Investigation of weld defects in friction-stir welding and fusion welding of aluminium alloys
© Kah et al. 2015
Received: 12 March 2015
Accepted: 15 December 2015
Published: 22 December 2015
Transportation industries are obliged to address concerns arising from greater emphasis on energy saving and ecologically sustainable products. Engineers, therefore, have a responsibility to deliver innovative solutions that will support environmental preservation and yet meet industries’ requirements for greater productivity and minimised operational costs. Aluminium alloys have successfully contributed to meeting the rising demand for lightweight structures. Notable developments in aluminium welding techniques have resolved many welding related problems, although some issues remain to be addressed. The present study attempts to give an overview of the key factors related to the formation of defects in welding methods commonly used with aluminium alloys. First, a concise overview of defects found in friction-stir welding, laser beam welding and arc welding of aluminium alloys is presented. The review is used as a basis for analysis of the relationship between friction-stir welding process parameters and weld defects. Next, the formation and prevention of the main weld defects in laser beam welding, such as porosity and hot cracking, are discussed. Finally, metallurgical aspects influencing weld metal microstructure and contributing to defects are tabulated, as are defect prevention methods, for the most common flaws in arc welding of aluminium alloys.
KeywordsDefects Friction-stir welding Aluminium alloys Laser beam welding Arc welding Process parameter effects
Aluminium alloys have been one of the primary candidates for material selection in many industries, including the commercial and military aircraft and marine sectors, for more than 80 years, mainly due to their well-known mechanical behaviour, design ease, manufacturability and the existence of established inspection techniques (Dursun and Soutis 2013). Increasing utilization of aluminium in various industrial sectors is the main driving force in the search for a viable and efficient technology for joining aluminium that does not cause deterioration in the desirable mechanical, chemical and metallurgical performance of the material.
In recent years, the growing concerns surrounding energy saving and environmental conservation have increased the demand for lightweight structures. While modern alloys such as advanced high-strength steel (AHSS) have allowed many industrial objectives to be met, for example, weight reduction while maintaining crashworthiness in vehicles, further significant reduction of weight, of the order of 30 %, is highly unlikely without the usage of multi-material structures (Sakiyama et al. 2013).
The best combinations for such multi-material structures are considered to be aluminium alloys and AHSS. However, such dissimilar materials are difficult to join by welding due to the differences in their mechanical and physical properties and due to the formation of large amounts of brittle intermetallic compounds (Ogura et al. 2012). Aluminium is unique as a weld metal when compared to ferrous alloys because aluminium lacks a solid-state phase transformation upon cooling. Therefore, only solidification determines its microstructure. However, the high temperatures found during fusion joining processes significantly affect the microstructure of the metals, which has a direct impact on the properties and the behaviour of the material (Courbiere 2008).
This work considers three welding methods: friction-stir welding (FSW), laser beam welding and arc welding. First, we focus on the factors contributing to the defects in FSW of aluminium alloys. As FSW is a complex hot shear and forging process, identification of the origin of defects is not straightforward. The defect population and residual stresses in the weld zone are greatly influenced by the complex plastic deformation process. Therefore, a long-standing problem has been a lack of clear information on the effects of friction-stir welding process parameters on weld defects that would enable relationships and correlations to be drawn and would assist optimisation of FSW. Relationships are identified between the plastic flow mechanism around the tool, process parameters (such as tool tilt or penetration into the joint) and FSW defects. In the second part of the paper, we focus on laser beam welds and investigate defect mechanisms in laser beam welding of aluminium. Laser beam welding is constantly evolving as laser beam power source technology advances. However, a number of problems and issues remain to be resolved. The formation and prevention of the main weld defects in laser beam welding of aluminium alloys, such as porosity and hot cracking, are discussed. The third part of the paper focuses on the weld related flaws in arc welding of aluminium alloys. Heat generated for joining can cause significant changes in material microstructure, thereby compromising the mechanical property of the base metal and causing weld distortion. For example, in fusion welding of aluminium alloys, the generated heat, which supports the joining of the metal, can lead to microsegregation of alloying elements such as copper, magnesium, silicon and manganese. Solidification cracking, weld porosity and heat-affected zone liquation cracking are some of the flaws examined.
