Corrosion Behavior of TIG Welded AlMg 6 Mn Alloy

The influence of TIG welding on the corrosion and mechanical properties of AlMg6Mn was investigated. The as-received hot rolled AlMg6Mn alloy plates (AR) with a thickness of 8 mm were welded by TIG welding with S-AlMg5 as a filler material. The microstructure, mechanical properties and corrosion behavior of the weld joint filled with AlMg5 welding wires, were studied by optical microscopy, scanning electron microscopy, hardness testing and corrosion NAMLT and ASSET tests. It was found that AR is more susceptible to intergranular corrosion than TIG welding specimens, while neither AR nor TIG specimens showed susceptibility to exfoliation corrosion


Introduction
EDIUM strength highly formable 5xxx Al-Mg alloys have been widely used, especially for structural applications, in the constructions of transportation due to different combination of strength and formability [1][2][3][4].Such properties can be achieved by the mechanism of solid solution hardening, and enhanced by deformation due to the high strain hardening behavior [5].Further improvements in properties, such as good weldability and high corrosion resistance, have been achieved by small additions of other alloy elements and/or modified processing routes [ 6-9].
Despite that, as concerning one of the mostly used commercial Al−Mg alloys, EN AW-5083 (Al−4.7Mg−0.7Mn), the further increase in its magnesium content has recently been in the focus of interest [10][11][12][13].Alloy with 6 wt.% of Mg possesses a good weldability and a good combination of strength and formability [11, 12 14].However, the increase of the Mg content, as well as the presence of different kind of intermetallic phases in the alloy, increases the strength, but leads to a higher susceptibility to the localized corrosion [3].Al-Mg alloys with more than 3% Mg, due to the limited room temperature solubility of Mg in Al matrix (1.9 wt.% Mg), become supersaturated and the excess Mg atoms precipitate out as anodic β-Al 3 Mg 2 phase.Because of the low melting points of these alloys substantial diffusion of Mg occurs at the temperatures approaching room temperature, β-phase precipitates preferentially at both grain boundaries and on preexisting particles which are enriched in manganese [15][16][17].It is considered that the grain boundaries are preferential site for βphase nucleation due to the low diffusion barrier (presence of defects, such as dislocations and vacancies) [18].Under certain conditions, either during the fabrication of the sheet plate Al-Mg products or in service, extended exposure in the range 70°C to 200°C, can cause the precipitation of Mg-rich β-phase, preferentially at the grain boundaries.It makes these alloys become "sensitized", susceptible to intergranular (IGC), stress (SCC), or pitting corrosion since, the β-phase is electrochemically more active than the aluminum matrix [19][20][21][22].
A specific form of IGC, often encountered in Al alloys, is exfoliation corrosion, characterized by lifting up of the metal surface by force of expanding corrosion products occurring at the grain boundaries [23].Exfoliation corrosion occurs when the grains are flattened by a heavy deformation during hot or cold rolling, and where no recrystallization has occurred [19].
In the environments where Al alloys are susceptible to IGC, a discrete pitting or pit blistering is to be expected also, i.e. the formation of small holes along the surface.
Despite that.most of the works examine the effect of sensitisation and influence of β-phase on the localized corrosion, the other intermetallics also play an important role in the localized corrosion attack ((Fe,Mn)Al 6 , Mg 2 Si, Al6Mn, Al 3 Mg 5 ).The influence of intermetallic particles on the resistance against localized corrosion is crucial for the material integrity in long-term service, but data of corrosion resistance of this alloy are lacking.
Tungsten inert gas (TIG) welding is one of the most common joining methods for aluminum alloys.During the welding process, the phase transformation and softening phenomena are induced due to the heat, providing differences in microstructure, mechanical properties as well as electrochemical potential difference between various zones of the weld.Many authors found that the corrosion resistance of the welded materials at different regions that formed during the welding process is not the same [24][25][26].
Nitric Acid Mass Loss Testing (NAMLT) [27], is the most common method for evaluating the susceptibility of Al-Mg alloys to IGC, since it provides a quantitative measure of susceptibility.There are numerous studies that have investigated the effects of welding on the IGC of Al-Mg alloys containing up to 5 % Mg [17,18].However, there is a M lack of evidence documenting the corrosion behavior of AlMg6Mn alloy.
The aim of this work was the evaluation of the corrosion resistance of hot rolled aluminum alloy AlMg6Mn, and TIG welded joints of the same alloy.

Material
The material used in this study was received as a hot rolled plate of AlMg6Mn alloy, 8 mm thickness in as-received condition (AR).The chemical composition of the used parent alloy is given in Table 1.The 50 mm dimension was parallel to the longitudinal direction of the product (L-S).

