Welding

Weldability can be defined as the ability to produce a weld free of discontinuities and defects that results in a joint with acceptable mechanical properties, either in the as-welded condition or after a post-weld heat treatment. Although aluminum has a low melting point, it can be rather difficult to weld for several reasons:21 (1) the stable surface oxide must be removed by either chemical methods or more typically by thoroughly wire brushing the joint area; (2) the high coefficient of thermal expansion of aluminum can result in residual stresses leading to weld cracking or distortion; (3) the high thermal conductivity of aluminum requires high heat input during welding further leading to the possibility of distortion or cracking; (4) aluminum's high solidification shrinkage with a wide solidification range also contributes to cracking; (5) aluminum's high solubility for hydrogen when in the molten state leads to weld porosity; and (6) the highly alloyed, high strength 2XXX and 7XXX alloys are especially susceptible to weld cracking.

For aircraft structural joints, mechanical fastening is the preferred joining method; however, for some launch vehicles, the 2XXX alloys are fusion welded to fabricate large cryogenic fuel tanks. The 2XXX alloys that can be fusion welded include 2014,2195, 2219, and 2519. These alloys have lower magnesium contents, reducing their propensity for weld cracking. Alloy 2219 is readily weldable, and 2014 can also be welded to somewhat less extent. Alloy 2319 is also commonly used as a filler metal when welding 2219. Although the weldability of the aluminum-lithium alloy 2195 approaches that of 2219, it tends to crack more than 2195 during repair welding. The 6XXX alloys can also be prone to hot cracking but are successfully welded in many applications. Post-weld heat treatments can be used to restore the strength of 6XXX weldments. For the 7XXX alloys, those with a low copper content, such as 7004, 7005, and 7039, are somewhat weldable, while the remainder of the 7XXX series are not fusion weldable due to weld cracking and excessive strength loss.

The ability to fusion weld aluminum is often defined as the ability to make sound welds without weld cracking. Two types of weld cracking can be experienced - solidification cracking and liquation cracking. Solidification cracking, also called hot tearing, occurs due to the combined influence of high levels of thermal stresses and solidification shrinkage during weld solidification. Solidification cracking occurs in the fusion zone, normally along the centerline of the weld or at termination craters. Solidification cracking can be reduced by minimal heat input and by proper filler metal selection. The 4XXX alloys, with their narrow solidification range, are often used as filler metals when welding the 2XXX and 7XXX alloys. Liquation cracking occurs in the grain boundaries next to the heat affected zone (HAZ). Highly alloyed aluminum alloys typically contain low melting eutectics that can melt in the adjacent metal during the welding operation. Similar to solidification cracking, liquation cracking can be minimized by lower heat input and proper filler wire selection.

Due to its high solubility in molten aluminum and low solubility in solid aluminum, hydrogen can enter the molten pool and, with its decreasing solubility during freezing, form porosity in the solidified weld. Hydrogen is approximately 20 times more soluble in the liquid than the solid. Hydrogen normally originates from three sources: (1) hydrogen from the base metal, (2) hydrogen from the filler metal and (3) hydrogen from the shielding gas. Hydrogen from the base metal and filler wire can be minimized by ensuring that there is no moisture present and that all hydrocarbon residues and the surface oxide are thoroughly removed prior to welding.

Welding processes that produce a more concentrated heat source result in smaller HAZs and lower post-weld distortions; however, the capital cost of the equipment is roughly proportional to the intensity of the heat source. The nature of welding in the aerospace industry is characterized by low unit production, high unit cost, extreme reliability, and severe service conditions; therefore, the more expensive and more concentrated heat sources such as plasma arc, laser beam, and electron beam welding are often selected for welding of critical components.22

2.11.1 Gas Metal and Gas Tungsten Arc Welding

Gas metal arc welding (GMAW), as shown in Fig. 2.37, is an arc welding process that creates the heat for welding by an electric arc that is established between a consumable electrode wire and the workpiece. The consumable electrode wire is fed through a welding gun that forms an arc between the electrode and the workpiece. The gun controls the wire feed, the current, and the shielding gas. In GMAW, the power supply is direct current with a positive electrode. The positive electrode is hotter than the negative weld joint ensuring complete fusion of the wire in the weld joint. In addition, the direct current electrode positive (DCEP) arrangement provides cathodic cleaning of the oxide layer during welding. When the electrode is positive and a direct current is used, there is a flow of electrons from the workpiece to the electrode with ions traveling in the opposite direction and bombarding the workpiece surface. The ion bombardment breaks up and disperses the oxide film to create a clean surface for welding. The DCEP arrangement also provides good arc stability, low spatter, a good weld bead profile, and the greatest depth of penetration. A shielding gas, such as argon, is used to protect the liquid metal fusion zone; however, the addition of helium to argon provides deeper penetration. GMAW has the advantage of good weld metal deposit per unit time.