Characteristics of friction-stir welding
Advantages of friction-stir welding (FSW) over traditional processes
Some aluminium alloys that are either not weldable or difficult to weld due to problems of brittle phase formation and cracking are now weldable by friction stir welding as it is a solid-state process.
Longitudinal and transverse distortion is minimised in the FSW process due to the lower peak temperature in FSW compared to arc welding processes.
FSW welds exhibit improved fatigue resistance during cyclic loading conditions due to the lower peak temperature and lower residual stress.
Filler material requirements
For some aluminium materials, no suitable filler material matching the strength of the base material is available for arc welding processes. FSW does not require filler material to join the metals.
The comparatively few process parameters involved and easy controllability make FSW a relatively stable process.
Issues in friction-stir welding of aluminium alloys
In FSW, several thermo-dynamical process interactions occur simultaneously, including the varied rates of heating and cooling and plastic deformation, as well as the physical flow of the processed material around the tool. Throughout the thermal history of a friction-stir weld, no large-scale liquid state exists (Grujicic et al. 2010). Flaws such as porosity and hot cracking are not found in friction-stir welding as it is a solid-state joining process (Arbegast 2003). When a metal is friction stir welded, joining occurs well below the melting point, and so the parent metal does not undergo bulk melting at the joint.
In most welding processes, the materials are generally joined by reducing the resistance to deformation by supplying the required amount of energy in the form of heat. However, the heat supplied tends to create conditions that cause microstructural changes such as recrystallisation, grain orientation growth, and coarsening or dissolution of the strengthening precipitates. Such microstructural changes occur at different temperatures for different materials and are dependent upon the chemical composition of the materials involved. Therefore, depending upon the chemical composition of the material, the processing conditions can be termed either ‘very hot’ or ‘very cold’ processing (Schneider et al. 2006). Friction-stir welding is still susceptible to flaw formation because it lacks the potential for imbalances between the distinct processing zones. Defects such as non-bonding or void formation can occur at very cold welding conditions, due to insufficient material flow, and flaws such as flash formation, collapse of the nugget within the stir zone and deterioration in the strength properties of the joint can occur at very hot conditions, due to excessive material flow (Annette 2007). In addition to these flow related defects, other geometry related defects also exist, such as lack of penetration and lack of joining, which mainly occur due to operator errors.
Factors related to imbalances in the material flow associated with the position of the tool in relation to the joint are the main reasons for flaw formation in friction-stir welding. For example, incorrect setting of the tool position to the joint line can lead to a lack of joining. Depending on the distance from the tool, phenomena like dissolution and coarsening of precipitates or recovery and recrystallisation can occur to different extents (Wanjara et al. 2013; Annette 2007). Additional problems can be expected if the gap between the abutting plates is not tightly controlled. Significant reductions in fatigue strength occur with increasing gap between the plates to be welded.
Formation of an onion-ring microstructure
Onion-ring structures in friction-stir welds of aluminium alloys can be observed as bands in the weld nuggets. Onion-ring structures have notable dark and bright bands, and the spacing between the bands is equal to the forward motion of the tool in one rotation. In an onion-ring structure, the spacing of the alternate bands increases with the increasing rotations of the tool and increasing material transport per measure of the weld length (Krishnan 2002). The rotary speed of the tool determines the amount of heat produced per unit time and the stirring and the mixing of the material around the pin (Peel et al. 2003). The rotary and traverse speeds of the tool govern the peak temperature generated during welding and the time required for welding of the material. The translation of the tool entrains the material from the advancing side, and the material is rotated around the pin and deposited on the rear of the retreating side. Material carried from the retreating side of the weld is deposited to fill in the material cavity in the wake of the pin (Krishnan 2002; Nandan et al. 2008). Thus, the FSW nugget consists of a mixture of two streams of material with different histories and mechanical properties, which often leads to an onionskin microstructure.
Increasing the process temperature significantly influences the formation and the subsequent roles that the bands play in the formation of a crack path in a weld nugget placed under cyclic loading. The differences in the size, shape and density of the intermetallic particles within the bands are the result of hotter welds. Crack initiation in the weld is affected by onionskin partial bonding defects, and the tool pitch directly influences these defects. For a constant rotational speed, softening of the weld nugget reduces as the feed rate or translational tool velocity is increased (Krishnan 2002). Hence, it is clear that the formation of an onionskin macrostructure is related to variation in the tool pitch along the weld joint. Consequently, the possibility exists for process optimization to modify the weld microstructure and improve material properties, including fracture resistance.