Welding
Specimens were welded using the TIG process under Ar (99.5 %) atmosphere and S-AlMg5 as a filler material, using following welding parameters: current of 115 A, voltage of 15 V, and speed of 100 mm/min.The chemical composition of the filler material is given in Table 2. Direction of the welding joint was perpendicular to the plate rolling direction.

Microstructure
The microstructures were characterized by the Leitz optical microscope (OM) and scanning electron microscope (SEM-JEOL JSM-6610LV).Metallographic samples were prepared using traditional grinding and polishing techniques using up to 1 μm diamond paste.The particle structure was evaluated in the polished samples.To reveal the grain structure, after the electrolytic polishing in perchloric acid, the samples were etched in the Barker's solution.

Hardness
The Vickers hardness distribution of the weld zone was measured on a cross section perpendicular to the welding direction using micro hardness tester with load of 4.9 N.

Corrosion tests
The corrosion resistance tests were performed on specimens containing the weld in the central zone and on the parent alloy.Tests were carried out by means of susceptibility to intergranular and exfoliation corrosion.
The susceptibility to IGC was determined by the nitric acid mass loss test (NAMLT) according to the ASTM G67 standard.Specimens, with dimensions 50 x 6 mm and the thickness of the hot rolled plate of 8 mm, were immersed in concentrated HNO 3 at 30°C for 24 hours, and the mass loss was measured.The 50 mm dimension of the specimens was parallel to the longitudinal direction of the plate (rolling direction).Three specimens were tested.In order to calculate the mass losses per unit area more precisely, face and root reinforcement was considered.
The susceptibility to exfoliation corrosion was determined by a visual inspection using the ASSET method according to the ASTM G66 standard (Method for Visual Assessment of Exfoliation Corrosion susceptibility of AA5xxx Series Al alloys).

Microstructure
Microstructure of the parent metal and weld joint before the corrosion tests is shown in Figures 2-4.All micrographs were taken on the L−S plane.The micrograph of a grain structure of the parent metal (Fig. 2(a)) shows the elongated grains after hot rolling.The particles are aligned in the rolling direction (Fig. 2(b)).The grains in the weld metal (WM) are equiaxed (Fig. 3a) with a low content of homogenous distributed small particles (Fig. 4a).The fusion zone (FZ) is clearly noticeable (Fig. 3b).Some porosity is observed in the weld metal and in the vicinity of the fusion zone (as pointed out by arrows in Figs.4a and b).The heat affected zone (HAZ) is narrow with slightly elongated, recrystalized grains (Fig. 3c) and precipitation of the second phase particles at the grain boundaries were observed (Fig. 4c).

Hardness
Hardness distribution along the weld joint is shown in Fig. 5.The hardness of the base metal was approximately 90 HV 0.5 , while the weld metal zone has the lowest hardness value, 80 -85 HV 0.5.This is the result of a dendritic microstructure occurred by the solidification of a molten filler material (AlMg5), coarser grain size, some porosity, as well as lower content of the second phase particles compared to the AlMg6Mn alloy.

