^ Super Liner Ogasawara: Technicians employed FSW to Weld its Aluminum Panels – Image Courtesy of Haruno Akiha (talk) at https://en.wikipedia.org/wiki/File:Super_Liner_Ogasawara.jpg
A Star on the Rise
When devised in December 1991 at The Welding Institute (TWI) in UK, many looked at Friction Stir Welding (FSW) as more of an experimental exploit confined to the bounds of laboratories. That was then.
Today, the process is spreading the tentacles of its application far and across the manufacturing world – shipbuilding, aerospace, automotive, railways, fabrication, defense, medical, electronics – you name it. After all, twenty five years is a lifetime in technological progress.
A plastic or solid state welding process, FSW does not melt materials. Instead, it heats them to the plastic stage and connects them by applying mechanical force via a welding tool. Such a technique provides high-strength, high-quality weld joints with low distortion.
In doing so, FSW sidelines the demerits of fusion welding processes incurred due to melting and solidification. Engineers choose FSW when they need sturdy joints but cannot undertake subsequent heat treatment because the welded material cannot stand such treatment.
Such materials include aluminum alloys that have seized the top spot in the list of desirables of many a designer. Blessed with high strength despite its low weight, aluminum and its alloys are replacing heavier steel – to a limited extent of course.
The Need for Friction Stir Welding (FSW)
As mentioned, aluminum and therefore its alloys are gifted with a high strength-to-weight ratio. Using such materials slashes the weight of structures. This is particularly useful for transport vehicles because it makes them burn less fuel.
With their own weight thus reduced sizably, vehicles can transport greater loads while guzzling as little fuel as possible. This not only generates greater revenues, but also cuts down on fuel costs. Burning less fuel, they emit fewer pollutants.
These days, you cannot overstate the dire necessity of cutting emissions. The very survival of life on this planet depends on how efficiently we cut emissions. Because, Global Warming holds the mighty potential of flooding half the world and sucking dry the remaining half.
Global transportation gobbles up 20% of the global energy consumption. The corresponding statistic for the U.S. stands at 28%. A staggering 22% of the total global greenhouse gas (GHG) emissions can be traced to transportation with 75% of the 22% coming from road vehicles.
Welding such materials with conventional welding processes is however notoriously hard. Let us take the case of welding aluminum with traditional welding methods, an exercise beset with a host of steep challenges that include:
- Hot Cracks
- Stress Cracks
- Poor Penetration
First, aluminum has high thermal conductivity and a low melting point. It conducts away the welding heat rapidly. You need to compensate by employing higher welding voltages and currents to generate more heat.
But because of its low melting point, using such high power means you have to speed up the process or risk burnthrough i.e. the formation of holes in base material. And that is not all.
If you speed up the process too much, there is lack of penetration or the weld joint not extending to the very bottom of the base materials. Striking the correct speed is a tightrope walk. Thicker sections routinely suffer from low penetration while thinner sections are exposed to burnthrough.
Next, aluminum eagerly forms oxides that restrict its melting during fusion welding. This is another reason for low penetration. The high solubility of hydrogen in molten aluminum escalates the hazard of porosity.
Finally, the low columnar strength of aluminum makes its wires vulnerable to birdnesting i.e. tangling of wire between the drive roll and liner. This limits the use of aluminum as feed metal for fusion welding. Phew! Seems welding fusion aluminum is quite a task.
The Process & Equipment Setups of Friction Stir Welding (FSW)
FSW uses a profiled probe with a broader cylinder shoulder as the tool. The control mechanism rotates the tool and feeds it along the length of the joint at a constant speed. The probe is just shorter than the required weld depth.
Being wear resistant, the tool withstands erosion as its motion generates frictional heat at the joint between the two clamped workpieces. The heat softens the workpieces. The contour of the probe is such that it forces the softened material together as it moves ahead.
The point is, FSW joins materials without melting them. A sophisticated version of forging heated materials with a hammer. By doing so, it avoids all the demerits of fusion welding that make their presence strongly felt particularly when welding aluminum and its alloys.
FSW is particularly compatible with:
- long and longitudinal joints of many types viz. fillet, butt, lap, and butt-lap combination, T-butt, both side butt, butt laminate, and lap laminate
- joining numerous metals with high melting points
- most light metals such as aluminum, magnesium, copper, lead, zinc, and their alloys
It can easily connect 2xxx series and 7xxx series aluminum alloys conventionally considered unweldable. Robotic FSW welds 5xxx series. Copper is the major alloying metal in the 2xxx series of aluminum alloys. Zinc is the main metal in 7xxx series and magnesium in 5xxx.
And its applications are not limited to welding aluminum alloys alone. It can also weld a whole range of other metals and alloys such as titanium, high strength steels, carbon steel, and stainless steel.
Then again, FSW is capable of binding dissimilar metals into a strong joint. Not for nothing are sectors such as shipbuilding, aerospace, defense, medical, electronics, and transportation harnessing the power of this maverick process.
