WELDING AND CHARACTERIZATION OF 5083 ALUMINUM ALLOY

Transcription

1 WELDING AND CHARACTERIZATION OF 5083 ALUMINUM ALLOY Maamar HAKEM 1, S. LEBAILI 2, J. MIROUD 2, A. BENTALEB 2, S. TOUKALI 2 1 Centre National de Recherche Scientifique et Technique en Soudage et Contrôle CSC. BP 64, Route de Delly-Ibrahim, Cheraga, Alger, ALGERIE. 2 Université des Sciences et technologie Bab Ezzouar, USTHB, Alger, ALGERIE. hakem_maamar@yahoo.fr Abstract A pipe of Aluminium Alloy 5083 for Liquefied Natural Gas (LNG) transport has been welded by the Gas Tungsten Arc Welding process (GTAW). The welding was conducted following the welding parameters in four passes and using an ER 5356 filler metal according to standard American Welding Society (AWS) and Argon as shield gas. A Metallographic studies (Optical Microscopy and Scanning Electron Microscopy) and mechanical tests (Micro Hardness Vickers test and Tensile Test) were made to determine microstructure evolution and mechanical properties of weld joint. Key words: Aluminium alloys welding,, hardness, tensile strength & yield strength, microstructures. 1. INTRODUCTION The necessity to reduce weight and exhaust emissions, improve fuel economy has led to increased use of lightweight materials such as Aluminium alloys. Among the alloys, AA 5083 is used in many fields of liquefied natural gas (LNG) transport and storage tanks, ships, vehicles, and high pressure vessels for his good strength and welding properties, etc. Welding is an important manufacturing technology in Aluminium alloy application. The optimisation of the welding process requires a good understanding of the microstructures generated by the rapid temperature rise in the heat affected zone. Improvements to welding technique are desirable as it is one of the methods showing greatest potential for application in industry. 2. EXPERIMENTAL PROCEDURE 2.1 Material and Gas Tungsten Arc Welding (GTAW) The studied material, 5083 Aluminium alloy pipe, 6 diameter and 8 mm thickness, used in LNG transportation for low temperature. The chemical compositions of base and filler metal are given in "Table 1". Table 1 Chemical composition of the Aluminium base material and filler metal, wt. %. Materials Al Si Mg Ti Cr Mn Fe Ni Cu Zn Sn Pb S AA ,3 0,45 3,95 0,03 0,41 0,48 0,22 0,01 0,03 0,03 <10-3 <10-3 0,01 ER / 0.10 / / / / Weld was made by joining two pipe coupon tests by means of Gas Tungsten Arc Welding (GTAW). The configuration of the joint groove was V-shaped and the weld was completed in four passes using ER 5356 filler metal in position 5G (pipe with its axis horizontal and with welding groove in vertical plane. Welding was

2 done without rotating the pipe) [2] "figure 1". Cracking and porosity are major concerns in welding Aluminium alloys. To reduce the defects and to have good weldability, Argon as shield gas have used, which play an important role in reduction of generation of defects and protection of weld pool from oxidation (Aluminium being very reactive with oxygen contained in the atmosphere). Additional requirements of the shielding gas are a stable arc root mechanism, efficient shielding of the weld pool and adjacent area, and good weld penetration with a smooth weld bead profile. But they have very different characters [1] The welding parameters used are on Table 2: Table 2 Welding parameters Fig. 1 Dimension and pass of work piece. Layers Root Fill Cap Welding Process GTAW GTAW GTAW Welding position 5G 5G 5G Current & polarity AC AC AC Filer metal ER 5356 ER 5356 ER 5356 Electrode (mm) Rod (mm) Amp. Range (A) Volt. Range (V) Gas Ar Ar Ar Flow Rate (l/min) Specimens and Experiment Laboratory investigations including microstructure examination, mechanical properties and Scanning Electron Microscopy (SEM) were performed. A radiographic examination (X-ray) was used after welding to see if the joint does not contain defects (bubbles, cracks and inclusions). Samples from the parent metal and the welded region were prepared for optical Microscopy. They were polished and etched with Keller s reagent. Micro hardness Vickers test was performed on the welded sample under a load of 300 g. The micro hardness was measured on an interval of 0.5 mm through the weld, 1 mm through the Heat Affected Zone and 1.5 mm through the base metal according to 3 profiles "figure 2". HV3 HV2 HV1 Fig. 2 Micro hardness tests The last part of the experimental work included tensile testing of the welded specimens and fracture analysis. Tensile test specimens were machined out of the welded pipe according to ASME Sect. IX standard as shown in "figure 3".

