Metallurgical and mechanical properties of marine grade AA5356 using wire arc additive manufacturing

In the current work, a Gas metal arc welding (GMAW)-based Wire Arc Additive Manufacturing (WAAM) procedure was used to build a wall construction of measuring Aluminium alloy (AA) AA5356 on an AA5083 base plate. The microstructure and mechanical properties of AA5356 were examined at two places along the wall structure’s horizontal deposition direction and in two deposition orientations (horizontal and vertical). Optical microscopy, SEM, EDAX, and fractographical examinations were used to analyse the microstructure. Tensile and microhardness tests were performed at two wall locations to evaluate mechanical parameters. A microstructure analysis reveals a mixture of columnar grain structure and coarser intermetallics in the remelting zone, with finer granular structure in the central region. The horizontal direction of AA5356 deposition exhibited a highest elongation and tensile strength of 4.4% and 249 MPa than the vertical direction. For the horizontal and vertical orientations, the average microhardness values were determined to be 80 HV and 72 HV, respectively. Fracture analysis of the tensile samples showed that the deposited metal had a ductile mode of failure with a predominance of dimples with tearing shape. This study provides valuable insights into constructing wall structures and analyzing their mechanical properties.


Introduction
Aluminum possesses a unique property that allows it to combine with other alloying elements (metals and nonmetals) to create light-weight materials with excellent strength to weight ratio as well as resistance to corrosion [1][2][3].Aluminium alloys (AA) are most likely among the most intriguing and cost-effective choices available to produce massive parts of alloys [4].The Wire arc additive manufacturing (WAAM) technique is appropriate for the aluminum alloy series Al-Cu (2xxx) [5], Al-Si (4xxx) [6] and Al-Mg (5xxx) [7].As a result, the application of additive manufacturing with these AAs are used in the aerospace (brackets, fittings, etc), automobile (suspension parts, body panels, etc), shipbuilding (propellers, offshore platform components, etc), medical, defence and precision parts industries is expanding.The investigation and application of WAAM for 5xxx series, including the aluminum alloys 5183 and 5356, has increased recently [8][9][10].This study used 5356 Al-Mg alloy, which is a material that is particularly appealing due to its excellent strength and resistance to corrosion.Magnesium is the main alloying element in the aluminum alloys in the 5000 series.This alloy is particularly well-suited for maritime environments since it contains trace amounts of magnesium to improve corrosion resistance and manganese to increase strength [11].
Additive manufacturing (AM) of aluminum alloys has gained popularity due to the advancement of lightweight applications and is expected to grow significantly [12].The marine grade is prone to corrosion and needs to be regularly maintained as well as repaired after prolonged usage.Although it is not financially feasible to replace the entire structure, there are some alternatives, like cutting the deposited region, putting fresh additive layers at the corroded surface, and sealing small gaps [13].The use of AM systems, which create 3D structures layer-by-layer deposition, has grown in popularity since they need less time to produce, waste less material and environmental-friendly process [14][15][16][17].This approach saves about half of manufacturing costs in contrast to the traditional techniques [18].WAAM is a kind of AM which employs wire as the raw material and an electric arc as its source of heat to produce thermal energy [19].WAAM makes it possible to increase deposit density, rate of deposition, and liberty with design [20].Direct deposition is a possibility for enormous scale metallic constructions and equipment costs are significantly decreased because of the GMAW/GTAW system's use [21].The WAMM technique, which is based on GMAW, exhibited appropriate mechanical characteristics, large-scale component manufacture, and reduced equipment costs.According to Babu et al [22] the GMAW process is the most effective method for controlling the mechanical and microstructure of high strength steel due to its increased deposition rate and enhanced process efficiency.
Nowadays researchers have focused more on examining the wall structure for aluminum alloys made using the WAAM technique in various applications.Cong et al [23] reported producing thin walls of WAAM Al-6.3% Cu alloy samples using CMT in its advanced and pulsed versions.The WAAM Al-6.3% Cu alloy's thin wall and block architectures were examined for pore distribution, microstructure, and microhardness.The results showed that different CMTs and deposition methods led to microstructure differences.The thin wall samples' microhardness values were approximately 75 HV from the bottom to the center and progressively dropped toward the top.Wang et al [24] studied the use of cable-type welding wire (CWW) in the production of thinwalled AA5356 by using CMT technology.The outcomes of the tests revealed that WAAM-CWW produced higher quality structures.The optical microscope's findings showed the formation of an equiaxed grain deposit free of defects.In contrast to the casting aluminum alloy, the UTS and yield strength of the components produced by WAAM with CWW increased by 19.8% and 22.5%, respectively.The study conducted by Evstifeev et al [25] examined the mechanical characteristics of rolling as well as WAAM-fabricated aluminum alloy 5056.EBSD and EDX were used to do a structural investigation on the material.Additionally, impact toughness tests and tensile testing under quasi-static stress were conducted.There are notable similarities in the materials' mechanical characteristics when subjected to quasi-static loading.In particular, the AA5056 IM produced industrially had a yield stress of 128 MPa, while the AA5056 AM had a yield stress of 111 MPa.According to the researches, there is a limitation of literature reporting the WAAM technique for constructing a wall using Al-5356 on an AA5083 base plate.
The objective of this research is to present a understanding of the mechanical characteristics, porosity, heat input, and microstructures of 5356-Al produced using the WAAM process.This will be crucial for determining the best way to use WAAM technology to create aluminum alloys, which can cut costs and enhance material usage over existing methods-especially for larger aluminum structural parts used in the marine industry.

