Application of friction stir welding in aircraft structures

This thesis was written with purpose of gathering and summarising new knowledge in friction stir welding (FSW) between years 2005 and early 2009, especially with respect to practical application of FSW in aircraft manufacturing processes. It should ease decision making of smaller aircraft (and other) producers when deciding whether to use the technology and asses its benefits.

Chapters Introduction, Welding process and its parameters and FSW application contain the most of the practical information based on solid data (see References). Most of the other parts contains example calculation and optimisation of design when substituting riveted joint by FSW joint.

Abstract

Aluminium alloys series 2XXX and 7XXX are not normally considered  weldable by traditional methods. They are widely used as a part of aerospace structures. The common practise joining technology is riveting. Emerging technology of friction stir welding (FSW) has shown promising results in welding of these alloys. In this thesis we have investigated application of FSW technology in aircraft structures mainly as substitution of riveting on stringer reinforced panels. Manufacturing process steps were determined and further optimisation procedures were suggested. Knowledge base for setting basic welding parameters has been created and comparison of a sample of riveting and FSW technologies on a simple final element method model sample was examined in order to identify differences in stress distribution character in the riveted and FS welded samples.

Introduction

Friction stir welding (FSW) [1] is an emerging technology invented in 1991 at The Welding Institute, Cambridge. Since then, it has developed from an experimental joining procedure to a technology used commercially. The potential of this technology has not been used fully as there is not enough practical information about it in open literature.
Aeronautical engineering can make use of FSW at many fronts. It can be used to weld items that are not possible to be welded by conventional fusion welding such as aluminium alloys of 2XXX and 7XXX series [2] and serve as a riveting replacement [3]. It can also replace conventional TIG and MIG welds to provide significantly better corrosion resistance [4, 5] and fatigue properties [6, 7].
In the Czech Republic research on FSW is conducted in Aeronautical Research and Test Institute in Prague as a part of progressive joining technologies research [8].
In this thesis, we focused on friction stir welds of aluminium alloys of 2XXX and 7XXX series and provided guidelines to the practical application of FSW on aircraft structures.

Benefits of Fricion Stir Welding (FSW)

Fricion Stir Welding (FSW) brings many benefits when comparing to traditional fusion welding and other joining methods commonly used on aircraft structures. Mishra [9] names many benefits of FSW. The most crucial of them for application in aircraft industry are:

  • Low distortion of work piece
  • Good dimensional stability and repeatability
  • Absence of cracking

From production technology point of view Mishra [9] states these benefits:

  • No shielding gas required
  • No surface cleaning required
  • Eliminate grinding waste
  • Eliminate solvents required for degreasing
  • Improved material use
  • Low energy needed for weld creation (compared to fusion welding)

It also generally offers better mechanical properties when compared to fusion welds, however, the resulting properties strongly depend on welding process parameters such as welding tool shape [10-15], tool revolutions [12, 16-18] and welding speed [16, 19, 20].

FSW as riveting substitution

FSW is a very promising technology in the field of riveting substitution on the aircraft structures. Using riveting technology in current aircraft design limits the productivity levels manufacturers can reach for the aluminium aircraft structures. Use of the rivets also introduces unwanted stress concentrators and can induce crevice and fretting corrosion.

Current use of FSW

Today, the FSW technology is used mainly in aerospace industry where its benefits can be utilized fully. The technology brings significant weight savings into aluminium fuselage construction where riveting is usually the main mean of joining. Generally, FSW is well suited mainly for long straight welds, like the ones on stringer-reinforced panels, which are a vital part of today’s most common aluminium fuselage, wing and other constructions in aerospace industry.
Automotive industry can also take advantage of FSW. Wider use of aluminium alloys and together with FSW joining techniques helps to create lighter and more economical car construction [21].

Welding process and its parameters

The basic concept of FSW is simple. Non-consumable rotational tool with pin and shoulder shaped to provide required weld properties is inserted in between abutted edges of plates to be joined and traverse along the joint line. Joint is created by stirring the material of the two pieces by the tool pin [1]; plastic deformation caused by the pin and friction between the base material and tool shoulder creates heat that softens the basic material, thus easies the stirring process. There is no melting of the base material involved. [9, 22] The main source of heat in the weld is the friction between shoulder and base material [9]. The principle is illustrated on the schematic drawing (Figure 1) [23].

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Figure 1: Schematic drawing of friction stir welding [23]
The main process parameters are (a) tool geometry, (b) welding speed and (c) tool revolutions. Their influence on resulting properties of the weld are discussed later.
Typical friction stir weld in its cross-section (Figure 2) consists of three main zones [9]: (a) Nugget, stirred zone, (b) thermo-mechanically affected zone (TMAZ) and (c)heat affected zone (HAZ). The three zones pose distinct mechanical properties [9, 24-27] and nugget and TMAZ has been often documented as being the weakest part of the joint [26, 28]. The weld is not symmetrical due to the rotational movement of the tool, thus moving material from advancing side to retreating side.

