Engineering Failure Analysis 34 (2013) 387–396

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Engineering Failure Analysis

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Welding failure of as-fabricated component of aluminum alloy5052

1350-6307/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.engfailanal.2013.09.007

⇑ Corresponding author. Tel.: +91 22 25595402; fax: +91 22 25505151.E-mail address: kchandra@barc.gov.in (K. Chandra).

K. Chandra ⇑, Vivekanand KainMaterials Science Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 26 March 2013Received in revised form 23 August 2013Accepted 19 September 2013Available online 27 September 2013

Keywords:Alloy 5052Failure analysisWeldingSolidification crackingPorosity

This paper describes the failure analysis of the ‘‘tray section’’ made up of aluminum alloy5052 which is used as a specimen holder in a research reactor. Fracture was observed in thecentral rod of alloy 5052 before it was taken for service. The fracture had occurred in a brit-tle mode without any gross plastic deformation at a location where the rod was welded tothe stopper plate. Detailed microstructural examination was done using both optical andscanning electron microscopy. The weld fusion zone showed presence of high porosityand eutectic phases mainly along the inter-dendritic regions. These low melting tempera-ture eutectics were rich in Si and Fe and led to weld cracking along the dendritic grainsduring solidification of the welds. Solidification cracking of alloy 5052 was related to purealuminum filler wire used for welding that shifted the composition of the welds towardspeak cracking sensitivity of 1.5 wt% Mg. The failure of the tray section was concluded tobe due to welding defects, e.g. high porosity and solidification cracks. Recommendationsto avoid this type of failure are also proposed.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

A ‘‘tray section’’ made up of an aluminum alloy (Alloy 5052) is used as a specimen holder for irradiation testing in theresearch reactor. Alloy 5052 is widely used in applications such as automobile, pressure vessels, armor plate and componentsfor marine and cryogenic service [1]. Alloy 5052 is basically an Al–Mg alloy (Mg in the range of 2.2–2.8 wt%) and is non-heat-treatable. There are two categories of Al alloys used in structural and plant applications – non-heat-treatable and heat-treat-able. The former is mainly alloyed with Mg and designated as the 5xxx series according to ASTM standard. 1xxx, 3xxx and4xxx series are the other non-heat-treatable aluminum alloys. 5xxx series alloys have substantially different Mg contentsranging from around 0.2–6 wt%. All these alloys are welded with a minimum loss of strength; however, they differ with re-gard to their crack sensitivity. As in all non-heat-treatable aluminum alloys, especially in alloy 5052 due to the lower Mgcontent, the weldment has a higher weld crack sensitivity [1,2].

The fracture of central rod of the ‘‘tray section’’ was noticed even before it was taken for service. A schematic diagram of asection of the ‘‘tray section’’ is shown in Fig. 1. A number of stopper plates (discs) of alloy 5052 were welded to the centralrod of alloy 5052 at regular spacing along the entire length. The diameter and the thickness of the circular plate were 56 mmand 6 mm respectively. The diameter of the rod was 10 mm. The fracture of alloy 5052 rod had occurred at one such weldedlocation. On-site inspection of the fractured component was carried out and samples were identified for failure analysis.

Fig. 1. A schematic diagram showing a section of the ‘‘tray section’’. A number of stopper plates at a regular interval were welded to the central rod.

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2. Experimental

A sample (�10 mm length) containing the fracture surface was cut from the failed rod and the fractured surface wasexamined using a scanning electron microscope (SEM). To examine the microstructure of the base alloy, a specimen of about15 mm length was cut from the rod of alloy 5052. The cross-sectional surface was ground on successively finer grits of emerypapers and mirror-polished to a finish of 1 lm using a diamond paste. Chemical etching of the polished samples was done byKeller’s reagent (1 ml HF + 1.5 ml HCl + 2.5 ml HNO3 + 95 ml water). Since the failure had occurred in the welded region, themicrostructure of the weldment was also examined. The specimen was cut along the center of the rod at the welded regionso that the specimen contained the weld fusion zone, the rod and the stopper plate. The as-polished and etched specimenswere examined using both SEM and an optical microscope. The chemical composition of the base alloy and of differentphases present in the weldment was analyzed using an SEM equipped with an energy-dispersive analysis of X-rays (EDAX).

