The Effect of the Cutting Depth of the Tool Friction Stir Process on the Mechanical Properties and Microstructures of Aluminium Alloy 6061-T6
Emad Toma Bane Karash1, Saeed Rajab Yassen2, Mohammed Taqi Elias Qasim3
1Department of Machines, Mosul Technical Institute, Northern Technical University, Erbil, Iraq
2Department of Mechanical Engineering,College of Engineering, University of Salahaddin, Erbil, Iraq
3Department of Manufacturing Metallurgy, Mosul Technical Institute, Northern Technical University, Erbil, Iraq
To cite this article:
Emad Toma Bane Karash, Saeed Rajab Yassen, Mohammed Taqi Elias Qasim. The Effect of the Cutting Depth of the Tool Friction Stir Process on the Mechanical Properties and Microstructures of Aluminum Alloy 6061-T6. American Journal of Mechanics and Applications. Vol. 3, No. 5, 2015, pp. 33-41. doi: 10.11648/j.ajma.20150305.11
Abstract: In this study, friction stir process (FSP) is used to enhance surface properties of the AA6061-T6 alloy. Friction stir process tool travel and rotation speeds effects on the surface topography, hardness, tension mechanical properties and microstructures of Aluminum alloy were studied. The cylindrical tool without pin diameter (20 mm), tool rotational speed (800 rpm) and travel speed (60 rpm) used in all friction stir processes (FSW) in this study. The test results and analysis of the current study indicated that the hardness increases with the cutting depth in the mixing friction processes. The crystal structure analysis revealed that the hardness increased in the case of two stages twice the case of one stage. It was also noted that the size of the engineering flaws granules became smaller and the size of these granules increased with the cutting depth. In addition, the ratio of granules size and the friction in the case of two stages was twice the case of one stage.
Keywords: Aluminum Alloys, Friction Stir Process, Hardness, Rotational Speeds, Travel Speeds, Tensile Strength
Aluminum alloys and steelsform an interesting combination from the viewpoint of industrial applications, especially in the automotive and airplane industry. However, Join these materials is a challenge and the traditional fusion welding does not provide a good welding because of significant differences in physical properties such as melting point and thermal expansion coefficients, which tend to provide various welding defects such as cracks sclerosis, liquation cracks and porosity.
In order to get fully welded connections, the filler materials were selected very carefully, which have a mineral good compatibility with all the basic materials indispensable.
The fundamental concept of friction stir welding (FSW) is considerably simple. A rotating tool, that is no consumable, with a properly designed shoulder and pin, is introduced into the adjacent edges of plates or sheets to be joined and thereafter traverses over the joint line . The tool has two primary functions a) heating of the workpiece, and b) materials movement for the production of joint .
It is worth mentioning that heating is produced within the workpiece by two ways, friction due to rotating tool shoulder and pin with heavy deformation of the workpiece, meanwhile, localized heating softens the material close to the pin and, combined with the rotation of tool and translation, causes the material movement from the forehead toward the rear of the pin, for that reason the hole in the wake of tool filling as the tool goes forward. The tool shoulder restricts metals flow to a level equivalent to the shoulder position, which is about the primary upper surface of the workpiece . Sun et al. (2009)  conducted microstructure examinations during friction stir processing of sand cast A206 aluminum alloy under different processing parameters. They concluded that the higher ductility was due to the elimination of porosity and the breakup of coarse second phase particles.Their work also revealed thatthe microhardness profile in the majority of the nugget after FSP improved because of grain refinement, which was in accord with the Hall-Petch relationship.
Johannes et al. (2007)  demonstrated the effectiveness of four consecutive FSP passes in creating large areas of superplastic material (AA7075) compared with single pass FSP. The results indicated that the highest elongation of 1255% was found in the one pass sample at 470°C. The elongations achieved in multiple passes of 7075 Al are superplastic, although the single pass material exhibits slightly greater elongations.
Surekha et al. (2009)  conducted an experimental investigation to investigate the effect of processing parameters (rotation speed and traverse speed) on the corrosion behavior of friction stir processed high strength precipitation hard-enable AA 2219-T87 alloy. The results revealed that the presence of finer particles further increases the solubility of second phase particles in Al matrix. Also the study indicated that the dissolution of the CuAl2 particles during FSP reduces the number of sites available for galvanic coupling and hence increases the corrosion resistance as well as the amount of dissolution increases with rotation speed and hence the corrosion resistance also increases with rotation speed.
Zahmatkesh et al. (2010)  presented an experimental investigation to analyze the effect of FSP as a means to enhance the near-surface material properties of hot-rolled aluminum alloy AA2024-T4. The results of the experiments showed that the high wear behavior in the Nugget Zone (NZ) is attributed to a lower coefficient of friction and the improved micro-hardness in this region. Furthermore, FSP was found to be beneficial in improving wear resistance under applied load of 10 N.