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Figure 2: A typical macrograph showing various microstructural zones in FSP 7075Al-T651 (standard threaded pin, 400 rpm and 51 mm/min) [9]
The asymmetry of the weld can be used to improve mechanical properties of the weld of dissimilar alloys. A study showed that placement of the tool axis out of the joint axis by 0.5 to 1 mm improved mechanical properties of the weld [29].
According to [9, 20] temperatures in the weld do not reach melting point of alloys. Higher rotational speed of the tool lowers friction between the tool and base material which is reflected in lower heat production [30] and this prevents material from being melted within reasonable welding process parameters. Obviously welding affects structure of precipitates in nugget area as well as in both TMAZ and HAZ [31, 32].

Mechanical properties of welds

FSW can be used for welding of virtually any metal. In this thesis, we focused on 2XXX and 7XXX aluminium alloy series. Literature provides broad range of data for 2024 [2, 19, 22, 26, 29, 32-51], 2219 [10, 12, 52-55] and 7075 [49, 56, 57] as well as for dissimilar weld 2024-7075 [2, 29]. These aluminium alloys are considered as non-weldable by traditional fusion techniques. Unfortunately, not many resources provide detailed information about the conditions under which the mechanical properties were gained, reducing the usefulness of the data.
For the 2219-T6 Xu et al showed that the joint efficiency of the welds under optimized welding parameters varies from 71 to 80 % [58]. Another study done by Zadpoor et al showed very good results for 2024-T3 where the joint efficiency varied between 89 to 99 % [49], for 7075 was 90 % [49].
Generally, the optimized welding parameters can create a weld reaching joint efficiency of 70 % and higher. This is a good result, as these alloys are considered as non-weldable by traditional welding techniques [9].

Fatigue properties

Although FSW provides generally better fatigue properties than traditional fusion welding methods [7], fatigue properties are still significantly lower than of the base material. Several methods have been developed to prolong fatigue life of friction stir welds. A few of them have proven capable of improving fatigue properties of the weld. The technique of root flaw removal[51] and application of oxide array removal [50] appear to be effective. The fatigue characteristics of the alloy 2024-T3 weld exhibited improvement of up to 100 % [50, 51].
Studies on using of laser and shot peening on materials 2195 and 7075 [59, 60] did not show improvement of fatigue properties and crack growth speed as the residual stresses after FSW are already much lower than after welding by a traditional technology, which involves material melting.

Crack propagation within the weld

Propagation of cracks within the weld are most likely to start at so called “zigzag defect” of the weld. The defect is always initiated within the root of the weld and spreads through the nugget.
Zigzag defect is an inherent feature of the weld [35] and is considered as the main cause of the reduced fatigue life of FS welds in comparison to the base material. The typical zigzag defect is shown on Figure 3.

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Figure 3: Propagation of fatigue crack within a FS weld in 2024-T4 alloy

Corrosion properties of welds

Several studies have been conducted on corrosion behaviour of friction stir welds [4, 40, 61]. Detailed study of corrosion behaviour of 2024-T351 alloy [40] showed that nugget and HAZ areas are both sensitised and susceptible to intergranular corrosion. Whereas study on 7108-T79 [4] has shown that this material is most sensitive in TMAZ and the character of the corrosion is also intergranular.
Dependence of corrosion behaviour on rotational speed of the welding tool was observed [40] and can be used as an explanation of the aforementioned difference in the observation of corrosion sensitivity of different part of the weld. For the lower rotational speeds of the tool, localised intergranular attack was observed in the nugget region; for the higher rotational speeds, attack occurred predominantly in HAZ.
Generally, sensitization of the microstructure that occurs during welding process is responsible for the corrosion susceptibility of HAZ region of the welds [61]. It was shown as well that the most of localized corrosion and environmental cracking in FS welds of aluminum alloys is of an intergranular-type [61].
The method how to improve corrosion resistance of the weld was proposed to be a short-term post-weld heat treatments with time–temperature exposures similar to that during the welding, as it would rehomogenize the sensitized microstructure of the welds [61].

