Wire Arc Additive Manufacturing of 5356 Aluminum Alloy Cylindrical Component: Mechanical Properties and Microstructural Characteristics

*1B.PRASANNA NAGASAI, 2S. MALARVIZHI, 3V. BALASUBRAMANIAN

*1Bellamkonda Prasanna Nagasai (Corresponding author)

Research Scholar, 

Centre for Materials Joining and Research (CEMAJOR)

Department of Manufacturing Engineering, 

Annamalai University, Annamalainagar – (P.O),

Tamilnadu (State),

INDIA – 608002.

Email: nagasaibellamkonda143@gmail.com

2Dr. Sudersanan Malarvizhi,

Professor,

Centre for Materials Joining and Research (CEMAJOR)

Department of Manufacturing Engineering, 

Annamalai University, Annamalainagar – (P.O),

INDIA – 608002.

Email:jeejoo@rediffmail.com 

3Dr. Visvalingam Balasubramanian,

Professor & Director,

Centre for Materials Joining and Research (CEMAJOR)

Department of Manufacturing Engineering, 

Annamalai University, Annamalainagar – (P.O),

INDIA – 608002.

Email:visvabalu@yahoo.com

Abstract

Wire arc additive manufacturing (WAAM) is a potential method for fabricating large metal structures. Cold metal transfer (CMT) welding based wire arc additive manufacturing technology was used to manufacture a Al-Mg alloy cylindrical component by synchronously feeding ER5356 (Al-Mg) wire. The mechanical properties and microstructural characteristics were evaluated. The yield strength (YS), ultimate tensile strength (UTS) and elongation of the deposited cylindrical component in the lower and upper zones were recorded as 164.5±3.5 MPa, 256±2 MPa and 48.06±1.13 %, respectively. The hardness and impact toughness of the component were recorded as 84.6±1.26 (Hv0.5) and 13±1 J along the building direction. The fabricated cylindrical component exhibited an isotropic feature of the mechanical properties. The CMT based WAAM technique revealed uniform microstructural characteristics in the lower and upper zones of the Al-Mg cylindrical component. From the results, it is understood that the Al-Mg alloy cylindrical component manufactured by CMT-WAAM process has better properties than the Al-5356 cast material, Al-5052 weld metal and ER5356 filler wire properties.   

Key words: Wire arc additive manufacturing, Cold metal transfer arc welding, Al-Mg alloy, Mechanical properties, Microstructural features. 

1. INTRODUCTION

With the advancement of industry, the requirements for high efficiency, light weight, and low cost are becoming more demanding. Additive Manufacturing (AM) technology is a novel method of producing near net-shaped metallic components with complicated geometry at a reasonable cost. Metal additive manufacturing is a technology that has received a lot of attention in recent years. Metal additive manufacturing is more efficient and produces more complex geometrical components than traditional manufacturing processes like casting, forging or welding [1]. WAAM is probably one of the most potential technique among the different metal AM methods for producing large components, due to its very high deposition rate compared with other processes based on heat sources of the laser or electron beam [2].

In order to melt a metallic filler wire, WAAM processes require a welding power source to create an electric arc. Given its outstanding mechanical properties, large aluminum alloy parts have been typically adopted in the aerospace, railway, automobile and other sectors. WAAM is a potential technique for producing large components of aluminium. In WAAM, various welding defects like hot cracks and porosity can occur in aluminum alloys [3]. Porosity is generated for various reasons, such as arc welding, process parameters like high current and voltage, quality of wire, alloying elements, and interpass temperature. The heat input of a new superimposed layer can promote the development of pores in a multi-layer WAAM process. However, Mg is a very active element and the Al-Mg alloy is highly heat sensitive (prone to porosity). Apart from hot cracking, porosity in WAAM of aluminium alloys is one of the major issue that significantly restrict the part’s mechanical properties [4].