Gas tungsten arc welding (GTAW) uses a non-consumable tungsten electrode to develop an arc between the electrode and the workpiece. A schematic of the GTAW process is shown in Fig. 2.38. Although it has lower metal deposition rates than GMAW, it is capable of higher quality welds. However, when the joint thickness exceeds 0.375 in., GMAW is probably a more cost-effective

Fig. 2.37. Gas Metal Arc Welding

Flowmeter ,

. Valve

Flowmeter ,

. Valve

Fig. 2.38. Gas Tungsten Arc Welding

method. For welding aluminum with GTAW, an alternating current arrangement is used, which like the DCEP arrangement for GMAW, provides cleaning of the oxide layer during the welding process. The alternating current causes rapid reversing of the polarity between the workpiece and the electrode at 60 Hz. For this welding arrangement, tungsten electrodes and argon shielding gas are used. In general, material less than 0.125 in. thick can be welded without filler wire addition if solidification cracking is not a concern.

The reduction of strength and hardness in a fusion welded HAZ is illustrated in Fig. 2.39. The degradation of properties within the HAZ usually dictates joint strength. High heat inputs and preheating prior to welding increase the

Distance from Centerline (in.)

* Estimated from hardness readings.

Fig. 2.39. Strength Across Fusion Weld Joint21

Distance from Centerline (in.)

* Estimated from hardness readings.

Fig. 2.39. Strength Across Fusion Weld Joint21

width of the HAZ and the property loss. The property loss within the HAZ can be minimized by the elimination of preheating, minimizing heat input, using high welding speeds where possible, and by using multi-pass welding. Post-weld heat treatments can also be employed to improve properties, either by complete solution heat treating and aging or by post-weld aging only. Solution heat treating and aging will restore the highest properties but water quenching may result in excessive distortion; therefore, polymer quenchants that produce slower cooling rates are often used for post-weld heat treatments. While post-weld aging at moderate temperatures does not achieve as high properties as full heat treating, it is often used because it does not result in excessive distortion or warpage. It should be noted that both full heat treatment and post-weld aging result in decreased joint ductility.

2.11.2 Plasma Arc Welding

Automated variable polarity plasma arc (VPPA) welding is often used to weld large fuel tank structures. Plasma arc welding, shown in Fig. 2.40, is a shielded arc welding process in which heat is created between a tungsten electrode and the workpiece. The arc is constricted by an orifice in the nozzle to form a highly collimated arc column with the plasma formed through the ionization of a portion of the argon shielding gas. The electrode positive component of the VPPA process promotes cathodic etching of the surface oxide allowing good flow characteristics and consistent bead shape. Pulsing times are in the range of 20 ms for the electrode negative component and 3 ms for the electrode positive polarity. A keyhole welding mode is used in which the arc fully penetrates the workpiece, forming a concentric hole through the thickness. The molten metal then flows around the arc and resolidifies behind the keyhole as the torch

Gas Shroud

Plasma Gas

Shielding Gas x Dl -irm-i

Plasma Efflux

Tungsten Electrode

Water Cooling

Plasma Gas

Copper Nozzle

Shielding Gas

Plasma Arc x Dl -irm-i

Plasma Efflux

Weld Direction

Fig. 2.40. Plasma Arc Welding Process traverses through the workpiece. The keyhole process allows deep penetration and high welding speeds while minimizing the number of weld passes required. VPPA welding can be used for thicknesses up to 0.50 in. with square grooved butt joints and even thicker material with edge beveling. While VPPA welding produces high integrity joints, the automated equipment used for this process is expensive and maintenance intense.

2.11.3 Laser Welding

There is considerable interest in laser beam welding (LBW) of high strength aluminum alloys, particularly in Europe,23-25 where limited aerospace production has been announced. The process is attractive because it can be conducted at high speeds with excellent weld properties. No electrode or filler metal is required and narrow welds with small HAZs are produced. Although the intensity of the energy source is not quite as high as that in electron beam (EB) welding, EB welding must be conducted in a vacuum chamber. The coherent nature of the laser beam allows it to be focused on a small area leading to high energy densities. Since the typical focal point of the laser beam ranges from 0.004 to 0.040 in., part fit-up and alignment are more critical than conventional welding methods. Both high power continuous wave carbon dioxide (CO2) and neodymium-doped yttrium aluminum garnet (Nd:YAG) lasers are being evaluated. The wavelength of light from a CO2 laser is 10.6 ^m, while that of Nd:YAG laser is 1.06 ^m. Since the absorption of the beam energy by the material being welded increases with decreasing wavelength, Nd:YAG lasers are better suited for welding aluminum.25 In addition, the solid state Nd:YAG lasers use fiber optics for beam delivery, making it more amenable to automated robotic welding. Laser welding produces a concentrated high energy density heat source that results in very narrow heat affected zones, minimizing both distortion and loss of strength in the HAZ. In 0.080 in. sheet, speeds up to 6.5ft/min are achievable with a 2kW Nd:YAG laser and 16-20 ft/min with a 5 kW CO2 laser.