Residual stresses

Due to complex thermo-mechanical process during FSW, residual stresses appear. It is believed that FSW produces lower residual stresses due to lower temperatures involved. Several studies were conducted on residual stresses in 2XXX and 7XXX alloys [36, 43, 62-64] and have shown that longitudinal stresses are generally higher than transverse ones not depending on pin diameter, tool revolutions or welding speed. Several methods have been developed to eliminate residual stresses by tensioning material during welding [43, 65]. It shows to be effective for longitudinal stresses whereas effect of transverse stresses on fatigue life can be eliminated by peening [33, 66].
A study on 7075-T651 alloy showed that the residual stresses associated with the weld nugget decreased, while those associated with HAZ increased with time [62].
For alloys 2024 and 7075 a study showed that unlike traditional welded joints, where the maximum residual stresses values occur close to the surface of a FSW joint, which is near the tool shoulder border of advancing side of the joint, the residual stresses are of negative values on the surface and become positive in the joint. Surface residual stresses vary from -15 to -40 MPa and increase with depth up to tensional 50 to 150 MPa [64].

Aging of friction stir welds

As the FSW process introduces heat into the aluminium alloy structure, changes in the microstructure occur [9]. Material has to return to the desired metastable state (with higher density of metastable precipitates) by natural or artificial aging. Natural aging is more commonly used as it does not require additional heating. Studies reported reaching metastable state (metastable precipitates) of the material after 7 [45] or up to 45 days [49] of natural aging for materials 2024 and 7075 depending on the thickness of the material and the welding parameters.
Study conducted by Linton [62] on 7075-T651 alloy has shown that residual stresses change during natural aging of welded material. Welding stresses associated with the nugget decrease, whereas the ones associated with HAZ increase. Mechanical properties in all areas of the weld increased with the time.

FSW application

Joint configuration

FSW allows the use of virtually any joint configuration as we know them from traditional fusion welding. The only limitation is the tool shoulder, which makes fillet joints more problematic than flat ones [9]. Typical FSW joints are shown (Figure 4).

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Figure 4: Joint configurations for friction stir welding: (a) square butt, (b) edge butt, (c) T butt joint, (d) lap joint, (e) multiple lap joint, (f) T lap joint, and (g) fillet joint.
The most commonly used types of joint are the butt joints, in particular, square butt joint.

Tool geometry

Tool geometry has a great influence on resulting mechanical properties of the weld [10-14]. It provides in-situ heating, stirs base material, and thus creates weld. There has been variety of tool shapes used. FSW can be performed with tool of a simple geometry (Figure 5) yet having good mechanical properties [10]. Advanced tool design (Figure 6) provides intensified material flow in the stirred zone and better weld quality [23]

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Figure 5: Basic FSW tool pin profiles

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Figure 6: Advanced FSW tools developed at The Welding Institute, Cambridge

Experiments performed by Elangovan on tools with simple geometry [10] showed remarkable influence on mechanical properties of the weld as illustrated in the Figure 7 [10]. The best results were produced using tool with square cross-section pin that assured best stirring of the material in the weld area.

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Figure 7: Dependency of material properties on welding speed and tool shape [10]

The Figure 7 shows that the mechanical properties of the weld strongly depend on the volume of material stirred by the tool. The more material the tool stirs, comparing to its volume, the better the quality of the joint [23]. This is also the reason why tool producers try to create specially tapered and threaded tools to raise the ratio [23].

Welding speed and tool rotational speed

These two parameters of the welding process are the most important ones and determine overall mechanical properties of the welds.
A comprehensive study investigating dependency of mechanical properties of the weld on the welding and rotational speed was done by Elangovan et al [10, 12]. Figures below show the obtained results.

Dependency of mechanical properties on welding speed

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Figure 8 Dependency of elongation on welding speed and tool shape [10]

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Figure 9: Dependency of elongation on welding speed and tool shape [10]
Even thought the number of different welding speed measurements is quite low, we can expect that there always is an optimal welding speed for the given material, rotational speed and tool shape. Optimal welding speed in the cases shown on the figures above does not strongly depend on the tool shape and optimal value of the welding speed for the given other parameters considering elongation and joint efficiency lies between 0.8 and 1 mm.s-1.

Dependency of mechanical properties on welding speed

Figure 10 and Figure 11 show the dependency as measured by Elangovan et al [12].

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Figure 10: Dependency of elongation on rotational speed and tool shape [12]

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Figure 11: Dependency of joint efficiency on rotational speed and tool shape [12]
In this case, the dependency of the value of elongation and joint efficiency on the tool shape is more significant than their dependency on rotational speed.
The joint efficiency graph in Figure 11 exhibits a peak around the rotational speed value of 1600 min-1, whereas elongation values grow with raising rotational speed throughout the measured range. The author does not specify aging time of the welds, so it is possible that higher values of elongation are caused by higher weld temperature at higher rotational speed of the tool. This could have lead to creation of unstable microstructure with higher elongation and would return to the metastable state with lower elongation and higher joint efficiency after aging.