In recent studies, Yinghui Zhou et al. [5] studied the influence of travel speed on mechanical and metallurgical characteristics of wire arc additively manufactured Al-2219 linear wall parts. The authors noted that the size and volume of equiaxed grain were reduced as the electrical arc travel speed increased. The high arc travel speed also developed precipitation of the θ (Al2Cu) phase during WAAM. When travel speed was 350 and 250 mm/min, the volume fractions of the θ ′′ and θ ′ phases achieved their highest values. The tensile strength and hardness of walls showed higher values at high arc travel speed due to the finer equiaxed grains with low porosity content. Porosity issues are the major problem limiting the use of the WAAM technology for manufacturing of aluminium parts pointed out by [6].  Horgar et al. [7] fabricated AA5183 parts using a gas tungsten arc welding based WAAM technique. The authors observed variation in tensile strength and hardness in horizontal and vertical directions due to the formation of cracks and porosity in reheated weld metal at higher heat input levels. Guangchao Liu et al. [8] also observed similar behaviour in Al-Cu ER2219 parts manufactured by using the double-electrode gas metal arc-additive manufacturing (DE-GMA based AM) method. The tensile strength and hardness of the parts decreased along the deposition direction (horizontal) and increased in the vertical direction due to fine grain size at higher bypass current ratio. The changes in the mechanical properties are due to manufacturing defects (porosity and cracks) and differences in the grain size at higher heat input levels. The cooling rate within the high-temperature range has a strong impact on phase changes or grain size, depending on the solidification behaviour of the processed material. Similarly, the mechanical properties of the material are affected directly. Therefore, a suitable WAAM method must be selected to manufacture large aluminum parts to possess better properties. 

Figure 1 CMT-WAAM platform used to fabricate Al-Mg cylindrical component

Figure 2 Schematic illustrations of the lower and upper zones and specimen extraction scheme

A controlled arc power source is required for the successful WAAM process of aluminium alloy. The wire retraction mechanism in the WAAM process may influence heat input and melt pool dynamics, which are important factors for metallurgical defects and microstructures. Researchers [9, 10] have introduced the new CMT method for WAAM of aluminium components to prevent the development of cracks, porosity and coarse grain during deposition. The CMT method offers connection between the optimum pulse arc waveform (in Fig.2) and the filler wire movement to help the droplet detachment (in Fig.1), reducing the heat input and producing a spatter free weld. A low temperature gradient through the solidifying melt and a high solid-liquid interface velocity promote the equiaxed fine grains [11]. Compared to traditional GMAW, CMT technology minimizes arc energy, resulting in a shorter, cooler, and faster cooling melt pool, which reduces pore percentage when using aluminium alloys [12]. Gu et al. [13] reported that Al-Mg-Mn alloy components made by CMT WAAM contain more refined grains than Al–Mg alloy components. Zhang et al. [14] obtained good mechanical properties in comparison with the wrought alloy for the WAAM Al-Mg alloy using the low heat input of the CMT process. The increment in hardness and tensile strength is due to decreased porosity and fine grain in the manufactured parts. Heard et al. [15] adopted a short-circuit metal transfer mode for WAAM of Al-Si alloy and observed that the Si-eutectic phase was refined, which increased the ductility of the deposited part compared to the as cast Al-Si alloy. Nie et al. [16] studied the mechanical and micro structural characteristics of Al-4043 alloy produced by the CMT based WAAM process. The authors reported that the fine bead geometry and low surface roughness of beads are achieved by controlling the metal transfer by the CMT process, and the microstructures and hardness of parts are improved by low heat input. 

From the literature review, it is understood that CMT process has many advantages to employ as WAAM method. However, there is no research work reported on using the CMT short circuit mode to produce thick walled cylindrical Al-5356 alloy components. Therefore, in the present study, the CMT based WAAM technology was used to manufacture Al-5356 alloy cylindrical component. This paper reports a detailed analysis on the evolution of mechanical and microstructure characteristics of the different zones of the Al-5356 alloy cylindrical component made by WAAM technique using the CMT arc welding process.