Formation of flash defects
The material being welded experiences very hot processing conditions as the tool pin rotates at very high speeds. Therefore, excessive heat generated, thermally softens the material near the boundary of the tool-shoulder and expels large volumes of material in the form of surface flash. Excessive tool-shoulder frictional heat softening of the material is the reason for the formation of the flash, and high tool-shoulder pressure leads to the ejection of an excessive amount of flash (Bo et al. 2011). Incorrect tool pin length relative to workpiece thickness and change in penetration depth due to variation in plate thickness along the weld line or due to a bowed plate can lead to a lack of penetration. When the pin plug depth is high, the plastic material near the pin is extruded, which results in weld flash. When the pin depth is very high, extruded flash can occur at the roots of the weld, near the pin. At larger tool tilt angles, insufficient plasticised material remains to fill the cavity left in the weld nugget and weld flash appears on the retreating side (Keivani et al. 2013).
Formation of tunnel defects
As mentioned earlier, if the processing conditions, i.e. weld travel speed, tool rotation, etc., fail to generate the required heat for bonding, inadequate material mixing and stirring can occur, resulting in the formation of tunnel defects (Grujicic et al. 2010). Rapid dissipation of heat from the immediate deformation zone can also lead to too cold welds. A weld produced under too cold welding conditions becomes macroscopically hard, and fracture can occur through the defect.
As the tool progresses along the weld, the plasticised material around the tool pin is transferred layer by layer. The width of the plasticised material around the pin and the material volume carried per rotation determines the restriction of the material to flow from the retreating side to the advancing side, inside the cavity. The cavity is created behind the tool pin due to the unconsumed volume of the plunged pin. In order to maintain a large heat input during friction-stir welding, the transverse speed can be reduced, thereby generating more heat and more plastic metal, which improves the flowability of the weld metal (Kumar and Satish Kailas 2008; Xiaopeng et al. 2014). Experimental results (Zhao 2014) suggest that the area in which tunnel defects can occur, increases greatly as the traverse speed increases. Increasing shoulder diameter significantly increases the heat input volume, which directly improves the flowability of the weld metal into the cavities. Therefore, optimised heat input and good flow patterns of the plastic material are necessary to avoid very cold processing conditions and thus eliminate tunnel defects. Hence, a welding tool with a relatively large shoulder can help reduce the occurrence of tunnel defects.
Formation of kissing bond defects or zigzag defects
At very high rotary speed, sufficient heat input supports proper stirring of the material with wide and diffused distribution of oxide particles. The average grain size of aluminium present in the weld nugget decreases with the increasing welding speed or decreasing rotary speed. Hence, the control of rotary speed allows significant reduction in zigzag line defects (Xiaopeng et al. 2014). It has been reported that the fatigue performance of friction-stir welded joints of 7075-T6 alloy was undermined by the presence of a zigzag line defect; a fracture initiated at the root along the zigzag line and caused failure from the weld nugget during tensile testing (Di et al. 2007). Effective selection of FSW parameters eliminates the formation of zigzag lines, contributing to improved mechanical performance.
Formation of crack-like root defects
Formation of voids
The presence of voids in the weld is a common defect in friction-stir welds. The fluid dynamics associated with plastic flow in the weld nugget plays a key role in the formation of such voids. Although high welding speeds promote more economical friction-stir welds and higher productivity, too high welding speeds lead to the formation of voids beneath the top surface of the weld or on the advancing side at the edge of the weld nugget. Further increase in speed leads to the formation of bigger wormhole defects (Crawford et al. 2006).
Characteristics of laser beam welding
Laser beam welding (LBW) is a promising and increasingly important joining technology for products made of aluminium alloys. Laser welding uses the radiant energy carried in a very small beam cross section of very high power density to weld the boundary surfaces of the two parts to be welded. Laser beam welding provides welds of high quality, precision and performance, and with low deformation or distortion. The tight focusability and high power density of lasers enable very good flexibility and very high welding speeds, narrow and deep welds, small heat-affected zones and good mechanical properties to be achieved. Advantageous characteristics such as reduced manpower demands, full automation and suitability for integration with robotic systems (Katayama 2005) make LBW appropriate for a wide range of applications and welding contexts.