Joint efficiency for optimised welding parameters

Attachment 2 summarises mechanical properties of butt welds done with optimised welding parameters from different source. The joint efficiency of yield strength is generally high, varies from 75 to 99 %. The table can be used as a base for setting welding parameters for FSW of 2024 and 7075 alloys. Unfortunately the detailed information about specific welding parameters resulting in specific mechanical properties are rare in open literature thus, the statistical base of the table is limited.

Welding forces

Welding forces are important for choosing appropriate machinery and tool design. Hattingh et al studied forces applied to variety of tools during friction stir welding [14]. At welding speed of 150 mm.min-1 and spindle speed 500 rpm the maximum radial forces never exceeded 6 kN. Maximal axial force varied from 4 to 14 kN for different tools.
The axial, also called vertical down force, is crucial for creation of sound weld, as it assures proper contact of the tool with welded work pieces and together with rotational speed induces release of frictional heat from the tool shoulder which is the main source of heat for the weld [9].

Machinery used

FSW is not demanding in regard of machinery requirements. There is no special welding machine necessary and utilisation of standard workshop equipment such as milling machines [67] is possible. Although specialised machines provide higher levels of productivity especially in the respect to the ease of gripping work pieces, welding speed (maximum speed around 2 m.min-1 for standard FSW machines [68]) and also number of welding heads allowing to work on multiple parts of the work piece at the same time.
Most of the currently used machinery is tailor made for the customers by several specialised producers that provide wide range of machine parameters suitable to customer needs.

Material preparation

Thanks to the principle of FSW there is no special preparation of welded pieces needed. The only requirement is degreasing [69] which prevents introduction of particles that would lower mechanical properties of the weld.

Heat treatment

Welding of heat treated material

Alloy 2024 with different heat treatment variants was by Aydin et al [70]. Results are summarised in Table 1 below.

Table 1: Comparison of heat treatment effects on FS weld mechanical properties

Sample

Base material

Joint

Joint efficiency

0.2% proof strength

Ultimate tensile strength

Enlongation

0.2% proof strength

Ultimate tensile strength

Enlongation

Rp0.2 [Mpa]

Rm [Mpa]

A [%]

Rp0.2 [Mpa]

Rm [Mpa]

A [%]

2024-W

171

336

28.4

165

318

5.0

0.95

2024-T4

351

492

21.9

279

389

9.0

0.79

2024-T6a(100  C – 10 h)

293

451

25.5

220

346

7.1

0.77

2024-T6b(190  C – 10 h)

403

496

8.6

341

416

5.1

0.84

2024-O

126

289

17.6

126

289

12.8

1.00

The best tensile properties were obtained for heat treatment T6a (aging at 190°C for 10 hours); for annealing the tensile strength of the weld reached the same level as the base material although the maximal elongation was lower. Strength efficiency of the welds made of heat-treated materials varied from 77 to 84 % and elongation efficiency from 28 to 60 %.
Generally, FSW of heat-treated materials does not encounter big problems as the temperatures during the welding process are relatively low.

Post-welding heat treatment

A study done by Paglia et al found that  25 minute at 180°C heat treatment after FS welding, shows a significant increase in the localized corrosion resistance [71].
On the other hand a study on precipitate microstructure of welded 2024-T351 with post weld heat treatment T6 (10 hours at 190°C) showed mixed results in different zones depending on their distance from the weld centre. Far from the weld line, the post-welding treatment increased the hardness of the 2024-T351 FS weld up to the one of the hardness of 2024-T6 alloy. However, at the TMAZ, at 4 mm from the weld centre, the hardness declines sharply after the post-welding heat treatment [37].
Generally, post-welding treatment cannot be recommended without detailed investigation of resulting mechanical properties of the weld after the heat treatment.

Economy of FSW

As the matter of fact, FSW is very efficient welding technology. The main reason is that it does not reach the melting point of the material during the process of welding, thus heat and energy delivery into weld is much lower in comparison with any other welding technology, where melting point of the material is reached due to absence of melting heat.
Other aspect of FSW economy is high speed of welding reachable on modern machines assuring high productivity and low production time. Easy gripping and modern CNC machinery keeps staff skill requirements low as well.

Speed

Speed of FSW can be generally higher than the ones in conventional welding. Modern machines allow welding speed up to 2 m.min-1 and some studies showed [72] that higher welding speeds produce welds with better mechanical properties. Although optimal welding speed has to be determined with regard to the shape of the tool as well as rotational speed with regard to material used.

Energy consumption

Energy consumed by FSW process has three main components [30].

  • 1. Stirring the material (deformation energy) of the work pieces being joined.
  • 2. Production of heat due to friction between the welding tool and the work pieces.
  • 3. Energy consumed by machine/work piece movement.