3. Failure analysis

3.1. Visual examination

Fig. 2 shows the fractured region of the tray section of alloy 5052. The weld joint of stopper plate disc and the central rodof the ‘‘tray section’’ are also visible in Fig. 2(a). Fracture had occurred in the central rod at the location where it was weldedto the stopper plate (Fig. 2(a)). The fracture surface shown in Fig. 2(b) appeared absolutely flat with no sign of deformation.This indicated that the alloy 5052 rod had failed by brittle fracture.

3.2. Fracture surface examination

Fig. 3(a and b) shows the SEM micrographs of the fracture surface at a lower and a higher magnification respectively. Noevidence of ductile fracture is noticed on the fracture surface, even though aluminum-based alloys are highly ductile in nat-ure. The fractograph of a typical ductile fracture shows dimples on the fracture surface. However, the fracture surface in thiscase was flat with no signs of plastic deformation (Fig. 3(a)). Since, the SEM examination was done after considerable time offailure; the fracture surface was covered with lots of oxides. This is due to highly oxidizing nature of aluminum and hence aproper fractographic examination could not be accomplished. At a higher magnification (Fig. 3(b)), a number of secondarycracks together with the dendritic structure of the weld fusion zone are visible on the fracture surface. Based on the fracto-graphic study, it was confirmed that the alloy 5052 rod was fractured in the weld fusion zone in a brittle mode.

Fig. 2. Photographs of (a) the fractured region of the ‘‘tray section’’ made up of aluminum alloy 5052 and (b) the fracture surface showing no sign ofdeformation.

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3.3. Microstructural characterization

Fig. 4 shows the microstructure on the cross-sectional surface of alloy 5052 rod specimen. The microstructure did notshow any defect in the material. The overall chemical composition of the rod as measured by SEM–EDAX showed 2.2 wt% Mgand balance Al. The other minor alloying additions could not be detected by this method. According to the ASTM standard[3], the major alloying element in aluminum alloy 5052 is Mg (in the range of 2.2–2.8 wt%) with minor alloying additions ofCr (0.15–0.35 wt%), Si (0.25 wt% max) and Fe (0.4 wt% max) etc. Therefore, the chemical composition of the alloy was equiv-alent to alloy 5052.

The intact piece of the weld joint of the rod and the stopper plate (sample 1 in Fig. 2) at the failed region was cut into halfalong the length of the rod and the cut surface was examined for microstructure. The optical micrographs in Fig. 5 show themicrostructure of the weldment as observed in the as-polished condition under an optical microscope. A large number ofpores were observed (Fig. 5(a)) all over the weld fusion zone of alloy 5052. The porosity in aluminum weldments is a com-mon defect and is associated with improper shielding and surface cleaning [1,4–6]. The maximum pore size was measured tobe around 200 lm, although there was large variation in the size of pores. A number of these pores joined together by weldcracks along the inter-dendritic regions leading to cracking in the fusion zone as is evident in Fig. 5(a). Fig. 5(b) shows themicrostructure of the rod in the partially melted zone (PMZ) of the weldment. The dendritic structure of Al-matrix is noticedtogether with remnant coring at several other locations. The formation of low melting eutectic particles along the dendriticboundaries is also seen in the microstructure. Two eutectic phases of different colors are noticed (Fig. 5(b)): the black-col-ored phase was Mg2Si while the gray-colored phase was FeMg3Si6Al8, as identified by SEM–EDAX examination. These phaseshave been reported [7,8] to form after welding or casting of Al–Mg–Si alloys. In addition, the boundary separation or crackinitiation at the inter-dendritic regions is also evident from the micrograph (Fig. 5(b)).

The circular stopper plates were welded to the central rod at a regular interval (Fig. 1) and fracture was observed in one ofthe weldments. Therefore, the microstructure was also examined on the weldment of the stopper plate and the rod locatednext to the failed region that had remained intact. For this, a specimen was cut along the center of the rod at the weldedregion so that the specimen contained the weld fusion zone, the rod and the stopper plate (Fig. 6). This specimen had a widecrack along the transverse direction of the rod in the PMZ of the weld, which was visible with the naked eye. The specimenwas subsequently ground, mirror-polished and etched. The specimen in the etched condition was examined using both opti-cal microscope and SEM. After polishing, it was found that the crack had almost propagated to the entire diameter of the rod(Fig. 6). In addition, large number of pores was also evident in the weld fusion zone (location ‘A’ in Fig. 6). The optical

Fig. 3. SEM micrographs of the fracture surface of the central rod (a) at a lower and (b) at a higher magnification showing a flat fracture with secondarycracks.

Fig. 4. The microstructure of alloy 5052 central rod as observed under an optical microscope showing a typical defect free structure.