Weld porosity and prevention methods
A critical problem in laser beam welding of aluminium alloys is porosity, which causes stress concentration effects. The two types of porosity occur in laser welding of aluminium alloys: metallurgical porosity and keyhole porosity. Metallurgical porosity mainly occurs due to the presence of hydrogen in the weld pool. Keyhole pores are comparatively larger and irregular in shape. These porosities are mostly present in the weld centre. Keyhole porosity has mainly been observed in partial penetration welds and is rarely observed in full penetration welds (Whitaker et al. 1993; Katayama et al. 2010). Matsunawa et al. (1998) suggest that the primary cause for porosity is the unstable nature of the hole drilled in the liquid pool. Other observations (Katayama et al. 2010; Seto et al. 2000; Menga et al. 2014) support this hypothesis, and it has been reported that keyhole instability is the main cause of bubble initiation especially in deep penetration welding. Katayama et al. (2010) present a mechanism of porosity formation during pulsed laser welding. Seto et al. (2000) report the same information for continuous laser welding. For pulsed laser welding, it is reported that when the laser is terminated, the melt surrounding the upper part of the keyhole flows downward to fill the keyhole. Porosity is formed when the upper part of the melt rapidly solidifies, preventing the melt from flowing down to fill the keyhole. With continuous laser welding, bubbles are formed at the bottom tip of the keyhole. Some of these bubbles are able to escape the molten pool, but others are trapped at the solidifying front, resulting in the formation of porosity at the bottom of the weld seam. At low welding speeds, porosity is formed from bubbles generated at the tip of the keyhole; whereas, at high laser-power densities, porosity is formed in the middle part of the keyhole (Katayama et al. 2010). Matsunawa et al. (2000) also reported that fluctuations of the keyhole resulted in the formation of bubbles at the tip of the keyhole, which in turn formed porosities. Hydrogen porosity can be effectively reduced by increasing the welding speed so that insufficient time is available for the hydrogen to accumulate because of the rapid cooling and solidification. Using a high-power fibre laser, Katayama et al. (2009) investigated penetration and defect formation in several aluminium alloys. They found that 10-mm thick plates of AA5083 were penetrated completely with a power density of 64 MW/cm2 and that nitrogen gas was more effective than argon at preventing porosity. Their research showed that keyhole-induced porosity can be avoided by using effective welding parameters and vacuum conditions. The results substantiated the work of Kawahito et al. (2007), who stated that processing parameters and surface conditions are responsible for porosity formation but can be effectively controlled by optimisation.
Other defects in laser beam welding
HAZ degradation is not severe in laser beam welding (LBW) of aluminium alloys as the process uses low power and low heat input. However, highly localised mechanical property variation can prove detrimental for structural materials due to localised deformation. In addition, some alloys are highly susceptible to weld metal or HAZ cracking, which is especially the case for 6xxx series alloys because of the formation of Mg–Si precipitates. Proper addition of filler wire can, however, mitigate this problem by reducing the freezing range of the weld metal and thereby minimising the tendency to solidification cracking. High-power density processes are not recommended for certain alloys, such as 6061 and some 5000, 6000 and 7000 series alloys, because the high power density can vaporise strengthening elements such as Mg and Zn. The presence of Mg is very important in 5000 series and 6000 series alloys; as is Zn in 7000 series alloys (Cross et al. 2003). Ramasamy and Albright (2000) found that vaporisation of magnesium and silicon occurred and metal hardness was reduced in welding of aluminium alloy 6111-T4 with a 2-kW Nd:YAG laser in the pulsed mode, a 3 kW continuous wave Nd:YAG laser, or a 3–5 kW CO2 laser.
Characteristics of arc welding processes
Special shielding gas mixtures used in arc welding of aluminium and key gas characteristics (based on (Regis 2008))
Shielding gas mixtures
Resultant positive weld features
Argon and helium (80 %)
Improvements in bead profile and fusion
Argon—low cost and better protection as its density is higher than air.
Argon and chlorine
Significant reduction in porosity & improved process tolerance
Chlorine—extreme toxicity limits its suitability for many applications.