Points 1 and 2 are closely connected and take the major part of the FSW energy requirements. As the energy in point 3 can be considered as roughly equal with the energy used for the same purpose in conventional welding the energy saving can be determined by investigating and comparing energy in points 1 and 2.
Studies conducted on material 7050 showed that energy consumptions vary from 800 to 2800 J.mm-1 [30] depending on used welding properties, mainly axial force applied to the tool. It is apparent that the most of the heat is delivered through the tool wear, as also other studies have shown [9].

There was a statistically significant correlation between the ration maximum weld temperature to solidus temperature and energy needed for one millimetre of the weld as shown in Figure12. The correlation is valid for different materials.

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Figure 12. Temperature ratio as a function of effective energy per weld length with linear regression curve added. [30]

Machinery cost

Machinery used for FSW process does not require the use of any new or very advanced technology. The technology level is comparable to standard CNC milling machines. There is very little information available about the costs of the machinery. Due to technological compatibility with milling machines, it can be presumed that the prices are also comparable.
There is a wide range of different types of FSW machines. The main differences lie in the gripping equipment and the maximum size of the work pieces and number of welding heads and their power output. Most of the current machines are armed with CNC unit allowing automated welding.
A study by Minton showed a possibility of utilizing usual milling machine as a FSW machine [67] with good results. Many companies are equipped with universal welding machines that can be easily transformed into FSW machines at very low costs. The main issue is to figure out FSW tool gripping. Bearing load capacity, machine stiffness and power unit performance are usually above standard FSW requirements. Nevertheless using adapted machines lowers machine productivity in comparison with its original purpose and does not allow high productivity levels of FSW to be reached either. Adapting of milling machines is preferably recommended for technology testing purpose only.

Staff requirements

FSW is a process with high possible level of automation and does not require special staff training or certification for running the welding machines. The level of skills and knowledge of worker is comparable to the one needed for running CNC milling machines. Specific required knowledge varies with complexity of the machinery i.e. degrees of freedom of welding heads and gripping equipment.
Complexity of programming of CNC programs is also on the similar level of difficulty as for milling machines.

Work piece preparations costs

As mentioned before, one of the advantages of FSW lies in the fact that there is no need for special preparation treatment of work piece surfaces that are being connected. The only requirement is making sure the surfaces are grease free [69] which usually does not require any special treatment or only a simple degreasing procedure with alcohol or acetone being required.
Low sensibility of FSW technology to corrosion spots and other particles other than the base material significantly lower the costs of work piece preparation, and thus reduce overall costs of application of FSW technology overall.

Practical use of FSW

FSW technology is being widely used in high-tech aerospace, maritime, automotive and railway industry and its users benefit mainly from the possibility of welding materials that cannot be welded by means of any other welding technology without compromising on the side of mechanical properties of the weld.

Typical machines and tools

There is several major FSW machinery producers throughout the world. The main ones are [73]:

  • ESAB
  • Crawford-Swift
  • Transformation Technologies
  • Friction Stir Link, Inc

All the manufacturers provide variety of highly customisable models of welding machines. Starting with basic models allowing only one axis welding tool movement to the six-axes FSW robots.
The situation with the commonly used tool is complicated by the fact that there is not much detailed information about produced tools available in open literature. TWI produces its series of tools Whorl, Triflute [74] and Trivex [75] but does not provide specifications needed for the appropriate welding parameters to its non-customers.
However, the basic parameters of a simple design tools are available [11, 13, 14, 76, 77]. For the FSW of aluminum alloys tool material of usual tool steel or W-Co steel is sufficient [78], so there is enough information available to produce customized design tool according to the manufacturer needs if needed.

Current large users of FSW technology

Boeing Co., USA

Boeing was one of the early adopters of FSW technology and has been utilizing its benefits mainly in their spacecraft division.

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A longitudinal welding machine, plus handling equipment for welding of fuel tanks for the Delta II programme of space rockets. [69]

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A complete plant designed for circumferential welding of fuel tanks, also including a friction plug welding unit. [69]

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A complete plant designed for circumferential welding of fuel tanks, also including a friction plug welding unit. [69]
Attachment 1 provides photographs of the machinery.

Marine Aluminium, Norway

FSW technology has users also in non-aerospace industries.
A plant for production of flat panels, 16×6 meters, freezing blocks, coolers, railway wagon parts, H-beams, road signs and barriers, as well as the production of complex extrusions. [69]
EADS CRC/IS, France
EADS and its division Airbus are also early adopters of the technology and conduct a wide research in the area as well.
FSW welds on fuselage panels of Airbus A380 [79] saving on stringer-to-panel joint 0.18 kg per meter of weld and 0.8 kg per meter of weld on panel-to-panel joint [79].
Eclipse Aviation
Eclipse aviation is one of the first aircraft producers introducing the FSW technology on large scale into smaller aircraft design.

