect of the present invention to provide a friction stir welding tool, comprising of: a primary tool holder; a rotatable secondary tool holder; a tool body; a shoulder at a distal end of welding tool body; and a pin extending downward from the distal end of the welding tool body beyond the shoulder, wherein the tool is configured to comprise a cooling element or cooling unit.

It is another aspect of the present invention to provide a friction stir welding tool, wherein the cooling unit comprises one of a liquid sprinkler, a blower or a fan removably and rotatably fixed at the intersection of the primary tool holder and the shoulder, and combination thereof.

It is another aspect of the present invention to provide a friction stir welding tool, wherein the tool profile of tool body is shoulder diameter 5-500 mm, pin diameter 1- 25 mm, and pin length 0.5-5 mm.

It is another aspect of the present invention to provide a method of forced cooling friction stir welding of alloys / workpiece, comprising of the steps: welding of alloys using a welding tool; and subjecting the weldment or workpiece to forced cooling conditions, wherein the weldment or workpiece is cooled by one of forced air cooling, forced liquid cooling and combination thereof.

It is another aspect of the present invention to provide a method of forced cooling friction stir welding of alloys / workpiece, wherein the forced air cooling comprises of cooling by means of air blower / fan.

It is another aspect of the present invention to provide a method of forced cooling friction stir welding of alloys / workpiece, wherein the forced liquid cooling comprises of cooling by means of liquid sprinkler.

It is another aspect of the present invention to provide a method of forced cooling friction stir welding of alloys / workpiece, wherein the forced cooling comprises of cooling by means of air blower / fan and liquid sprinkler. It is another aspect of the present invention to provide a method of forced cooling friction stir welding of alloys / workpiece, wherein the air blower comprises of a blower / blades or fan rotating at a speed of 100 - 5000 rpm.

It is another aspect of the present invention to provide a method of forced cooling friction stir welding of alloys / workpiece, wherein the parameters of welding are rotational speed of blower at 100 - 5000 rpm, welding speed of 10 - 1000 mm/min, and tilt angle 0- 5 degree.

It is another aspect of the present invention to provide a method of forced cooling friction stir welding of alloys / workpiece, wherein the force rate of water sprinkling is 5 - 1200 m³/min, for 100 - 5000 rpm.

It is another aspect of the present invention to provide a method of forced cooling friction stir welding of alloys / workpiece, wherein the force rate of air is 5 - 2000 m³/min, for 100 - 5000 rpm.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the features, advantages and objects of the invention, as well as others which will become apparent, may be understood in more detail, more particular description of the invention briefly summarized above may be had by reference to the embodiment thereof which is illustrated in the appended drawings, which form a part of this specification. It is to be noted, however, that the drawings illustrate only a preferred embodiment of the invention and is therefore not to be considered limiting of the invention's scope as it may admit to other equally effective embodiments.

Figure 1 illustrates water sprinkled friction stir welding.

Figure 2 illustrates forced air cooled friction stir welding. Figure 3 illustrates the tool of the present invention along with essential features.

Figure 4 and 5 illustrates comparison of tensile properties.

Figure 6 illustrates micro-structural analysis of forced air cooled FSW with water.

Figure 7 illustrates fractography image of tensile sample and dispersion of magnesium and silicon in AA6061 T6.

Figure 8 illustrates micro hardness of different weld region.

Figure 9 illustrates macrostructure.

DESCRIPTION FOR DRAWINGS WITH REFERENCE NUMERALS:

[100] Friction Stir Welding Tool [101] Primary tool holder

[102] A rotatable secondary tool holder [103] Shoulder at distal end of welding tool body

[104] A pin extending downward from the distal end of the welding tool body, beyond the shoulder

[105] Cooling element: A blower / a fan [106] Cooling element: Liquid sprinkler

[107] Alloy / Work piece / Weldment

[108] Fasteners

[109] Tool body

The present invention relates to the field of welding. More particularly, it relates development of novel friction stir welding process and apparatus to improve mechanical and metallurgical properties of the joints / weldment. According to the main essential aspect of the present invention, a friction stir welding tool [100] comprises a secondary tool holder [102], one or more fastening means / fasteners [108], primary tool holder [101], shoulder [103], cooling unit / element [105, 106], tool pin [104] attached to the distal end of tool body [109],

According to a preferred embodiment of the present invention, the apparatus is configured to have a primary tool holder [101], a rotatable secondary tool holder [102], shoulder [103] at a distal end of the welding tool body [109], a pin [104] extending downward from the distal end of the welding tool body [109], beyond the shoulder [103], and the tool is configured to comprise a cooling element or cooling unit [105, 106],

According to one embodiment of the present invention the friction stir welding tool comprises cooling unit / element preferably a fan / blades [105] removably and rotatably fixed at the intersection of the primary tool holder [101] and the shoulder [103],

According to second embodiment of the present invention the friction stir welding tool comprises cooling unit / element preferably consisting of a fan / blades [105] removably and rotatably fixed at the intersection of the primary tool holder [101] and shoulder [103], and liquid sprinkler / blower [106],

It is another aspect of the present invention to provide a method of welding with varying parameters by subjecting the alloys/workpiece [107] to forced cooling conditions.

