Feasibility of Joining Al-20%Mg2Si In-Situ Composite ...

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Abstract: The feasibility of joining an in situ Al-20%Mg2Si composite by TIG is ... Joints welded with a welding current in the range of 80-85 amp displayed the ...

Proceedings of Iran International Aluminum Conference (IIAC2012) May 15-16, 2012, Arak, I.R. Iran

Feasibility of Joining Al-20%Mg2Si In-Situ Composite By TIG Welding Amirreza Jahangiri*,1, Mohd Hasbullah Idris1,2, Saeed Farahany2 1

Department of Manufacturing Engineering, FKM, Universiti Teknologi Malaysia, Johor, 81310, Malaysia 2 Department of Materials Engineering, FKM, Universiti Teknologi Malaysia, Johor, 81310, Malaysia

Abstract: The feasibility of joining an in situ Al-20%Mg2 Si composite by TIG is investigated. Welding current was varied at five different values between 80 and 100 amp. Different areas were identified in the weldment region due to effect of welding specification. Mechanical properties of joints were evaluated by Tensile and microhardness tests. Joints welded with a welding current in the range of 80-85 amp displayed the highest degree of hardness. The fracture produced in the welded specimens was in the base metal MMC, indicating a strong interface between the base MMC and the weld. Therefore, the study finds that in situ Al-Mg2Si MMC can in fact be welded using TIG and Al-Si filler metal.

Keywords: Aluminium; In-situ composite; Joining; TIG

Introduction Composite materials are engineered materials made from two or more component materials with different physical or chemical properties which remain separate and distinct on a macroscopic level within the finished structure. In the past decade they have been viewed and applied as engineering materials. The growing demand for more fuel -efficient vehicles to reduce energy consumption and air pollution is a challenge for the automotive industry. Recently, Al-Mg2Si metal matrix composites (MMCs) have been developed for high performance applications in automotive industries. Al- and Mg- based composites, reinforced with particulates of Mg2Si have been recently introduced as a new class of ultra light materials. They are attractive candidate materials in the automotive and aerospace industries [1-2] due to their low density and excellent castability, as well as their good wear resistance and mechanical properties [3-4]. Moreover, Al-Mg2Si metal matrix composites are good candidates to replace Al-Si alloys used in aerospace and engine applications. Suitable joining technologies will be required if the materials are to be used as structural components. Given that the machining of MMCs is difficult, welding can instead be used for mechanical joining. Past studies primarily focus on investigating joining methods such as gas shielded metal arc welding [5], laser welding [6], tungsten inert gas welding (TIG) [7], metal inert gas welding (MIG) [8], electron beam welding, and friction stir welding [9]. However, some drawbacks associated with these forms of fusion welding have been reported for metal matrix composites [10-11]. Because of the different properties of components in composite materials, the joining of them is indeed a *

Corresponding author: Tel./Fax: +98 9123892254 E-mail address: [email protected]

challenge, and limits the widespread application of these materials because of their poor weld ability when using the conventional fusion welding process [12]. Joining methods such as mechanical bonding of these materials result in excessive tool wear and are relatively expensive. Furthermore, nontraditional welding process like laser welding beam welding and friction stair welding are found effective but they are very expensive. Using traditional welding process are better way to joining these kind of materials because most of them are economical and flexible but as mentioned earlier conventional methods still have some problems. In the present study, the feasibility and integrity of joining Al-20%Mg2Si composites by TIG welding is investigated.

Experimental Procedure The base composite with chemical composition listed in Table 1 was used. The welding process used was gas tungsten arc welding (TIG) and the current was regulated at five different values between 80 amp and 100 amp. The MMC was machined into 50 × 50 ×5 mm specimens and was set up for welding as shown in Fig.1. Before the welding was conducted, the edges of the specimens were ground for a better surface finish, and mechanically cleaned with a metallic brush to remove the oxide film on the surface of the specimens. The specimens were then cleaned with acetone to eliminate oil and grease, and were preheated in an oven at 150 °C immediately before the welding process was begun. The filler used for this study was ESAB ER4043 welding wire, the chemical composition of which is shown in Table 1. The specimens were cut 5 mm from the end of the weld line, in a direction that is transverse to the weld direction. Parts that were cut were used for the microstructure

Proceedings of Iran International Aluminum Conference (IIAC2012) May 15-16, 2012, Arak, I.R. Iran

analysis and hardness test, with the rest saved for the tensile test. Table1. MMC and filler metal (wt.%)

Mg and Si diffusion in the liquid around the Mg2Si particles. (a)

100µm

(b)

Fig.1 Configuration of the specimens for experimentation The microstructure of the welded region was observed under an optical microscope (Olympus BX60F5) after the samples were prepared, which consisted of grinding, polishing and etching with 0.5 ml Hydrofluoric acid and 99.5 distilled water. Tensile properties were evaluated using a 100 kN universal tensile testing machine (Instron10) at a constant crosshead speed of 5 mm/minute. In addition to tensile testing, hardness measurements were also made using a Vicker’s hardness machine with an applied load of 5 N. The reported value is an average of the five measurements.