Al-Danaf et al. (2010)  conducted an experimental study to test the effect of FSP on commercial AA5083 rolled plates. Samples were subjected to friction stir processing with a tool rotational speed of 430 rpm and a traverse feed rate of 90 mm/min. They found that the ductility of the friction stir processed material was enhanced by a factor ranging from 2.6 to 5 compared to the ductility of the as-received material, in the range of the strain rates tested. The results showed that the Vickers hardness number of the base metal was about 80 Hv, whereas at the center of the nugget the hardness increased to about 95 Hv.
Karthikeyan and Kumar (2011)  conducted an extensive experiment 0to investigate the surfaces of an AA6063-T6 aluminum alloy which were friction stir processed. The experimental study involved studying the effects of process variables such as axial force, tool feed, and rotational speed. A tool with a threaded pin was used. The results of the experiments showed that all the specimens processed with an axial force of 8kN force displayed defects such as voids, pinholes and tunnel defects in the nugget zone either on the advancing or retreating or both sides, while those processed at an axial force of 10 kN and most of the specimens processed at an axial force of 12 kN did not display any microstructural defects in the nugget or the surrounding zones.
The aim of this work is to study the ability of AA6061-T6 to be friction stir processed. The effect of FSP parameters on the topography and mechanical properties of the processed layer and the effect of multi-passes FSP will be studied.
2. Experimental Work
The material used in this study is an AA6061-T6 alloy with the nominal composition in (Wt.%) as shown in Table 1. The AA6061-T6 plates (250×20×3 mm in size) were friction stir processed, with a classic vertical milling machine at constant rotation and travel speeds of (800 rpm and 80 mm/min) with a non-consumable tool made of a steel shoulder diameter of 10 mm without the pin. The tools inserting depths were 25, 50, 100, 150, 200, 400 µm for the both stages. The first stage is one pass and the second is double passes in a 3 mm thick plate. The mechanical properties are shown in Table 2, and Table 3 shows hardening exponent values (n) and the strength coefficient (k) of aluminum alloy AA 6061 - T6.
The friction stir process tool was fabricated from steel tool labeled as X12M and was heat treated to have a hardness level of 62.85 HRC. The tool had a concaved shoulder (20) while the tool pin was made cylindrical with right-hand threads of (1) mm pitch and had a round bottom. The tools geometry used for this study is shown in figure (1). The details of the metal friction stir tool in table 4 were selected according to Table 2.
|Model||Specification||Dimensions Tool||Shoulder diameter (mm)||Pin diameter (mm)||Pin height (mm)|
|Alloy steel||ISO 6508||20||-||-|
|1||Aluminum alloys||< 12||Tool steel, WC – Co|
|2||Magnesium alloys||< 6||Tool steel, WC|
|3||Copper||< 50||Nickel alloys, PCBN (a)|
|4||Copper alloys||< 11||Tool steel|
|5||Titanium alloys||< 6||Tungsten alloys|
|6||Stainless steels||< 6||PCBM, tungsten alloys|
|7||Low – alloy steels||< 10||WC, PCBN|
|8||Nickel alloys||< 6||PCBN|
|PCBN, polycrystalline cubic boron nitride|
A classic vertical milling machine was employed to carry out all the friction stir processes at the University of Nahreen, with a simple fixture of carbon steel to fix the plate during processing.
In the current study, two parameters were selected among the friction stir process parameters, which are the tool travel and rotation speeds. The parameters were chosen in consideration with the machine capabilities. Rotational speed (800) rpm and three travel speeds (80) mm/min were selected; the tool was rotating in the counterclockwise direction to carry out all the friction stir processed. Figure 2 shows some of the aluminum samples through and after processing.
Many trials were made using pin, free shoulder with 15 mm diameter to find out its effects on the processed area, all those trials failed because of the limited load of the machine used, where the effect of stirring was not homogeneous on the surface and the processed region did not completely appear (less than the shoulder diameter).
Multi-passes (two passes) FSP was made on the surface with an overlap of 50%, as shown in figure 3 to find out whether a continuous surface layer can be obtained on the specimen. Also double passes of FSP were undertaken and its effects on properties were investigated.
The microstructural analysis of the friction stir process samples was undertaken on both the one pass and the double passes samples by optical microscope and scanning electron microscope (SEM). The samples were etched with Keller's reagent to reveal the grain boundaries.Microhardness measurements were carried out using (Zwick/Roell Z HV) Vicker's microhardness tester at 27g load for the 30s according to ASTM-E384. The readings were taken for each specimen at three points, one of these three points at the center of friction stirred zone near the surface and the two other readings were distributed as a straight line to the right and left of the center, the interval between the neighboring measurements is 1mm as shown in Fig. (4). Transverse tensile specimens were prepared for the base metal and friction stir processed samples at 800 rpm rotational speed and 80 mm/min travel speed with different cutting depths ( 25, 50, 100, 150, 200, 400 µm) in two stages. The first stage is one pass and the second is double passes, (According to ASTM (B557M)). The test was carried out by (Tinius 200 super L) computerized universal testing machine.