3 Fig. 3 Tensile test specimen 3. RESULTS AND DISCUSSION The AA 5083 is an Aluminium alloy that relies solely upon cold work and solid solution strengthening for his strength properties. It differs from heat treatable alloys in that it is incapable of forming second-phase precipitates for improved strength. Before examining the weld, a metallographic examination of the base metal presents a granular structure slightly stretched out "figure 4". The size of the grains varies between 20 and 40 µm. The second phase has a same size that the matrix. Analyses performed with Energy Dispersed Spectrum (EDS) indicate that some are rich in magnesium and others contain iron and manganese "figure 4". These second phases are the Al 3 Mg 2 and an intermetallics were either Al 6 (Fe, Mn) or Al 6 Mn and Al 3 Fe.[3] Fig. 4 Microstructure of base metal (X500) and EDS analyses. Applying the welding parameters, weld was performed by GTAW process, shielded argon for four passes. Visual aspect was good and the NDT control (X-ray) made on the welded joint to verify the welding defects did not reveal anything. Metallographic study of the different regions of the weld shows that the Heat Affected Zone () has a coarse grain structure than the base metal "figure 5". This is due to thermal cycle caused by welding. While, the weld metal microstructure consists of columnar, epitaxial grains with a cellular or columnar-dendritic substructure that has inter-dendritic eutectic constituents primarily Al 3 Mg 2 and an the same intermetallics like a base metal Al 6 (Fe, Mn.) or Al 6 Mn and Al 3 Fe. "Figure 6". EDS-SEM analysis revealed the same chemical elements as the base metal.

4 Hardness HV 0,3 0,3 Hardness HV 0,3 0,3 Hardness HV 0,3 0, , Brno, Czech Republic, EU Fig. 5 Microstructure of Heat Affected Zone (X500) and EDS analyses. Fig. 6 Microstructure of the weld metal (X500) and EDS analyses. Selecting the best filler alloy for a given application depends on the desired performance relative to weldability, strength, ductility, and corrosion resistance. In general, the filler alloy selected should be similar in composition to the base metal alloy. Similarly, 5xxx filler alloys are used to join 5xxx-series base metal alloys. An exception to this rule is encountered when weldability becomes an issue. Others Problems are with hot cracking encountered when welding under highly constrained conditions or when welding certain alloys that are highly susceptible to cracking. Such is the case when welding the alloys that have low range magnesium content. To avoid cracking, use of a high-magnesium filler alloy is recommended [4]. These aluminium alloys are susceptible to hydrogen-induced weld metal porosity, as are all aluminium alloys in general. This porosity forms during solidification due to the abrupt drop in hydrogen solubility during solidification. Porosity can best be avoided by minimizing hydrogen pickup during welding. This can be accomplished through proper joint preparation, use of high-grade shielding gas as argon with low-dew-point, and careful storage of filler wire (that is, protection from exposure to moisture and oil). It has been determined that welding filler wire is often the primary source of hydrogen contamination. The 5xxx-series filler alloys, in particular, are susceptible to the hydration of surface oxides, which can result in porosity [4]. In "figure 7", it can be observed that the values of micro hardness tests varies between 80 and 100 HV, and a softening is apparent for the area. The hardness values are generally scattered, so that it is very difficult to discern a. This can be attributed mainly to recrystallisation in the weld and that had taken place during welding. HV 1 HV 2 HV WM 40 WM 40 WM Distance (mm) Distance (mm) Distance (mm) Fig. 7 Micro hardness profile as shown in figure 2 Figure 8 shows tensile strength vs. deformation. It is known that with mild steel there is a clearly defined point on the stress strain curve at which the elastic limit is reached; this yield point is followed by a sharp