EXAMPLE 1 For the purpose of exemplification, friction stir welding of AA6061 is carried out and described herein.

Rectangular sheets of 2 mm thickness with dimensions (100mmx60mmx6mm) are welded under three conditions namely normal cooling, air cooling and water cooling. Nine FSW joints are made with varying welding parameters and samples are subjected to tensile testing [Figure 4], hardness [Figure 8], microstructure analysis [Figure 6], SEM [Figure 7], The welded joints characterization is done to identify the sample which yielded comparatively better mechanical properties among the nine combinations Water cooled process was found to be the best among other cooling process.

Constructional feature of the device in detail:

The forced cooling friction stir welding element comprises of a primary or stable tool holder [101] in which the complete setup is attached by means of motor rod to which the secondary tool holder [102] or rotatable tool holder [102] is attached. The rotatable tool holder [102] is over assembled by a cooling fan [105] by means of fasteners [108], The bottom portion of the rotating tool holder is attached to a tool [109] which has specific shoulder [103] to pack the extruded or pulled away material by the tool pin [104] during welding. As the welding starts forced air cooling is provided by the cooling fan [105] attached to the rotating tool holder [102] and other mean of cooling is by external water sprinkling [106] method as shown in the Figure 2. This cooling effect contribute the micro structural generation after welding improving the efficiency greater than normal friction stir welding process.

The cooling element is basically classified into two sources in this invention.

1) Forced Air Cooling system and

2) Forced Water Cooling System Forced Air Cooling

In this system a fan [105] attached to the rotating tool holder [102] that helps in altering the heat exchanging rate during welding which in turns results with better weld.

Forced Water Cooling

This setup is similar to Forced Air Cooling Setup and in addition to that water is sprinkled [106] over the forced air [105] which sprays over the welding area as tiny droplet for enhancing the cooling rate helping in better recrystallization increasing the efficiency of the processer weld.

Experimental setup:

AA 6061-T6 alloy of size 120 mm x 50 mm x 2 mm sheet is used for butt joining in air cooled and water cooled medium. The mechanical property and material composition of AA6061 T6 material is shown in Table 1 and table 2 respectively. The parameters used in various levels for butt joining by friction FSW are the welding speed, rotational speed and tilt angle. Further parameters such as tool profile, load applied and plunge depth are kept constant throughout the process. The complete parameter is repeated for three conditions normal cooled, forced air cooled and water cooled conditions.

The different cooling systems are shown in Figure 1 and Figure 2. The tool is made up of EN-32 material with shoulder diameter 5-500 mm, pin diameter 1-25 mm, and pin length 0.5-5 mm, preferably shoulder diameter 9 mm, pin diameter 3 mm and pin length 1.7 mm. The different welding parameter used for welding are represented in Table 3. To identify the surface defect the sample is tested with Liquid Penetrant test. To identify the internal defect in welding the welded samples are tested with radiography testing and results with zero defects and immersed ultrasonic testing is carried out to find further defect analysis. On confirmation the defect free samples are cut by means of wire EDM for mechanical and metallographic testing. The mechanical testing is done with universal tensile testing machine with the sample cut by means of ASTM E8 standard in dog bone shape and metallographic sample is cut to the size of 10 mm x 40 mm x 2mm. The prepared samples are mounted by means of cold setting component and polished. The polishing undergoes three stages namely emery polishing, alumina polishing and diamond polishing. Then the component is etched by means of Poultant reagent (30 ml Hydrochloric acid, 40 ml nitric acid, 2.5 ml hydrogen fluoride, 12 gm chromic acid and 42.5 ml distilled water) to capture the macro and micrograph at different regions of the weld sample. Microhardness of the welded samples was measured using Matsuzawa Micro Vickers MMT-X Series load ranging from 1-2000 gf. A load of indentation was fixed to be 500 gf for the dwell time of 15 seconds. The fractography is examined using Hitachi (S-3000H) Scanning Electron Microscopy equipped with Electron Dispersive Spectroscopy (EDS) Detector. Table- 1 Mechanical Properties of the Aluminium alloys

Table-2 Chemical Composition of the Aluminium alloys

Table-3 Friction Stir Welding Levels and Parameters

Results and Discussion

The mechanical and metallurgical properties are investigated and resulted are tabulated in Table 4.