3. Results and Discussion
The as-received aluminum 6061-T6 alloy consisting of large elongated grains with an average grain length of 187 μm and a length /thickness ratio (aspect ratio) of 2.97, with large particles of precipitates distributed non-homogeneously as shown in figures 5 and 6.
Due to the rotating and pressing effects of the friction stir tool shoulder on the plate surface and the heat generated due to plastic deformation; a surface stirred layer (modified surface) will be produced underneath the tool shoulder to about a micrometer level.
In the primary trials of the current study, the rotational speed was 800 rpm and the travel speed was 80 mm/min to produce the stir layer and cutting depths (25, 50, 100, 150, 200, 400 µm) in two stages. The first stage is one pass and the second is double passes. Figures 7 to 19 summarize all surfaces stirred layers produced from the selected 800 rpm rotational speed with 80 mm/min travel speed.
In the trials of this study, the cutting depths for both the one pass and the double pass were applied with 800 rpm rotational speed and 80 mm/min travel speed. The cutting depths were 0, 25, 50, 100, 150, 200, 400 µm. The granules in the friction zone be equal axes and soft center and smaller than the size of the metal base granules, due to the occurrence of the process of re-soften the crystalline sizes of the minutes deposited in this region as a result of the plastic deformation caused by high rotational motion of the welding tool and the occurrence of recrystallization as a result of the heat generated during the friction head bump stitches with the surrounding metal. The study of the crystal structures of the alloy under the microscopic structures revealed that the process of friction one pass diminished engineering flaws in the surface of the metal as shown in figures 7 to 13. The size of the pieces on the surface of the original metal was as follows: when a depth of cut was 25 µm the decreased defects ratio was 3.03% size, at a depth of cut 50 µm the decreasing proportion was about 3.32%, at a depth of cut 100 µm the attrition increased clearly to reach 21.61%, at a depth of cut 150 µm was 30.42%, at a depth of cut 200 µm the amount of the decreasing proportion was 38.58% and the highest decrease recorded was 45.85% at a depth of cut 400 µm.
Moreover, the study revealed that the process of friction double pass diminished the size of defects in the metal surface relative to the surface of the original metal as shown in figures 14 to 19. The defects size ratio was as follows: when a depth of cut was 25 µm it was 4%, at a depth of cut 50 µm it was 6%, at a depth of cut 100 µm it was 10%, at a depth of cut 150 µm it was 35%, at a depth of cut 200 µm it was 38% and at a depth of cut 400 µm it was 40%.
From the crystal structure analysis and note that the size of the engineering flaws granules least getting smaller and increase the size of these granules cutting depth and the ratio of small and decreasing friction in the process for more than double pass the friction process for one pass.
Figure 20 shows the relationship between the cutting depths and the value of hardness for the process stir friction.
Figure (20 - a) shows the relationship when the friction stir process one pass, is evident from the figure that the increasing in cutting depth lead to increase the value of the hardness significantly this increase and to remain a great depth (300 µm, 8.34%) and after the increase are very few. Figure (20-b) also shows the relationship between cutting depth and hardness in the case of the friction stir process double pass and seen from figure substantial increase in the value of hardness increase of cutting depth to the limit (300 µm, 17.71%) this increase is less significantly. The Figure (20-c) shows a comparison between one pass and double pass friction stir process where the hardness value increases with increasing cutting depth and is in the double pass friction stir process more increase than one pass friction stir process. The reason is due to the homogeneity of the crystals have been more as well as less engineering defects and impurities as a result of the high temperature of the friction stir process of the surface of the metal.
Friction Stir Process is a complex thermo-mechanical process in which the combination of temperature, strain and their period are responsible for the creation of a new surface layer. In FSP of AA6061-T6, due to a mixing action of the tool, second phase particles are refined, so properties of the base metal are changed. The microhardness of stirred layer is almost increased compared to that of the base metal. The maximum increment of this property is about 8.34 - 17.71%. The yield and tensile strengths of Friction Stir Process samples are 288 Mpa and 330.6 Mpa, with an increment of 13% and 7% respectively. The toughness of the surface layer increases to some extent since its ductility is almost unchanged during Friction Stir Process with the little increase in strength.
This research was supported by Engineering Science Research Program through the Mechanical department - College of engineering the University of Mosul funded by the Ministry of Higher Education and Scientific Research / Republic of Iraq. (No. 2015-00333).