2.11.4 Resistance Welding

Resistance welding can produce excellent joint strengths in the high strength heat treatable aluminum alloys. Resistance welding is normally used for fairly thin sheets where joints are produced with no loss of strength in the base metal and without the need for filler metals. In resistance welding, the faying surfaces are joined by heat generated by the resistance to the flow of current through workpieces held together by the force of water-cooled copper electrodes. A fused nugget of weld metal is produced by a short pulse of low voltage, high amperage current. The electrode force is maintained while the liquid metal rapidly cools and solidifies. In spot welding, as shown in Fig. 2.41, the two parts to be joined are pressed together between two electrodes during welding. In seam welding, the two electrodes are replaced with wheels. While the 2XXX and 7XXX alloys are easy to resistance weld, they are more susceptible to shrinkage cracks and

Fig. 2.41. Resistance Spot Welding

porosity than lower strength aluminum alloys.26 Alclad materials are also more difficult to weld due to the lower electrical resistance and higher melting point of the clad layers. Removal of the surface oxide is important to produce good weld quality. Both mechanical and chemical methods are used, with surface preparation being checked by measuring the surface resistivity.

2.11.5 Friction Stir Welding

A new welding process which has the potential to revolutionize aluminum joining has been developed by The Welding Institute in Cambridge, UK.27 Friction stir welding is a solid state process that operates by generating frictional heat between a rotating tool and the workpiece, as shown schematically in Fig. 2.42. The welds are created by the combined action of frictional heating and plastic deformation due to the rotating tool.

A tool with a knurled probe of hardened steel or carbide is plunged into the workpiece creating frictional heating that heats a cylindrical column of metal around the probe, as well as a small region of metal underneath the probe. As shown in Fig. 2.43, a number of different tool geometries have been developed, which can significantly affect the quality of the weld joint. The threads on the probe cause a downward component to the material flow, inducing a counterflow extrusion toward the top of the weld, or an essentially circumferential flow around the probe.28 The rotation of the probe tool stirs the material into a plastic

Sufficient Force to Maintain Contact

Advancing ! of Weld

Shoulc

Sufficient Force to Maintain Contact

Advancing ! of Weld

Shoulc

Trailing Edge of Weld

Fig. 2.42. Friction Stir Welding Process27

Leading Edge of Weld

Trailing Edge of Weld

Fig. 2.42. Friction Stir Welding Process27

Oval Shape

Paddle Shape

Re-Entrant

Changing Spiral Form

Fig. 2.43. Sample Friction Stir Welding Tool Geometries Source: TWI

Oval Shape

Paddle Shape

Re-Entrant

Changing Spiral Form

Fig. 2.43. Sample Friction Stir Welding Tool Geometries Source: TWI

state creating a very fine grain microstructural bond. The tool contains a larger diameter shoulder above the knurled probe which controls the depth of the probe and creates additional frictional heating on the top surface of the workpiece. It also prevents the highly plasticized metal from being expelled from the joint. Prior to welding, the workpieces have to be rigidly fixed with the edges butted to each other and must have a backing plate to withstand the downward forces exerted by the tool. A typical welding operation is shown in Fig. 2.44.

The larger the diameter of the shoulder, the greater is the frictional heat it can contribute to the process. Once the shoulder makes contact, the thermally softened metal forms a frustum shape corresponding to the tool geometry with the top portion next to the shoulder being wider and then tapering down to the probe diameter. The maximum temperature reached is of the order of 0.8 of the melting temperature.29 Material flows around the tool and fuses behind it. As the tool rotates, there is some inherent eccentricity in the rotation that allows the hydromechanically incompressible plasticized material to flow more easily around the probe.27 Heat transfer studies30 have shown that only about 5% of the heat generated in friction stir welding flows into the tool with the rest flowing into the workpieces; therefore, the heat efficiency in FSW is very high relative to traditional fusion welding processes where the heat efficiency is only about 60-80%.