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micrographs in Fig. 7 show the microstructure of the weldment. The main crack (location ‘B’ in Fig. 6) was located at theboundary of the fusion zone and the base alloy (Fig. 7(a)). Fig. 7(b) depicts the microstructure of weld fusion zone (location‘A’ in Fig. 6) indicating large porosity and weld cracks similar to that observed at the failed region of the weldment. Themicrostructure of the fusion zone consisted of well-developed cast columnar structure near the fusion boundary of the weld(lower part of the micrograph in Fig. 7(b)). In contrast, equiaxed dendritic grains were formed at the center of the weld(upper part of the micrograph in Fig. 7(b)). This type of solidification structure in the welds is the result of epitaxial solid-ification, which grows towards the center in a direction along the maximum thermal gradient. The growth rate increasesfrom zero at the fusion boundary to a maximum value at the weld center resulting in a weld solidification structure ofcolumnar grains at the fusion boundary to equiaxed grains along the weld center [1,4,9].

Fig. 5. Optical micrographs showing the microstructure of the weldment where failure was observed (a) high porosity and solidification or weld cracking inthe fusion zone and (b) eutectic particles and crack initiation along the inter-dendritic regions in the partially melted zone (PMZ) of the rod.

Fig. 6. Photograph of the weld joint of the stopper plate and the rod located next to the failed region where failure was not observed. A wide crack is seenalong the transverse direction of the rod in the partially melted zone of the weld. ‘A’, ‘B’ and ‘C’ are the locations at which detailed microstructures arepresented.

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The cracks or boundary separations and porosity in the weld fusion zone are more clearly evident in the SEM micrographs(Fig. 8) that show the location ‘C’ in Fig. 6 of the weldment. It is evident from the micrograph (Fig. 8(a)) that porosity andweld cracks were restricted to the weld fusion zone only and these defects were not present in the base alloy (i.e. alloy5052 rod). At a higher magnification (Fig. 8(b)), the porosity and cracks or boundary separations along the dendritic grainsin the welds are clearly evident. The formation of some eutectic phases (light colored phase in Fig. 8(b)) was also observed

Fig. 7. Optical micrographs showing the microstructure of the weldment located next to the failed region where failure had not occurred: (a) the main crackalong the fusion boundary (location ‘B’ in Fig. 6) and (b) high porosity and cracking in the weld fusion zone (location ‘A’ in Fig. 6). Dendritic grains areequiaxed at the center and columnar at the boundary of the fusion zone.

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along the dendritic grains. EDAX analysis was performed on these eutectic phases in the weld fusion zone to ascertain thetype of phase formed.

The overall composition of the weld fusion zone using SEM–EDAX was determined to be: Mg: 0.54, Si: 2.59 and Al: bal-ance, all in wt%. The results indicated substantial amount of Si in the welded regions, although it is restricted to less than0.3 wt% in alloy 5052. In addition, the eutectic phases present along the dendritic grains were rich in Mg, Si and Fe. As re-ported earlier, two different eutectic phases were identified in the weld microstructure. Fig. 9(a) shows gray-colored phase(appeared dark in the optical microscopic image in Fig. 5(b)) on which EDAX analysis was carried out. The correspondingEDAX spectrum is shown in Fig. 9(b). The chemical composition of the phase was: Mg: 21.3, Si: 13.6, and Al: balance, allin wt%. The overall composition of this phase appears to be the result of eutectic decomposition of liquid phase (L) to a(Al) and Mg2Si [8]. The chemical composition of the other white-colored phase (Fig. 9(c)) at the dendritic boundary (ap-peared gray in color in the optical microscopic image in Fig. 5(b)) was: Mg: 2.26, Si: 12.53, Fe: 11.90 and Al: balance, allin wt%. The corresponding EDAX spectrum is shown in Fig. 9(d). In some of the precipitates, the Si content was measuredup to 26 wt%. This phase may be related to FeMg3Si6Al8 which is also the product of eutectic decomposition of liquid phase[8]. The incorporation of Si in the weld fusion zone might be through the filler wire or through the stopper plate because thechemical analysis of the rod using EDAX did not show any Si content. It is to be mentioned here that welding of rod andstopper plates was done by gas tungsten arc welding (GTAW) process using pure aluminum filler wire.