Welds of construction of Eclipse 500 airplane. [80]

FSW as riveting replacement on a fuselage panel

FSW replaced traditional riveting on an example of an outer fuselage panel. Usual design of fuselage outer panels consists of stringers riveted onto the panel surface. Riveting is a well known and widely used technology however its usage on the outer panels brings problems with impermeability.
Expected improvements lie mainly in productivity and speed of production, better tensile properties of the joint, better fatigue properties and corrosion properties.
New panel design
The new panel keeps all the outer dimensions of the riveted panel, only the construction of the stringer reinforcement has been changed. Details of the new panel design are in Attachment 3.

Production process

The stringers are held in the gripping device and the panel plate will be put on them. Positioning parts of the gripping device will assure correct position of the panel plate. This all will be gripped down to the workbench by 3 rods using clamps a shown on the scheme below.
Due to deviations from the steady welding state in the first and last 20 mm of the weld [22] the work piece has 20 mm allowance on the both sides as the approach and drifting area. After the welding has been performed the allowance will be cut off.

The movement of the welding tool was designed in order to assure highest possible productivity as shown on Figure 43.

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Used tool

As the known tools produced by TWI are suitable for welding of aluminium plates thicker than 6 mm, the custom made tool was chosen to be used. The basic parameters based on the tools used on similar jobs in Attachment 2 are following:

  •     Tool type                Conical threaded
  • Pin diameter 1                   5 mm
  • Pin diameter 2                   3 mm
  • Pin length                           4 mm
  • Shoulder diameter           15 mm
  • Tool material              W-Co tool steel

Detailed design of the tool is not within the scope of this thesis and would require more detailed analysis.

Welding parameters

Determination and optimization of the welding parameters is a process requiring testing on the actual samples of the material, machine and the tool used. The parameters used as the starting point in the welding parameters optimization process is based on the welding parameters of similar welds as in Attachment 2.

  • Rotational speed            1400 min-1
  • Welding speed               110 mm.min-1
  • Tool tilt                            2.5°
  • Vertical down force        3.5 kN

Before introducing a serial production, the welding parameters have to be optimized. The proposed optimization would consist of a series of experiments for welding speeds in range of 100 to 150 mm.min-1 by step of 10 mm.min-1 and rotational speed in range of 1200 to 1600 by step of 200 min-1. For each of the combinations 5 samples should be done and their tensile strength in the axis perpendicular to the stringer plane would be evaluated. Also the joint would be examined visually for presence of cracks and examination of overall weld quality.
These optimization steps are vital for successful and efficient application of the technology on the panel and further investigation like fatigue tests might be needed to comply with certification requirements.

Machine used

The welding machine chosen is ESAB FSW SuperStir, which allows welding of large work pieces. Parameters of the machine are following [82].

  • Maximum work piece dimensions         2500 x 4000 mm
  • Maximum welding speed                       2000 mm.min-1
  • Maximum vertical down force               12.5 kN
  • Maximum revolutions of the tool          2500 min-1
  • Maximum welding depth                       6 mm

The machine parameters are sufficient for the expected range of welding parameters.

Gripping equipment

Gripping equipment was designed to assure the correct positioning of panel plate and the stringers as well as stiffness of the whole work piece needed for the welding process involving high welding forces.
Figure 44 shows the scheme of the welding device. Stringer holders together with the stringer stop assure the appropriate position of the stringers. Corner and side stops guarantee the right position of the panel plate with the 20 mm allowance on the both sides towards the stringers. The panel plate together with the stringers are clamped down using the beams.
The low distance between the stringer holders together with the fact that the welded stringer always carries the vertical down force initiated by the tool, should assure low deformation of the work pieces during the welding process. Further examination of the deformation of the work pieces under effect of welding process should be done before introducing the gripping device as a part of serial production.
The full welding force should not be applied in the first and the last 20 mm of the weld, as stringer do not support the welded panel plate in that area. The time behaviour of the vertical down force, induced by the welding tool, during the first and last 20 mm of the weld should be a part of the further optimization of the welding process.

Attachment 4 contains the system drawing of the gripping device.