Experimental Procedure

Fabrication of Cylindrical Component

Table 1 Chemical composition of ER5356 filler wire

Element	Si	Cu	Fe	Mn	Mg	Cr	Zn	Ti	Be	Al
Wt %	0.25	0.1	0.4	0.05-0.20	4.5-5.5	0.05-0.2	0.1	0.06-0.20	0.0003	Bal.

Table 2 GMAW-WAAM process parameters 

Parameters	Value
Wire feed speed (m/min)	6.4
Current (A)	105
Voltage (v)	13.2
Travel speed (mm/min)	250
85Ar+15CO2  (lpm)	18

Table 3 Dimensions of manufactured cylindrical component

Geometry	Unit	Value
Average wall thickness	mm	7.4±2
Average single layer height	mm	2.85±1
Diameter of cylindrical component	mm	127±7
Height of cylindrical component	mm	160

ER5356 filler wire with a 1.2 mm diameter was used to manufacture the cylindrical component. The chemical composition of the filler wire used in this study is presented in Table 1. A welding power source (CMT Advanced 4000 R) was used in connection with a 3-axis automated motion system and rotating table to manufacture component using WAAM technique, (shown in Figure 1). The ER5356 filler wire was deposited on the aluminium 6061 alloy substrate (250 mm x 250 mm x 10 mm). The welding gun was kept perpendicular to the substrate during the deposition, and the substrate plate rotated with the rotating motor setup. Table 2 shows the optimised process parameters used to manufacture the cylindrical component. The two zones (lower and upper) of the manufactured cylindrical component were separated. The schematic diagram shows the separated zones at the lower (from the substrate to 75 mm in height) and upper (from 75 mm to 150 mm in height) along with scheme of specimen extraction (Fig. 2). Table.3 presents the geometry of the manufactured cylindrical component and Figure 3 shows the photograph of produced cylindrical component. The lower and upper zones of the produced cylindrical component were separated and machined with a CNC lath machine, as shown in Figure 4 with a 4 mm wall thickness. 

Figure 3 Photograph of the manufactured cylindrical component

Figure 4 Photograph of lower and upper zones of the cylindrical component (after machining)

4 mm wall thickness. 

Mechanical Properties Characterization

The smooth and notch tensile specimens were extracted from the lower and upper zones as per ASTM E8 standard. Tensile testing was performed using a universal testing machine capable of 100 kN. Using the engineering stress curve, the yield strength was calculated using the 0.2% offset technique and strain was calculated from 25 mm gauge length. In the lower and upper zones, three tensile samples were tested, and their average value was reported.

The subsize charpy impact specimens were prepared as per the ASTM A370 standard and tested using a pendulum type impact testing machine. Three impact specimens were tested in the lower and upper zones of the component, and mean values are given in Table 4.

Table 4 Mechanical properties of manufactured cylindrical component 

Zone	UTS (MPa)	0.2% YS (MPa)	Elongation (%)	NTS  (MPa)	NSR  (%)	Average Hardness (Hv0.5)	Impact Toughness@RT (J)
Lower	254	161	46.93	261	1.02	83.34	13
Upper	258	168	49.20	276	1.06	85.86	14

The hardness values in the lower and upper zones of manufactured cylindrical component were measured using the Vickers Microhardness Testing Machine. The hardness testing was conducted with a 500-gram load and a 15-second dwell time.

Macro and Microstructural Analysis

Both the samples (extracted from upper and lower zones) were polished with different grades of emery paper and diamond paste was used to obtain a fine finish. The macro and microstructural features were revealed by using Keller’s reagent. The macro-structural characteristics of the lower and upper zones were examined using a stereozoom microscope. A light optical microscope was used to examine the microstructural features of different zones of the manufactured cylindrical component. 