Corrosion properties
The effect of TIG welding on the IGC susceptibility of the tested alloy was carried out by means of the NAMLT corrosion testing.Samples of the parent material, as well as of the welded plates before and after the corrosion test, are shown in Figures 6 and 7.It is clear that the surface of both the base metal and weld joint is rougher after the 24h exposure to HNO 3 .However, it is interesting that more intensive attack was observed in the base metal (Fig. 6b) than in the base metal of the welded specimens (Fig. 7b).The mass loss of the specimens with and without the weld joint is presented in Table 3.It was found that the mass loss in the base material specimens was much higher (aver.89.4 mg cm -2 ), compared to the TIG welding processed specimens (15.45 to 30.48 mg cm -2 ).It shows that the IGC susceptibility was strongly affected by TIG process, i.e. the welded specimens showed higher resistance toward IGC.According to ASTM G67, a large mass loss (>75 mg cm -2 ) can be related to distribution of the β-phase in a continuous network along the grain boundaries [27].The mass loss measured for the welded specimens is considered according to ASTM G67 as an intermediate value.This indicates the lower content or/and random distribution of the β-phase particles and both of them decrease the IGC susceptibility.The lower content of Mg in a weld metal compared to base metal, as well as fast cooling during solidification of the weld metal, result the lower content of the β-phase, since the β-phase precipitates below 200°C and with a slow growth rate [18].Unlike samples of the parent metal where the mass loss was almost equal among all three samples, the mass loss among TIG samples varies.It is supposed that this is the result of different amount of porosity and gas bubbles in the weld zones, which causes slightly different intensity of the corrosion attack.In support of the statements a different morphology is also observed after the corrosion test (Fig. 7b).Surface morphology after NAMLT test Figures 8a and 8b show typical surface of base metal after the NAMLT test.Severe dissolution along the grain boundaries is observed in both L-S and L-T plains.It can be assumed that pits were initiated at anodic β-phase particles, which is preferentially dissolved and then grown along the grain boundaries, supported by other pits or dissolved matrix (model proposed by ref. 31), the presence of Mg 2 + ions in solution increases the rate of dissolution of aluminum).
Numerous small pits on the grain surface are observed (Fig. 8b).It is supposed that those pits are caused by dissolution of β-phase particles.This could be checked by revealing the distribution of the β-phase in the microstructure.The typical morphology of the TIG welded joint surface after NAMLT is shown in Fig. 9.It can be noted that the surfaces of all zones, even the base metal, are covered with dimples.Unlike the AR specimens (Fig. 8), in any of the welded joint zones intergranular attack was not observed, which was confirmed by the analysis at higher magnification (Fig. 10).Such difference in the surface morphology between the AR and TIG specimens after corrosion test is a little unexpected, and requires further investigation, special distribution of the β-phase, since the preferential attack of approximately continuous β-phase particles at the grain boundaries is believed to be responsible for IGC [19].
Large pores, mostly in the fusion zone and weld metal, were observed (Figs.9c, d).Comparing these pore size with the pores size in Fig. 4, it seems that pores are preferential sites for corrosion attack.However, the mass loss in the TIG specimens was lower than in AR specimens (Table 3).Since the mass loss in the TIG welded specimens is a sum of both, mass loss in the weld metal, HAZ and base metal, on the base of the previous results it has not been possible to calculate the fraction of each of them.But, according to the surface morphology after the NAMLT test, corrosion of the base metal of TIG specimens is less intensive than in AR specimens.
While severe corrosion attack along the grain boundary was observed in AR specimens (Fig. 8), in the base metal TIG specimens an intergranular attack was not observed (Fig. 9).It can be attributed to different microstructure of the weld metal zone (primarily lower content of the β-phase, and lower content of Mn-bearing phases), but it should be investigated thoroughly.

Exfoliation and pitting corrosion susceptibility
Exfoliation and pitting corrosion susceptibility were estimated after a visual inspection according to the ASTM G66 standard.Fig. 11 presents the surface appearance of the base metal and TIG welded specimen samples after the exfoliation corrosion test.In both specimens, the surface was etched, but without any pitting, pit blistering or a visible lifting of the surface.According to the ASTM G66, both samples passed the test with no appreciable attack.In addition, it is interesting to note that AlMg6Mn alloy is very susceptible to intergranular attack in HNO 3 , as evidenced by ASTM G67, but on the contrary, it satisfies the highest requirements regarding the exfoliation and pitting resistance, as concluded after the ASTM G66 procedure.

Conclusion
The susceptibility to intergranular (IGC) and exfoliation corrosion of TIG welded and as-received specimens of AlMg6Mn alloy were investigated according to the ASTM G67 and ASTM G66 standards.As-received AlMg6Mn alloy has shown susceptibility to IGC, as the mass loss determined by NAMLT test was 89,4 mg/mm 2 .The TIG welded specimens have shown better IGC resistance than as-received specimens, since the mass loss in NAMLT test was between 15.45 mg/mm 2 and 30.48 mg/mm 2 .
There is a clear correlation between the mass loss and surface morphology after exposure to nitric acid (NAMLT).In as-received specimens intergranular corrosion attack was observed, which is attributed to severe dissolution of β-phase along the grain boundaries.On the other hand, corrosion attack in the TIG welded specimens was not intergranular.The increase of corrosion resistant of TIG weld compared to as-received alloy is a result different of microstructure: lower content of Mg in the filler material (AlMg5), and thus, lower content and size of the second phase particles, cast microstructure of the weld metal, and absence of the phase at the grain boundary.
The contribution of particular zones of weld to increase corrosion resistance of the weld will be the subject of further investigation.
Exfoliation corrosion, determined by a visual inspection using the ASSET method, was not observed neither in base metal nor in TIG weld specimens.

Figure 1 .
Figure 1.The deformation direction with respect to rolling direction of the plate: L-longitudinal (rolling direction); Т-transversal; S-short transversal.

Figure 5 .
Figure 5. Hardness distribution across the weld joint.

Figure 8 .
Surface morphology of AR specimen after NAMLT test, in L-S plane, at different magnification: a) 100x; b) 500x.

Figure 9 .
Surface morphology of the weld joint after the NAMLT test: a) base metal; b) HAZ; c) FZ; d) WM.

Figure 10 .
Figure 10.Surface morphology of the weld joint after the NAMLT test: a) base metal; b) HAZ; c) WM.

Figure 11 .
Figure 11.Base metal and TIG welded specimen of AlMg6Mn alloy after the exfoliation susceptibility test.

Table 3 .
Metal loss in NAMLT test