Once the tool has penetrated the workpieces, the frictional heat caused by the rotating tool and rubbing shoulder results in frictional heating and plasticization of the surrounding material. Initially, the material is extruded at the surface but as the tool shoulder contacts the workpieces, the plasticized metal is compressed between the shoulder, workpieces, and backing plate. As the tool moves down the joint, the material is heated and plasticized at the leading edge of the tool

Direction

Direction

Fig. 2.44. Friction Stir Welding Source: ESAB Welding Equipment AB

and transported to the trailing edge of the probe, where it solidifies to form the joint.31

The advantages of friction stir welding include (1) the ability to weld butt, lap and T joints, (2) minimal or no joint preparation, (3) the ability to weld the difficult to fusion weld 2XXX and 7XXX alloys, (4) the ability to join dissimilar alloys, (5) the elimination of cracking in the fusion and HAZs, (6) lack of weld porosity, (7) lack of required filler metals, and (8) in the case of aluminum, no requirement for shielding gases.30 In general, the mechanical properties are better than for many other welding processes. For example, the static properties of 2024-T351 are between 80 and 90% of the parent metal, and the fatigue properties approach those of the parent metal.32 In a study of lap shear joints, friction stir welded joints were 60% stronger than comparable riveted or resistance spot welded joints.33

The weld joint does not demonstrate many of the defects encountered in normal fusion welding and the distortion is significantly less. A typical weld joint, as shown in Fig. 2.45, contains a well-defined nugget with flow contours that are almost spherical in shape but are somewhat dependent on tool geometry. TWI has recommended the microstructural classification shown in Fig. 2.45 be used for friction stir welds. The fine-grained recrystallized weld nugget and the adjacent unrecrystallized but plasticized material is referred to as the thermomechanically affect zone (TMAZ); therefore, the TMAZ results from both thermal exposure and plastic deformation and extends from the width of the shoulder at the top surface to a narrower region on the backside. A series

Nugget (D)

A - Parent Metal B - Heat Affected Zone (HAZ) C - Unrecrystallized Area D - Recrystallized Nugget

C + D - Thermomechanically Affected Zone (TMAZ)

Nugget (D)

A - Parent Metal B - Heat Affected Zone (HAZ) C - Unrecrystallized Area D - Recrystallized Nugget

C + D - Thermomechanically Affected Zone (TMAZ)

Fig. 2.45. Friction Stir Fusion Weld2'

of concentric rings, called onion rings, are frequently observed within the weld nugget, possibly as a result of the swirling motion of the plasticized material behind the advancing tool probe. The unrecrystallized portion of the TMAZ, which has also undergone thermal exposure and plastic deformation, has a grain size similar to that of the parent metal. The HAZ is typically trapezoidal in shape for a single pass weld with a greater width at the tool shoulder due to the heat generated between the shoulder and the top of the workpieces. The HAZ results primarily from thermal exposure with little or no plastic deformation present.

Grain sizes in the weld nugget are extremely fine, much finer than in the base metal. Some preliminary investigations have indicated that the extremely fine grain size may promote superplasticity in friction stir "processed" material, possibly leading to some interesting applications such as friction stir welded/SPF aluminum unitized structure.34-36 On the other hand, under certain conditions, it has been reported that abnormal grain growth can result in the center nugget area and on the surface under the shoulder during post-weld solution heat treatment. However, this may be a function of welding parameters. In another study,37 it was reported that both tool rotation and feed rate influenced grain growth during heat treatment, with tool rotational speed being the predominate variable. Higher rotational speeds (i.e., 1000rpm vs. 500rpm) and slower feeds (i.e., 0.08in./s vs. 0.20in./s) decreased abnormal grain growth in 2095 that was subsequently solution heat treated and naturally aged to the T4 condition. It should be noted that speeds and feeds will influence the heat input during welding; at a constant feed rate, the amount of heat input will increase with increasing tool rotational speed, while at a constant tool rotational speed, the heat input will decrease with slower feed rates.

The friction stir welding process has already been adapted to a number of industrial applications. In 1999, the fuel tanks on the Boeing Delta II rocket were launched with friction stir longitudinal welds. Based on this early success, Boeing purchased the large friction stir welding unit shown in Fig. 2.46 to weld

Fig. 2.46. Friction Stir Welding Applications for Delta Source: ESAB Welding Equipment AB and The Boeing Company
Fig. 2.47. Large Friction Stir Welder Being Used in Marine Industry Source: ESAB Welding Equipment AB

Delta IV fuel tanks. The machine is equipped with one milling head and one welding head traveling on a vertical main beam. The parts to be welded are loaded on an indexing fixture, the edges are milled, welding is carried out, and the final length is then milled. Tanks up to 40 ft long by 16 ft in diameter can be welded. Another early adapter of friction stir welding has been the marine industry as shown in the welded ship skin in Fig. 2.47. Construction of high speed trains is another important potential application for friction stir welding.

Was this article helpful?

0 0

Post a comment