FSW setups vary from small, table sized equipment to giant ones used for welding spacecraft components:
- Fabrication Shop FSW Setups: are modular with rack and pinion drive systems. Heavy duty bearings support the motion of the welding head
Capable of welding metals of 0.5 to 65 mm thickness, these machines work well with exchangeable clamping systems and larger fixtures
- Robotic FSW Systems: are more intricate. These can weld 5xxx series aluminum alloys of maximum 6 mm thickness
With the ability to apply a vertically downward force of 13 kN, they can weld along complex paths and adjust well with modifications to the path
- Custom Designed FSW Robots: can be column & boom, gantry type, and seam welder robots. These deal with high-volume production of aluminum alloy parts
Totally automated FSW robots find application in:
- Shipbuilding: for panel, stiffened panel, and long welding
- Aerospace: to weld space capsules, cryogenic tanks, and airframes
- Transportation: for welding truck structures and railcars
- Oil & Gas: to weld storage and fuel tanks
Determining the optimum values of diverse parameters requires judicious tradeoffs. For example, designers look to minimize welding forces to prevent breaking and wear of tools and setup. This requires large heat input and low traverse speed that erode FSW’s quality and productivity.
Important design parameters for FSW are:
- Welding Forces: because FSW uses very high welding forces. These include:
- Downward Force: maintains the tool position in relation to the weld material
- Transverse Force: acts along the direction of tool motion
- Torque: rotates the tool to generate friction
- Lateral Force: acts perpendicular to the direction of tool motion
Designers devise the welding cycle and forces so as to minimize the possible wear, fracture, or breakage of tools and machinery
- Tool Design: directly affects the strength and quality of the weld joint as well as the sped of welding because it controls the heat generation and penetration of the oxide layers on both materials
And because welding force is so important in FSW, tools must be hard, wear-resistant, tough, and strong. They must retain their hardness for long durations at elevated temperatures
Tools must also possess low thermal conductivity to minimize heat losses and thermal damage to the machinery. Plus, they must be oxidation resistant
Hot-worked tool steel such as AISI H13 has served well to weld aluminum alloys of 0.5 to 50 mm thickness. FSW of advanced materials such as metal matrix composites will require better tool material as will the FSW of diverse steels and other hard alloys
- Traverse Speed and Tool Rotation Speed: depends on the welding tool, materials and their thickness, and joint type. Generally, low traverse speeds and higher rotation speed provide hotter welds
Sufficient high temperatures around the joint facilitate plastic flow for better welds by minimizing the required downward force. Excessive heat however can cause all the aforementioned defects of fusion welding
- Surface Contact: is necessary to generate the correct level of friction for better quality welding. FSW apparatus with excellent downward force control give best results
- Plunge Depth and Tool Tilt: plunge depth is the depth of the lowest point on the tool shoulder below the surface of the weld material. Tool tilt is the angle of the tool
Such plunging hikes pressure and provides proper welding at the rear of the tool and so does tilting the tool by 2-4 degrees in a manner that keeps the front end of the tool higher than the rear
Pros & Cons
Although an automated FSW system costs more than a conventional welding setup, it will not cost more than a cutting edge, automated laser cutting machine.
Cost however is only one parameter, not the only parameter. In order to appreciate the virtues of FSW, we need to place the entire process on to a broader canvass.
At the root of most of its qualities is the fact that FSW is a plastic welding process. Unlike fusion welding processes, it does not melt the base materials and therefore uses low heat input.
This endows the process with a host of advantages:
- Joints with Higher Tensile Strength: because FSW is free of fusion, it induces minimum stress-causing shrinkage and distortion in joints. This transforms into joints with higher tensile strength
- Better Corrosion Resistance and Fatigue Protection: because it is free of high temperatures that attract corrosion as well as of fusion-solidification that boosts fatigue levels
- Power Efficient: uses lesser energy than gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW)
- Faster: outpaces GMAW many times over
- Improved Safety: as it does not generate sparks, fumes, or noise
Being safer, you do not have to seek regulatory sanctions that inevitably require installation of diverse equipment and compliance with umpteen standards
- More Aesthetic: joints look better than those welded by conventional processes
Other merits include:
- Requires No Accessories: such as wires or shielding gas. This cuts operating costs and more than compensates for the higher capital investment
- Welding from One Side: it can weld materials of between 0.5 mm and 65 mm from one side without creating voids or porosity
- Automation Compatible: because its setup is simpler
But the absence of melting-solidification that imparts FSW with multiple assets also handicaps it on numerous frontiers:
- Tunnel-Like Longitudinal Defects: in the weld material result from low temperature operations that render FSW incapable of accommodating large welding deformations
- Kissing Defect: is the very light and insufficient contact between the weld materials. This is similar to the low penetration defect in fusion welding. Insufficient depth of the probe creates this defect
What sets off the alarm bells ringing in case of kissing defect is that you cannot detect it with ultrasonic or X-ray testing. This one’s a real silent killer
Other weaknesses of FSW include:
- Requires Large Downward Forces
- Leaves Exit Hole: at the location where the tool is withdrawn
- Less Flexible: than arc and manual welding processes
All said and done, friction stir welding (FSW) is a process with great many capabilities that will blossom with further technological breakthroughs. And being a greener, more eco-friendly process, it will certainly command greater acceptance.
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And for a demonstration on how various welding processes work in practice, contact Kemplon Engineering, a top notch provider of marine fabrication services, marine pipe fitting, and large scale custom metal fabrication.