Argon and Freon
Improved arc stability and weld bead geometry
Argon/Freon mixtures—non-toxic so Freon can substitute chlorine yet obtain similar effects to argon/chlorine mixtures.
Helium—welding Al leads to high levels of ozone. Small amounts of nitric oxide can control ozone formation.
Arc welding cracks and prevention methods
Common problems encountered in fusion welding of aluminium alloys, metallurgical aspects and prevention strategies (based on (Kou 2003))
Type of alloy
Metallurgical aspects promoting the defect
Higher strength alloys (e.g. 2014, 6061, 7075)
◦ Solidification temperature range
◦ Grain structure
◦ Primary solidification phase
◦ Quantity of eutectic liquid at the end stage of solidification
◦ Coarse columnar dendritic structure—higher susceptibility
◦ Fine equiaxed dendritic structure with abundant eutectic liquid—lower susceptibility
♦ Appropriate dilution ratio
♦ Appropriate control of minor alloying elements
♦ Grain refinement—using agents
♦ Magnetic arc oscillations
♦ Reduce strains—preheating
♦ Improve weld bead shape
Loss of strength in HAZ
Work hardened materials and heat-treatable alloys
◦ Increase in heat input/unit length—increases the size of HAZ and retention time above effective recrystallisation temperature
◦ Deformed grains (due to work hardening) that tend to recrystallise (forming strain free, soft grains)—softens the HAZ
♦ Reduce heat input—weld process like EBW or GTAW
◦ Wide PMZ—high thermal conductivity and wide freezing temp range
◦ Large solidification shrinkage
◦ Large thermal contraction
◦ Grain boundary (GB) liquid—weakens the PMZ
♦ Appropriate filler material
♦ Reducing heat input—multipass welding, etc.
♦ Decrease in degree of restraint
♦ Oscillating arc method
Aluminium alloys are most attractive solutions for many industrial sectors, including the aerospace, marine and other transportation industries, where demand for lightweight structures exists. FSW avoids problems related to melting, formation of cast microstructure and solidification of weld shrink zone that are associated with conventional fusion welding. Weld defects found in friction-stir welds are quite different from conventional welding flaws. FSW defects include an onionskin microstructure, tunnel voids, porosity, defective tightness, excessive flash, ‘kissing-bond’ defects and crack-like root flaws. In order to avoid such defects, the thermo-physical and mechanical properties of the welded material should be identified and the processing temperature and processing rates manipulated accordingly. Tool rotary speed and tool traverse speed govern the peak temperature generated during FSW and the time required to weld the material. The way in which temperature affects material properties varies significantly for different aluminium alloys. Hence, friction-stir welding parameters suitable for processing one series of aluminium alloys differ considerably from those suitable for other series alloys.
Innovations in power source technology for laser beam welding are expanding the range of suitable applications. The availability of higher power lasers and higher power densities has enabled the formation of stable keyholes and improved beam qualities and have mitigated problems related to high surface reflectivity and high thermal conductivity. As a result of these developments, both CO2 and Nd:YAG lasers can now be used for a wide variety of aluminium alloys. Shorter wavelengths mean a slight advantage in welding speeds for Nd:YAG lasers compared to similar power CO2 lasers. Two types of porosity occur in laser welding of aluminium alloys: metallurgical porosity and keyhole porosity. Keyhole instability is the main cause of bubble initiation especially in deep penetration welding; however, this can be reduced by using effective welding parameters and vacuum conditions. Metallurgical porosity mainly occurs due to the presence of hydrogen in the weld pool; therefore, to reduce hydrogen porosity, increased weld speed should be used, which results in insufficient time for hydrogen to accumulate due to rapid cooling and solidification.
Arc welding is a widely used joining method for aluminium alloys. Intense variations of energy, mass and momentum transfer occur from time to time at the terminating end of the weld, resulting in unsteady temperature and fluid flow fields. The lack of metal ductility and tensile stress promote crack formation. Utilization of an appropriate dilution ratio and control of minor alloying elements, grain refinement and magnetic arc oscillations can minimise its occurrence. At certain compositional limits, the amount of eutectic liquid is large enough to form continuous films at grain boundaries. This combined with high shrinkage leads to solidification cracking. In solute-rich aluminium alloys, crack sensitivity is very low since eutectic is abundant that it backfills and heals incipient cracks.
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