Results and Discussion

Mechanical Properties

Table 4 presents the lower and upper zones of the tensile properties of the Al-Mg cylindrical component. The lower and upper zones of the cylindrical component showed better tensile properties than ER5356 filler wire properties. The Al-Mg cylindrical component exhibited isotropic tensile properties from the lower to the upper zone. The yield strength (YS), ultimate tensile strength (UTS) and elongation were found to be 164.5±3.5 MPa, 256±2 MPa and 48.06±1.13 % in the lower and upper zones, respectively. The notch strength ratio (NSR) is greater than 1 for all the specimens and it implies that the manufactured cylinder falls under notch ductile category. 

Table 4 presents the average impact toughness of the lower and upper zones of the cylindrical component. The impact toughness was found to be 13±0.5 J at room temperature, irrespective of zones. Impact toughness of the Al-Mg WAAM-CMT cylindrical component exhibited isotropic behaviour from the lower to the upper zones.

The lower and upper zones of the average hardness value were found to be 84.6±1.26 (Hv0.5). The average microhardness values of the lower and upper zones of the Al-Mg alloy cylindrical component manufactured using the CMT technique are shown in Table 4. The CMT-WAAM Al-Mg cylindrical component exhibited isotropic mechanical properties (UTS, hardness and impact toughness) from the lower to the upper zone.

Macrostructure and Porosity of the Cylindrical Component

LOWER ZONE                          UPPER ZONE

Figure 5 Macrostructures of the cylindrical component

LOWER ZONE                                         UPPER ZONE

Figure 6 Distribution of pores in the lower and upper zones of WAAM component

Figure 5 shows the macrostructure of the lower and upper zones of the cylindrical component. The deposited layers are properly merged with one another, and free from faults and defects.  The distribution of pores in the lower and upper zones of the cylindrical component is shown in Figure 6 and it is found that the pores are uniformly distributed. Figure 6 shows the range of pores in the component that have a diameter greater than 10 µm. In the Al-Mg WAAM-CMT cylindrical component, there were no pores larger than 60 µm, and the porosity was less. The burning loss of elemental Mg is reduced by using CMT arc. Because Mg is a highly active element and it will produce loose MgO when it is burnt in the WAAM process. These MgO float on the surface of each successive deposition layer. This oxide absorbs moisture from the atmosphere, causing hydrogen to develop in the deposition and responding to the formation of pores. Since heat input of the CMT-WAAM process is lower, magnesium undergoes a low evaporation rate. Lower heat input reduces Mg evaporation, resulting in reduced porosity.

Microstructure of Al-Mg Cylindrical Component

Figure 7 Optical micrographs of the lower and upper zones of A1-Mg cylindrical component

Figure 7 shows the microstructural features of the WAAM-CMT Al-Mg cylindrical component.  Figure 7 (a-d) shows the microstructure of the lower zone. The microstructure of the upper zone is shown in Figure 7 (e-h). The microstructure shows the interlayer boundary between each layer, which is formed by the alternating overlaying of layers. The top layer of deposited bead re-heats the subsequent layer of deposited bead WAAM-CMT cylindrical component is separated into two parts (the inter-layer region and the inner-layer region). In the lower and upper zones, three distinct zones with different metallurgical characteristics in the layer structure were identified based on microscopic examinations (Figure 7 a) and e)). The fine grain microstructure near the fusion line boundary is depicted in Figures 7(b) and (f). Figures 7 c) and g) show coarse grain microstructure with higher secondary phase segregation (Al3Mg2) at grain boundaries. The grain growth in the lower and upper zones is due to the thermal effect of the subsequent layer in WAAM. The CMT based WAAM technique produced uniform microstructural characteristics in the lower and upper zones of the Al-Mg cylindrical component. 