Materials and WAAM fabrication process
Aluminum alloy measuring 200 mm by 200 mm by 8 mm (thickness) was used as a base material and ER5356 (length = 41.2 mm) wire was used as the filler material to complete the thin-wall manufacturing process.Table 1 displayed the wire's and substrate's chemical composition.
The substrate was physically polished to eliminate the oxide coating from its surface using a stainless steel acetone cleaning brush before the test.Lastly, it was placed in a vacuum drying oven and the welding wire was used to dry it simultaneously.The main elements of the experimental setup were a GMAW torch (PRO MIG-530), mixing chambers, controls, shielding gas, and a wire feeder.The filler material used was AA5356 alloy, which has a diameter of 2.4 mm which is shown in the figure 1. Table 2 displays the most desirable set of parameters obtained by simultaneous optimization.The built volume of the WAAM configuration measures 100 × 50 × 25 mm 3 .Shielding gas is made of argon at a steady flow rate of 20 l min −1 to create walls 35 layers high.It is important to guarantee the quality of argon gas, particularly for scientific or industrial uses.Precise management and validation of the gas quality are made possible by quantitative purity monitoring.For this, a variety of analytical methods are frequently used, including mass spectrometry and gas chromatography, which offer precise assessments of contaminants down to trace levels.Such quantitative evaluation contributes to the integrity and dependability maintenance of argon gas-using operations.After fixing the substrate plate, the wall structure was deposited on the plate using a torch.The schematic representation of WAAM machine is shown in the figure 2. When depositing material, a GMAW torch can move in all three axes.It was favored to use G code programming to provide responses via the controller unit.To heat and melt metallic wire, the COLTON iFLEX 350 power sources were employed.Figure 3 shows the WAAM as built structure with substrate in (a) front view (b) side view and (c) top view.Figure 3(d) numerous feasibility studies have been carried out, and the findings   have been extended in these studies, demonstrating superior mechanical qualities, higher-quality structures, and the absence of defects.Mechanical evaluations of tensile testing, micro hardness (MH), and fractography have been conducted along the horizontal and vertical depositing direction.Figure 4 shows the sample preparation WAAM deposited alloys using Power hacksaw cutting and Wire Electrical Discharge Machining (WEDM) was used to prepare the specimens for tensile test (sub size) as per ASTM E8M standard.The M-100 universal tensile test apparatus was utilized to assess the tensile characteristics of the specimens with cross head speed of 1 mm min −1 .Figure 5(a) indicates the schematic diagram of tensile specimen extract in the milling of WAAM wall surface for sample preparation.Figure 6 illustrates the test sample dimension as per ASTM standard.The wall structure's MH was measured using a Vickers micro hardness tester (ESEWAY EW-150) in accordance with ASTM E384 standards.For every MH measurement, a 150 gf load and a 10-s dwell period were used.Multiple sites recorded MH readings.Every place received three indentations, and the mean value of those indentations was calculated for analysis.The fractography of the tensile specimens was examined using Zeiss Ultra 55 scanning electron microscopy.