To analyze for the increased Si content in the weld, EDAX analysis was also performed on the filler wire and the stopperplate. The EDAX spectrum of the filler wire did not show any Si peak and was confirmed to be pure aluminum. However, incase of stopper plate, the EDAX analysis showed the presence of Si- and Fe-rich precipitates. These precipitates are clearlyseen in the microstructure of the stopper plate (Fig. 10(a)). The EDAX spectrum of one of the precipitate in Fig. 10(b) shows Siand Fe peaks and the chemical composition of the precipitate was measured to be Si: 5.91, Fe: 9.13 and Al: balance, all inwt%. Therefore, the stopper plate was confirmed to be the source of Si in the welds. In addition, the overall compositionof the stopper plate did not conform to alloy 5052. To accurately determine the chemical composition of the stopper plate,wet chemical analysis was performed. The chemical composition of the stopper plate was determined to be: Mg: 0.65, Si:0.53, Fe: 0.3 and Al: balance.

Fig. 8. SEM micrographs of the weldment located next to the failed region where failure had not occurred (location ‘C’ in Fig. 6): (a) the main crack at thefusion boundary and high porosity in the weld fusion zone and (b) eutectic phases and crack initiation along the dendritic grains at a higher magnification.

Fig. 9. SEM micrograph of the welded region of the rod (location ‘C’ in Fig. 6) showing (a) gray-colored and (c) white-colored precipitates at the dendriticgrain boundaries. The corresponding EDAX spectrums are shown in (b) and (d) respectively indicating these precipitates to be Mg, Si and Fe rich.

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Fig. 10. (a) SEM micrograph of the stopper plate showing small size precipitates in aluminum-matrix and (b) the EDAX spectrum of the precipitateindicating it to be Si- and Fe-rich.

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4. Discussion

Based on the results, it is obvious that the failure in the central rod of tray section of aluminum alloy 5052 had occurred inthe regions where it was welded to the stopper plate. The fracture of the rod along the transverse section at the weldmentwas associated with both very high porosity and weld cracks along the inter-dendritic regions. The presence of cracks alongthe dendritic grains in the fusion zone or near to the fusion boundary are the result of hot cracking or solidification crackingthat occur during the welding process [4,8,10–12]. Generally, solidification cracking occurs by liquation mechanism wherebylow melting temperature eutectic phases are formed especially along the dendritic grains during the welding process. As thesolidification of the melt start, it leads to a high level of thermal stress and solidification shrinkage, while the eutectic phasein the alloy remains liquid. Strain accumulated due to solidification shrinkage and thermal contraction serves to pull weldmetal grains apart in the mushy zone, resulting in the separation of grain boundary liquid films [4,8,10–13]. Hot crackingtakes place in a brittle mode without any gross plastic deformation. This is the reason of observing brittle fracture in thiscase, although the aluminum-based alloy is highly ductile.

It is reported [1,2,10] that aluminum-based alloys with lower Mg content (<2.5 wt%), e.g., alloy 5052 are particularly sen-sitive to hot cracking during welding. As small alloying additions (Mg or Si) are made, the crack sensitivity becomes moresevere, reaches a maximum and then falls off to relatively low levels with increase in alloying additions. At high solute levels,there is sufficient eutectic liquid available for the back-filling of tears or cracks that form during solidification [2]. In fact,most of the 5xxx series alloys (Al–Mg alloys) show little crack sensitivity. The peak of cracking sensitivity is at approximately1.5 wt% Mg. All of the 5xxx alloys, except alloy 5052, contain considerably more than 1.5 wt% Mg and lie well away from thecracking peak. Alloy 5052, however, lies right on the cracking peak and shows high cracking sensitivity amongst the 5xxxalloys. The crack sensitivity during welding can be reduced by selecting a proper filler wire so that the chemistry of solid-ifying weld pool is altered. For materials that exhibit high crack sensitivity, a filler wire is used which has a very differentchemistry so as to get a weld chemistry away from the cracking peak (1.5 wt% Mg). Therefore, for alloy 5052, a high-Mg con-tent aluminum filler alloy such as 5356, 5183 and 5556 (5.1 wt% Mg) should be used for welding to bring the chemistry ofthe solidifying weld away from the cracking peak.