Discussion

Fig 8 Stress vs Strain curve

Table 5 Comparison of tensile properties of WAAM specimens with widely used cast 5356-Al alloy, Al-5052 weld metal and 5356-Al filler wire

Material	Specimen location	YS (MPa)	UTS (MPa)	Elongation (%)
Al-5356 Wire specification	/	83-150	200-250	10-18
Al-5356 cast alloy [21]	/	87	202	23
Al-5052 weld metal [22]	/	175	195	8
Al-5356 CMT-WAAM component	Lower	161	254	46.93
	Upper	168	258	49.20

Figure 8 shows the typical stress versus strain graph measured along the building direction at room temperature from WAAM-Al-Mg alloy cylindrical component specimens. Table 4 also presents the tensile properties (UTS, YS and % elongation) of the WAAM-Al-Mg cylindrical component in the lower and upper zones. The additively manufactured Al-Mg cylindrical component showed relatively equal YS and UTS values in both zones (164.5±3.5 MPa, 256±2 MPa, respectively), indicating isotropic behaviour in tensile properties. The uniformity and homogeneity of the microstructure, as evidenced by the evenly distributed microhardness values in all depositions from the lower to the upper of the WAAM-CMT cylindrical component, is mainly responsible for the uniformity in the tensile properties of the lower and upper zone specimens. The equiaxed fine and columnar grains in the inner-layer area are responsible for the isotropic properties of the WAAM-CMT Al-Mg cylindrical component. Similarly, isotropic tensile properties for WAAM aluminium components have been found in a few recent investigations [17, 18]. Chengde Li. [17] observed isotropic tensile properties in the horizontal and vertical directions of the Al-7 Si-0.6Mg alloy part at room temperature. The authors noticed only 0.5-1% variation in the UTS, YS and % elongation from the horizontal to the vertical direction due to the completely spheroidized and uniformly distributed Si phase in the primary phase (α-aluminum matrix) with the precipitated magnesium-silicon phase. Zhi-qiang Liu et al. [18] additively fabricated 4043-aluminum thin-walled parts by the CMT process. The authors observed isotropic tensile properties, including UTS, YS, and % elongation in the longitudinal and transverse direction samples of the thin-walled components. Similar isotropic tensile properties are reported by Zewu Qi et al. [19]. The tensile properties in deposited and building directions reveal an isotropic nature, with only 3-7MPa difference in UTS, 2-5MPa difference in YS, and 0.1-0.4% difference in elongation. The authors reported that equiaxed fine and columnar grains in the inner-layer area are responsible for the isotropic properties of Al-Cu-Mg parts. Table 5 summarises the average values of the produced WAAM Al-Mg cylindrical component, Al-5052 weld metal and cast Al-5356 alloy, as well as the Al-5356 filler wire properties. The lower and upper zones of the WAAM-CMT Al-Mg cylindrical component showed higher values of UTS, YS and % elongation than the properties of the filler wire material. According to previous studies [20, 21], the Al-Mg alloy cylindrical component manufactured by WAAM-CMT has higher performance than the Al-5356 cast material and Al-5052 weld metal.

Conclusions

The novel CMT-WAAM system was used to fabricate the Al-Mg alloy cylindrical component. The microstructural features and mechanical properties of the lower and upper zones of the component were investigated. The following are some of the key findings:

1. The CMT-WAAM technique is found to be very much suitable for producing aluminum alloy based cylindrical components. The deposited layers are properly merged with one another, and free from faults and defects. 
2. The additively manufactured Al-Mg cylindrical component showed relatively equal values of YS, UTS (164.5±3.5 MPa, 256±2 MPa, respectively) and hardness (84.6±1.26 (Hv0.5)) and impact toughness (13±0.5J) in both zones, indicating isotropic behaviour in mechanical properties.
3. The microstructural variations (inner-layer coarse grain and inner-layer fine grain) in the deposited beads are due to the alternating overlaying of layers. The CMT based WAAM technique revealed uniform microstructural characteristics in the lower and upper zones of the Al-Mg cylindrical component. 

Acknowledgements

The first author is grateful to the Department of Science and Technology (DST), Ministry of Science and Technology, Government of India, New Delhi, for the financial assistance provided through the PURSE-Phase-2 Fellowship scheme. The authors express their gratitude to M/s. Fronius India Pvt. Limited, Chennai, for their technical assistance.

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