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Figure 44: Griping device scheme: 1. Clamped beams holding the weld pieces, 2. Stringer holders attached to the main desk, 3. Corner stop, 4. Side stops, 5 Stringer stop

Manufacturing steps

The process of manufacturing the reinforced panel has following steps:

  • Cutting of panel plate work piece
  • Cutting stringer work pieces
  • Clamping down the gripping device to the welding machine workbench
  • Positioning the stringers into the device
  • Positioning the panel plate into the device
  • Clamping the work pieces down to the machine work bench using the beams
  • Defining the work piece position towards the tool in the known base position for given CNC welding program (in serial production for the first piece only)
  • Automated welding program execution
  • Removing the clamping beams
  • Removing the weldment from the griping device
  • Basic visual control of the weld
  • Cutting the 20 mm material allowance off
  • Cutting the side U shaped notches into the panel

Discussion

Assessment of knowledge available in open literature

FSW, as a relatively new technology, has not yet gained standardised procedures for setting basic welding parameters and tool selection for given types of joints and materials. Open literature provides a wide range of information about microstructure of FS welds, mechanical, fatigue and corrosion properties of welds manufactured under a low range of specific welding parameters for a low selection of materials. Availability of specific information about welding tool design is very limited. In addition, paper presenting results of testing of mechanical properties present used tool detailed characteristics in an incomplete way. All these facts together cause that the value of the information in open literature for setting welding parameters and deciding about effectiveness of FSW implementation into the manufacturing processes of aircraft producers is low. It makes adoption of FSW technology more expensive, because optimisation of welding parameters requires further detailed investigation by the adopting manufacturers.
The lack of standardisation in the area of FSW can bring additional obstacles into the certification process of the aircraft, which uses FS welds in its construction, because no standardised calculation of fatigue life which can be used as a proof of the lifespan of the welded part. In addition, the effect of corrosion and residuals stresses in the welded area and their influence on the fatigue life have not yet been fully understood.
Generally, there are significant gaps in the literature leaving the field open for larger scale research allowing easier setting of welding parameters and making assessment of benefits of FSW technology implementation less problematic and more accurate.

Substitutability of riveting by FSW and its application

Riveting is a technology substitutable by FSW if the specifics of the technology are respected and preferably design changes are executed. However, there are some areas where riveting technology could be used more conveniently.
The design changes in the transition from the riveted construction to FS welded construction are the vital part of adoption of FSW technology ensuring its effectiveness. Changes in design utilizing the FSW technology benefits make the resulting lighter and possibly stiffer than the original construction.
The main benefit of FSW in comparison with riveting lies in the fatigue life field. Riveting introduces many stress concentrators into the construction and thus, raises risk of crack initiation. In contrary, FSW technology does not create additional stress concentrators. The only risk factors towards the fatigue life introduced by FSW are residual stresses and crack in the non-optimised welds.
The bending test performed on the FEM modelled samples of riveted and FS welded panels showed that, for the construction of this type, FSW variant could bear the same load as the riveted construction with 6 to 9 % less deformation, depending on loading direction, and weight 7 % less than riveted construction. Switch from riveted to FSW technology saved 3 kg per panel of this size, again, comparing with riveted construction.
Economical benefits of introducing the FSW technology as riveting substitute lay mainly in the possibility of higher automation of manufacturing process and requirement of less skilled labour. For the example of FS welded panel the time consumption of the welding process is nearly 140 min, which is comparable with time consumed by production of riveted construction design. However, the FSW technology is fully automated, not requiring labour during the welding process. Comparison of energy requirements would need detailed investigation of riveting process requirements, which is not in the scope of this thesis.
The area of substitution of riveting by FSW requires further investigation. We suggest to perform examination of real samples of riveted and FS welded construction with accent on the static mechanical properties and mainly fatigue life of FS welded structures.

Evaluation of the use of FSW as rivet substitution on the stringer-reinforced panel

The substitution of riveting on the stringer-reinforced panel can be considered as successful. The FS welded panel demonstrates higher stiffness and weights approximately 3 kg less than its riveted version, which gives promise of further significant weight savings on other parts of the fuselage if FSW technology is used.
Fatigue and corrosion resistance properties of the used FS welds require further investigation and high cycle fatigue mechanical testing in order to allow aircraft certification.
From the technological point of view, the set welding parameters require further optimisation and experimental confirmation to acquire the ideal welding and rotational speed of the tool. Another set of experiments would be required to determine ideal shape of the welding tool, alternatively paid consultation with some of FSW tools and machines manufacturers can fasten and ease the process of determining the ideal welding parameters.
Used technology provides possibility of full automation of the welding process, thus, improving production efficiency.