Boundary layer 100µm

(c) Eutectic

aluminium

Results and Discussions The different regions developed in the welded area were shown in Fig.2. Fig.2a shows the fusion zone which comprises the larger part of weld region, in which aluminium dendrite and eutectic Al-Si phases were observed. Moreover, Fig.2b shows the narrow area due to penetrate of Al dendrite of filler in the matrix of MMC. Indeed, this area indicates boundary layer between filler in the fusion zone and base metal. Furthermore, Fig.2c illustrates the area contains primary Mg2Si particulate reinforcements and binary eutectic Al-Mg2Si. A local increment of the aluminium phase was observed, which can be attributed to the effect of heat input during TIG welding and heat affected area (HAZ). According to the pseudo-binary phase diagram, the Al-Mg2Si eutectic phase has a lower melting point than primary Mg2Si, and as such, it can be expected that the area will contain more microstructural changes. It has been reported [13-14] that the existence of α-Al is related to the non-equilibrium solidification of the alloy, which leads to a limited rate of

Primary Mg2Si 100µm

Fig.2. Different area in welded region: (a) fusion zone, (b) boundary area and (c) Heat affected and base metal Fig.3 shows influence of welding current on dendrite arm spacing (DAS) in fusion area. The change in size of Al dendrite was measured according to the intercept method. As seen, DAS increased linearly with increasing the current from 80 to 100 amp. Welding current is one of the variables that influence the microstructure and the quality of weld features. The melting rate is directly proportional to the amount of heat energy supplied, and therefore, current must be maintained within limited ranges. In the other word, increase of input heat through high current,

Proceedings of Iran International Aluminum Conference (IIAC2012) May 15-16, 2012, Arak, I.R. Iran

decreases the cooling rate and therefore allow to growth of Al dendrite. On the other hand, there are variations of hardness values as a function of welding current in the fusion zone, as shown in Fig.4. As is evident, the hardness value decreased by increasing the current. This can be a correlation between the DAS values and variations of hardness, when welding current changes from 80 to 100 amp. Owing to the differing inherent properties of MMC and filler, it can be expected that a fracture will occur at the welding region during tensile tests. The failure of most welded joints consistently occurs in the base MMC (Fig.5). This indicates a stronger interface between the weld and the base MMC, as well as higher joint efficiency.

Fig.3. Influence of welding current on DAS in fusion area

Fig.5 Fractured samples in tension Fig.6 shows the changes in the ultimate tensile strength of the MMC during the tension test for different welding currents. It is observed that the measured values are closely related to features of parent materials. The UTS values varied from 155.12 to 172 MPa. In addition, the elongation percentages of welded samples reveal a lack of ductility, which is mainly related to the nature of the MMC, in terms of the existence of coarse and unrefined Mg2Si particles. These coarse and unrefined Mg2Si reinforcement particles prevent the materials becoming plastic from deformation, and therefore, plasticity was reduced. Furthermore, brittle fractures occurred in the composite [15]. Given these disadvantages, previous studied have explored the refinement of the primary Mg2Si phase by way of adding elements [13, 16-18]. The reduction of this reinforcement phase should then lead to increased strength [15]. Accordingly, samples that have lower levels of Mg2Si display higher ductility. Irrespective of welding specification, some defects were observed during welding process that influences the quality of welding. Formation of gas porosity can be attributed to humidity of filler metal or higher volume of the reinforcement material results to undesirable chemical reactions.

Fig.4 Influence of welding current on hardness in fusion area

Fig.6 Influence of welding current on UTS

Proceedings of Iran International Aluminum Conference (IIAC2012) May 15-16, 2012, Arak, I.R. Iran

base MMC indicate a stronger interface between the base MMC and the weld. On the whole, this study demonstrates that in situ Mg2Si-Al MMC can be welded using TIG and Al-Si filler metal.

Acknowledgment The authors would like to thank Dr. Emamy for the manufacturing of the MMC, as well as Universiti Teknologi Malaysia for their provision of research facilities.

Fig.7 Influence of welding current on elongation to fracture (a)

Gas Porosity

(b)

Crack

Fig.8 Defects observed in welded samples :(a) gas porosity and (b) crack

Conclusion The microstructure analysis shows the fusion zone, the interface region, HAZ and unaffected areas, which are due to the influence of weld specifications. The integrity of joints was investigated by analyses of its microstructure, tensile and hardness testings. The higher hardness values were obtained for welding current in the range of 80-85 amp. Most of the joints fractured in the

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Proceedings of Iran International Aluminum Conference (IIAC2012) May 15-16, 2012, Arak, I.R. Iran

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