Results and discussion
3.1.Microstructure Figure 5 indicates the micrsotructure image (250 X) of thin wall built with AA5356 alloy (central zone).Figure 6 shows the microstructure image (250 X) at the interface region of two passes (remelting zone).The central layers and the remelting layer of the samples exhibit varying types of grain structures based on their respective positions.Equiaxed grains are presents above the slant line of figure 6 predominate in the inner layers of the deposits, whereas coarse column grains are marked below the slant line of figure 6 make up the interlayers, as seen in.The unique layered microstructure is a result of the cooling pace and thermal gradient that occur within the molten pool.During additive manufacturing, the first layer deposited on the substrate has exceptionally high cooling rates due to cold environment and deposited plate, resulting in fine microstructure.The formation of multiple layer microstructure is a result of the rate at which it cools and thermal gradient that surround the molten pool.Heat dissipates more quickly in the initial layers of deposition because there is less heat build-up and a shorter channel for heat to get to the substrate.This indicates that the maximum temperature gradient is vertical to the substrate and is an excellent prospect for columnar grain generation.As height increases, heat  builds up and dissipation decreases, creating a thermal equilibrium.As a result, the grains grow in all directions at the same time, and in the central layer area, the equiaxed grains eventually form.The re-melted zone is heated and repeatedly melts, creating a coarser microstructure than the inner layer region.

Microstructure analysis using scanning electron microscope
The figures 7(a) and (b) shows the SEM images of interface region and central region, respectively.Figure 7(a In general, the chemical composition and velocity of solidification will have a major influence on the as-cast microstructure.For instance, higher solidification velocities and the presence of grain refinement make it easier to produce intermetallic compounds.The high magnesium concentration, can also lead to the preferential diffusion of magnesium to grain boundaries and the formation of the β phase  precipitate.At about 450 °C, a eutectic reaction takes place in the Al-Mg binary system, forming the α-Al and βphase Al 3 Mg 2 [9] shown in the figure 8(d).
The WAAM process has a quick solidification rate and high cooling.Mg elements tend to segregate to grain borders and the interdendritic area during solidification [27].Lower solidification speed in the melting zone is caused by higher heat input compared to the interface region.As a result, particles in the remelting zone are more easily formed and developed, which results in a somewhat lower magnesium concentration and a weaker solute strengthening effect in the matrix.The remelting zone's secondary phases are somewhat bigger than those in the interface zone.
As thermal convection increases, so does the cooling rate, which in turn causes undercooling to increase and enhances the nucleation rate [28,29].The molten pool with the decreased supercooling level is formed by the remelting region, which receives heat continually and limits down the nucleation rate.The ability of atoms to migrate and disperse improves with heat accumulation, and the pace of annexation between grains is accelerated, causing the grains to grow significantly.As a result, the remelting region has larger grains than the central zone.Porosity is the result of hydrogen being prevented from escaping by the coarse grain structure.
One of the main causes of the lower mechanical qualities near the interlayer compared to the central region is the distribution of micropores at the interlayer.Porosity is the result of hydrogen being prevented from escaping by the coarse grain structure [30].The only gas that is known to dissolve in liquid aluminium is hydrogen.Both the oxidation of aluminium and the breakdown moisture in the air cause hydrogen to absorb into liquid aluminium.Since there is a noticeable difference between hydrogen's solubility in liquid and solid aluminium at its melting temperature, hydrogen has been identified as the primary cause of pore formation in aluminium castings, despite the fact that hydrogen is far less soluble in aluminium than Fe, Cu, and Mg.Many researchers have theorized that the picked-up hydrogen concentration rises over the equilibrium concentration as a result of aluminium solidifying into the mushy zone and rejecting hydrogen, which causes pore formation to occur suddenly [31].

Mechanical properties 3.3.1. Tensile strength
The tensile strengths of the samples are shown in both horizontal and vertical directions in figures 9(a) and (b), respectively.Table 3 indicates the tensile test results of the deposited alloy and substrate on which it is deposited.The stress strain graph of the tensile samples is shown in the figure 10.The samples' tensile, yield, and elongation values were 245 MPa, 127 MPa, and 4.2% in the longitudinal direction and 249 MPa, 131 MPa, and 4.4% in the horizontal direction, respectively.For the sample, there is a small variation between the two directions.The longitudinal tensile samples exhibit a somewhat lower strength in comparison to the transverse samples.Aluminum alloys' mechanical characteristics are dependent on their microstructure and micro pore concentration.Therefore, pores and cracks between the layers will adversely affect the tensile qualities.
The fracture morphologies of the WAAM manufactured samples in vertical and horizontal orientations are displayed in figures 11(a) and (b), respectively, which make the existence of pores evident.It is discovered that test samples exhibit ductile failure [32] and demonstrate a predominance of dimples with ripping morphology in the metal that is being deposited.The silver secondary particles disperse evenly across the Al-matrix, as was previously indicated.It is known that particles in the second phase might cause fracture [33].The failure of metal components can result from microcracks that start in the micropores and eventually merge together.Additionally, pores can greatly lower the tensile strength and are more susceptible to applied stress [34].The samples exhibited reduced strength in the vertical direction due to the presence of larger pores throughout the interlayer areas.