Table-4 Friction Stir Welding Levels and Parameters

The parameter set 2 is found to be optimal among all 18 trials conducted with normal cooling and water cooling then further three trails were made for confirmation in all three cooling stages.

Optimized condition under different cooling system: The Table 5 provides the optimized result for the parameter set 2 with varying cooling conditions such as normal cooled, forced air cooled and forced air with water cooled. The consecutive graphs for ultimate tensile strength and yield strength are represented in Figure 4 and 5. Table-5 Optimized parameter at different cooling condition

The metallurgical inferences of the welded samples are presented by microstructure, macrostructure and fractography. The Figure 6 gives the clear microstructures of different region such as weld nugget, heat affected zone, thermomechanically affected zone and base metal. The micro structural analysis supports the study with variation of grain size, metal flow and crystal orientation by means of micro structural studies. Stir zone is found to have fine and equiaxed grain structure compared to other region.

Figure 7 represent the fractography of the tensile tested sample the first image shows the first mode of metal transfer which is found to be brittle due to the sudden cooling at the top region and the second image is taken at the second mode of metal transfer which has elongated dimples with spores and inclusions. This appearance makes this to be ductile mode. The Figure 7 represents the EDS elemental mapping taken at the fracture surface which clearly gives the different material composition available in the material such as silicon, magnesium and manganese which are the major reason for the improvement in mechanical and metallurgical properties.

The Figure 8 shows the harness profile supports the tensile results by making a clear stand of improved hardness for water cooled sample than other process. This is also gives the clarity on the failure location by least hardness area which is coinciding with the fractured samples.

Conclusion

In this invention, a novel friction stir welding process to improve mechanical and metallurgical properties of the joints / weldment has been successfully developed.

The Tensile properties of welds obtained by both forced air cooled and forced air cooled with water are found to be better than normal friction stir welding.

Claims

WE CLAIM:

1. A friction stir welding tool [100], comprising of: a primary tool holder [101]; a rotatable secondary tool holder [102]; a tool body [109]; a shoulder [103] at a distal end of welding tool body [109]; and a pin [104] extending downward from the distal end of the welding tool body [109], beyond the shoulder [103], characterized in that the tool [100] is configured to comprise a cooling element or cooling unit [105, 106], and wherein the cooling unit comprises one of liquid sprinkler [106], a blower or a fan [105] removably and rotatably fixed at the intersection of the primary tool holder [101] and the shoulder [103], and combination thereof.

2. The friction stir welding tool [ 100] as claimed in claim 1 , wherein the tool profile of tool body [109] is shoulder diameter 5-500 mm, pin diameter 1-25 mm, and pin length 0.5-5 mm.

3. A method of forced cooling friction stir welding of alloys / workpiece, comprising of the steps: welding of alloys [107] using a welding tool [100]; and subjecting the weldment or workpiece [107] to forced cooling conditions, wherein the weldment or workpiece is cooled by one of forced air cooling, forced liquid cooling and combination thereof, wherein the forced cooling comprises of cooling by means of air blower / fan [105], liquid sprinkler [106] and combinations thereof.

4. The method of forced cooling friction stir welding of alloys / workpiece as claimed in claim 3, wherein the forced air cooling comprises of cooling by means of air blower / fan [105],

5. The method of forced cooling friction stir welding of alloys / workpiece as claimed in claim 3, wherein the forced liquid cooling comprises of cooling by means of liquid sprinkler [106],

6. The method of forced cooling friction stir welding of alloys / workpiece as claimed in claim 3, wherein the air blower comprises of a blower / blades or fan [105] rotating at a speed of 100 - 5000 rpm.

7. The method of forced cooling friction stir welding of alloys / workpiece as claimed in claim 3, wherein the parameters of welding are rotational speed of blower [105] at 100 - 5000 rpm, welding speed of 10 - 1000 mm/min, and tilt angle 0- 5 degree.

8. The method of forced cooling friction stir welding of alloys / workpiece as claimed in claim 3, wherein the force rate of water sprinkling [106] is 5 - 200 m³/min, for 100 - 5000 rpm.

9. The method of forced cooling friction stir welding of alloys / workpiece as claimed in claim 3, wherein the force rate of air [105] is 5 - 2000 m³/min, for 100 - 5000 rpm.