Conclusion

FSW technology brings substantial advantage to the area of joining of aluminium alloys series 2XXX and 7XXX compared to riveting technology. Design changes in the riveted construction have to be done in order to use FSW as a direct substitute of riveting technology. The changes in design have to respect different character of the joint. Fatigue tests might be required to allow certification of the construction.
Availability of data required for design of FSW technology processes in open literature is limited but provides sufficient base to estimate roughly possible benefits of the usage of the technology or eventual switch from riveting joining technology to FSW. Information about basic welding parameters is sufficient to be used as starting values for optimisation process on the real construction, materials and tools. This causes higher financial requirements for new adopters of the technology.
Production of the stringer-reinforced panel using technology of FSW provides possibility of improvement of mechanical properties of the panel, reducing weight of the plane construction, introduction of fully automated production into the manufacturing process and raising its efficiency. Further investigation is needed, however, to determine fatigue properties of FS welds in order to allow certification of the aircraft structure.

Author: Martin Srubar

Acknowledgements

On a personal note, I would like to thank my parents for their continuous support throughout my studies, for which I am extremely grateful. In addition, I would like to thank my partner Alana for her permanent emotional support.
Also would like to thank all the professors at the Institute of Aerospace Engineering and the institute head, prof. Ing. Antonín Píštěk, CSc., for creating a great learning atmosphere, which motivated me a lot during my study years there.
Foremost, I would like to thank my supervisor doc. Ing. Josef Klement, CSc. for his support, valuable advices and the fact that he has always been exceptionally helpful.

Source: China Fittings Manufacturer – Yaang Pipe Industry (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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Symbols and units

  • Jx,R        mm4              Modulus of rupture of the stringer used in riveted construction
  • Jx,i          mm4              Modulus of rupture of respective sub-areas of stringers
  • a              mm               Width and height of riveted stringer
  • t               mm               Thickness of bottom part of riveted stringer
  • w              mm               Width of the upper part of riveted stringer
  • r               mm               Diameter of riveted stringer reinforcing extension
  • a’              mm               Height of stringer joined by welding
  • t’               mm               Thickness of stringer joined by welding
  • Jx,FSW      mm4            Modulus of rupture of the stringer used in FS welded construction
  • r2024        kg.m-3          Density of 2024 material
  • wp             mm               Width of the both riveted and FS welded panels
  • ws              mm               Width of the panel sample used for FEM simulation
  • Fs              N                   Force applied to the panel sample
  • FT              N                   Total equivalent force applied to the full-scale panel
  • Ms,c           N.mm            Moment introduced into the symmetry plane of the panel sample
  • Ms              N.mm            Moment introduced into the the panel sample
  • lp                 mm               Length of the both riveted and FS welded full-scale panels
  • ls                 mm               Length of the both of the panel samples used for FEM simulations
  • tp                 mm               Panel plate thickness
  • Jx,p            mm4              Modulus of rupture of the part of the panel plate used in FEM simulations
  • Mst,R          N.mm            Moment carried by the stringer sample of riveted construction panel in the FEM simulation
  • Mp,R           N.mm            Moment carried by the panel plate sample of riveted construction panel in the FEM simulation
  • Mst,FSW     N.mm            Moment carried by the stringer sample of FS welded construction panel in the FEM simulation
  • Mp,FSW      N.mm            Moment carried by the panel plate sample of FS welded construction panel in the FEM simulation
  • SR,st           mm2              Cross section surface area of the stringer for riveted panel
  • SFSW,st     mm2              Cross section surface area of the stringer for FS welded panel
  • Fs,R,st        N                   Shear force introduced into the stringer of the riveted panel
  • Fs,R,p         N                   Shear force introduced into the panel plate of the riveted panel
  • Fs,FSW,st   N                   Shear force introduced into the stringer of the FS welded panel
  • Fs,FSW,p    N                   Shear force introduced into the panel plate of the FS welded panel
  • ta                sec                Tool approach time
  • sta               mm               Tool approach distance
  • vff                mm.min-1      Fast feed speed
  • tw                sec                Welding time
  • nw                –                    Number of welds on stringers
  • sa,d              mm               Slow approach/drifting to/out of the work piece distance
  • sal                mm               Material allowance length
  • sw                mm               Weld length
  • vw               mm.min-1      Welding speed
  • ttm              sec                Time for tool movement between the welds
  • stm              mm               Overall distance travelled between the welds
  • tbrp             sec                Tool return to base position time
  • sbpr             mm               Distance from the last weld to the tool base position
  • tt                 sec                Total time of welding process
  • q                N.mm-1          Value of loading applied on the panel in FEM simulation
Summary
application of friction stir welding in aircraft structures - Application of friction stir welding in aircraft structures
Article Name
Application of friction stir welding in aircraft structures
Description
Basic information about friction stir welding (FSW) technology, which is a new high-tech way of metallic material jointing used mainly on aircraft and spaceship structures. The technology itself is quite simple and cheap. It hasn't been spread more widely because accessibility of information about FSW is rather low.
Author
Publisher Name
https://www.steeljrv.com
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