SEM fractography
The tensile test specimen depicted in figure 12 was subjected to a scanning electron microscope examination to investigate the fracture surface morphology.Multiple dimples with a homogeneous distribution were shown on the surface of fracture components in figure 12.This illustrates the well-built component's toughness [35].Furthermore, the presence of large dimples indicated that the multi-walled component's ductility was appropriate.The fracture surface that has been dimpled could have been caused by particles from the second phase.The uniformity and small size of the pits in the core area are evident from the pit size, indicating better   UTS.Its elongation is improved but its UTS are marginally less in the interlayer area because its pits are bigger and deeper than in the central region.The formation of larger dents may arise from the clustering and expansion of second phase particles in the interface regions due to inadequate heat dissipation and accumulation of heat.

Vickers micro hardness test
Figure 13 displays the microhardness graph for WAAM's thin walled 5356 aluminum alloy components.The hardness is 80 HV on average.The remelting zone has the lowest average hardness, while the center area has the highest average hardness, according to the average hardness of the two sections.The primary reason for the greater hardness is the fine grains in the central areas.Heat dissipation proceeds more quickly when the bottom and central layers are put on the base plate.The remelting region, being positioned between the inner layers, stays hottest which results in formation of coarse grains.In the WAAM process, β phase precipitates as temperature rises.Both the nucleation rate and the degree of sub-cooling rise as the cooling rate does.Despite this, the WAAM's residency duration in the hot region is extremely brief.Specifically, the generated β-phase is quite minimal when producing aluminum alloys.On the other hand, atoms may diffuse sufficiently slowly when the rate of cooling is relatively modest [24,35].The β-phase has enough time to develop and eventually joins forces with its neighbouring β-phase to generate an increased number of β-phase.β phase enhances the dislocation density and prevents dislocations from slipping, improving the hardness of the material.This suggests that the dislocation density is more influenced by the size of the β-phase than by its amount.But the brittle, highly hard intermetallic combination that makes up the β-phase limits the material's flexibility.

Conclusions
The following conclusions stem from utilizing the Wire Arc Additive Manufacturing (WAAM) technique for constructing walls, employing 5356 Al alloy wire, and 5083 Al alloy as the support plate.
1.The central region and the remelting region displayed distinct grain architectures based on their positions.Equiaxed grains predominate in the inner layers of the deposits, while coarse column grains make up the interlayers.
2. The tensile test results reveal that WAAM-deposited 5356 Al alloy exhibited higher tensile strength, yield strength, and elongation values in the horizontal direction, measuring at 249 MPa, 131 MPa, and 4.4%, respectively, compared to the longitudinal direction, which measured at 245 MPa, 127 MPa, and 4.2%, respectively.
3. Fractography results show a prevalence of dimples with tearing morphology in the metal that is being deposited, as well as ductile failure.
4. The hardness survey shows that the WAAM-deposited 5356 Al alloy ranged from 72 to 80 HV.

Figure 3 .
Figure 3. WAAM as built structure with substrate (l × b × t) (a) front view (mark two alloys in the picture itself) (b) side view and (c) top view (d) WAAM trials.

Figure 5 .
Figure 5. Micrsotructure image of thin wall built with 5356 Al alloy (central zone).
) displays coarse grains, while figure 7(b) displays fine grains.The high heat input in conjunction with the repetitive cyclic heating of the preceding layer facilitates the development of coarse grains at the interface.To describe the intermetallic phase depicted in figures 8(a)-(d), SEM and EDAX characterization were employed.SEM imaging of the interface region at 3500× magnification is shown in figure 8(a), and EDAX analysis was performed on three locations, shown in figure 8(a), to ascertain the chemical composition of those materials.It is evident from figure 7 that the matrix contains a significant amount of white intermetallics.In the case of Al-Mg alloys, intermetallic combinations (Mn, Fe)Al 6 , Mg 2 Si, and Al 5 Mg 8 (Al 3 Mg 2 ) with different solidification processes might constitute the secondary phases [26].As shown in figure from the SEM and EDAX analysis (figures 8(a)-(d), it is shown that the common Fe-rich phases identified in the Al-Mg-Mn alloys are Al 6 Mn, Al m Fe, αAl(Fe, Mn)Si, and Al 3 Fe which are indicated as spots 1, 2 and 3 in figure 8(a).These spots are shown as EDAX images in figures 8(b)-(d).

Figure 9 .
Figure 9. Dog bone shaped specimens for tensile test.

Figure 11 .
Figure 11.Macrostructure of the fractured tensile surfaces of the samples (a) vertical and (b) horizontal orientations.

Table 3 .
Summary of tensile tests results with substrate.