5 UTS (Mpa) , Brno, Czech Republic, EU reduction in the stress before the metal exhibits a plastic flow region with stress again increasing with strain until the ultimate stress is reached and the stress reduces to the point of failure. In most cases, the elastic limit or yield point is not clearly defined on the stress/strain curves for aluminium alloys, this is apparent by looking at figure 8. For this reason the point of departure from the elastic range has to be defined arbitrarily. Today, however, 0.2% is the international norm. The failure for the tensile test occurred at the welded joint. The tensile strength value is 257,73 Mpa, a yield strength is 246,59 Mpa and a deformation about 17,01 %. Furthermore, the tensile test can ensure that the minimum strength requirements, as there are defined by the ASME Sec IX standards [2], are met after welding. For AA welds, the minimum strength prerequisite is the same with that of the parent metal, set at 270 Mpa for the AA The obtained value is less about 4.5 % of the limit value. The weld metal of these aluminium alloys is typically the weakest part of the joint and is the location of failure when the joint is loaded in tension. This is in contrast to most heat-treatable aluminium alloys, where the heat affected zone often is the weakest link. The absence of precipitate-forming elements in this alloy becomes a positive attribute when considering weldability, because many of the alloy additions needed for precipitation hardening can lead to liquation or hot cracking during welding. In addition, joint efficiencies are higher in this alloy because the heat-affected zone () is not compromised by the coarsening or dissolution of precipitates. When these alloys are welded, microstructural damage is incurred in the. Unlike the case of heat treatable alloys, whose strengthening precipitates may dissolve or coarsen, the damage in non-heat-treatable alloys is limited to recovery, recrystallisation, and grain growth. Thus, loss in strength in the is not nearly as severe as that experienced in heat-treatable alloys. Deformation % Fig. 8 Tensile test curve Fracture analysis of the tensile specimen revealed a surface usually consisted of elongated dimples, a pattern indicating failure via ductile fracture mechanisms figure 9. Fig. 9 SEM micrograph of the fracture surface

6 CONCLUSION In this paper, the effect of the microstructural changes that accompany a common welding technique, on the mechanical properties is investigated. Based on the analysis described above, the following conclusions are drawn: Metallographic examination of the base metal presents a granular structure a slightly stretched out with a grains size varies between 20 and 40 µm. Applying the welding parameters, weld was performed by GTAW process, shielded argon gas. Argon as shielding and purging is the most common gas, which play an important role in reduction of generation of defects and protection of weld pool. The has a coarse grain structure than the base metal. The microstructure observed in the was the successive thermal cycle of multi-pass. Cumulative effect of thermal cycles after each passes resulted in softening of the lower pass and of the adjacent parent metal. Hardness tests values vary between 80 and 100 HV, and a softening is apparent for the area. The weld metal is the weakest part of the joint and is the location of failure when the joint is loaded in tension. The as-welded specimens were subjected to uniaxial tensile tests. The UTS value is MPa. REFERENCES [1] J.H. Kim, D.H. Park, J. Korean Weld. Soc. 12 (1) (1994) [2] ASME Boiler & Pressure Vessel Code, Section IX. Welding and Brazing Qualifications. [3] Calcraft RC, Wahab MA, Viano DM, Schumann GO, Phillips RH, Ahmed NU. The development of the welding procedures and fatigue of butt-welded structures of aluminium-aa5383. J Mater Process Technol 1999; 92 93:60 5. [4] R.P Martukanitz and P.R. Michnuk, Sources of Porosity in Gas Metal Arc Welding of Aluminium, Trends in Welding Research, ASM INTERNATIONAL, 1982, P