Incorporation of Si in the weld further aggravates the hot cracking problem. For Al–Si welds, the peak cracking is devel-oped at 0.8% Si [2,10]. Alloys of aluminum containing Mg and Si, i.e. 6xxx series Al–Mg–Si alloys are highly susceptible to

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solidification cracking [5,14]. Especially, low additions of Si and Mg, up to around 2 wt% each in aluminum, have a very poorresistance in this regard [5]. Maximum cracking is reported to occur in the vicinity of 1% Mg2Si and decreases with furtheralloy content. Mg and Si rich eutectic particles have been reported [11] to form along the dendritic grains and cause liquationduring welding of these alloys. However, cracking can be lowered by alteration of the weld chemistry with excess Mg (Al–Mgfiller wires) or excess Si (Al–Si filler wires). Therefore, ingress of Si in the weld fusion zone due to any reason should beavoided. As reported earlier, the incorporation of Si in the welds in the present case was from the stopper plate. Si and Ferich precipitates were present in the microstructure (Fig. 10(a)) of the aluminum alloy of the stopper plate. Fe is an ever-present impurity in aluminum and is reported to be present in aluminum-based alloys due to its low solubility in solid alu-minum [11]. In addition, the chemical analysis of the stopper plate did not conform to alloy 5052 as was previously reportedto be the alloy for the stopper plate. Based on the wet chemical analysis, the material of stopper plate was identified as alu-minum with impurities of Fe and Si. The welding of this stopper plate with alloy 5052 rod led to dilution of Mg in the weld.The use of pure aluminum as the filler metal for welding further diluted the Mg content in the welds to well below 2.5 wt%present in the rod of alloy 5052. This might have led to the composition of the weld moving to the peak cracking sensitivityof around 1.5 wt% Mg causing solidification cracking in the welds.

Porosity in aluminum weldments is one of the most common and undesired defects. Aluminum and its alloys, welded byGTAW process, are particularly sensitive to porosity formation [6]. In the present case, very high porosity (pore size up to200 lm) was observed in the welds (Figs. 5(a), 7(b) and 8(a)). Generally, increase in porosity is associated with high humid-ity, improper shielding of inert gas (water vapor) and poor surface cleaning (water vapor and hydrocarbons) during welding[1,4–6]. It is postulated that hydrogen is produced in the arc atmosphere by dissociation of water vapor and the hydrocar-bons. Porosity may form when hydrogen gas is entrapped during solidification. Hydrogen is absorbed into the molten poolduring welding because of its high solubility in molten aluminum at high temperatures. Since solubility of hydrogen is muchlower in the solid state, hydrogen atoms are rejected of the advancing solid–liquid interface causing gaseous hydrogen pores.Porosity could also come from the nucleation of hydrogen bubbles ahead of this interface caused by the supersaturation ofthe liquid and bubbles becoming frozen in the melt [1,4,6]. The presence of porosity severely reduces the mechanical prop-erties (reduction in ductility and ultimate tensile strength) of the weld [12,15] and may lead to fracture in the presence oftensile stresses during welding. Porosity at a level of 4% has been shown to reduce the ductility by 50% from its highest levelin the welds of Al–Mg alloy 5086 [15]. The pores, besides being stress concentration sites for crack initiation, formed thepreferred path for crack propagation and get interlinked during fracture. Fig. 5(a) clearly suggests that the pores presentalong the path of solidification cracks can join together in the weld fusion zone. This can lead to much faster crack propa-gation under tensile stresses and hence easier fracture in the weld fusion zone having high porosity.

5. Conclusion

Failure investigation was carried out on a ‘‘tray section’’ made up of aluminum alloy 5052 in which fracture occurred inthe central rod even before it was taken for service. Fracture was located in the region where alloy 5052 rod was welded tothe stopper plate. Fracture had occurred in a brittle mode with no gross plastic deformation. Very high porosity and solid-ification cracks were concluded to be the main reason for the failure after detailed microstructural examination of the weldfusion zone. Silicon and iron rich eutectic phases were formed along the dendritic grains that led to cracking under tensilestresses during the solidification of the welds. The source of silicon and iron was the stopper plate that was welded to thecentral rod.

6. Recommendations

� It is recommended to use the same grade of alloy for the stopper plate as that for the rod material (i.e. alloy 5052).� Pure aluminum filler wire should not be used for the welding of the rod and the stopper plate, instead, filler wire contain-

ing higher Mg, e.g., alloy 5356, 5183 and 5556 should be used. The use of these filler wires would shift the composition ofthe welds away from the peak cracking sensitivity of 1.5 wt% Mg, thus avoiding weld cracking.� Si in the base alloy lead to formation of Si-rich eutectic phases along the inter-dendritic regions causing high susceptibil-

ity to weld cracking. Therefore, Si should be minimized to a low level in the base alloy.� To reduce the porosity in the welds, inert gases should be used to shield the weld pool and special attention must be paid

with regard to surface cleaning.

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