New Design of Aluminum 6061 Welding Joining Using Friction Stir Welding Method

Parallel, lap, corner, and T joints are commonly used in fusion welding and solid welding. Other joints made are lap and butt joint. This study aims to evaluate a new design of fitting model single U and double U for friction stirr welding of Aluminum. Aluminum sheet 6061with a thickness of 10 mm, will be cut to a length of 150 mm and a width of 100 mm, then a locking groove will be made with an inner size of 5 mm and a length of 150 mm The flow is used to be paired in the FSW process. This experiment results show the high tensile strength that occurs in the double joint u, is 13.4 kN while the average is 12 kN. Single connection experiment u the highest tensile strength is 12.89 kN, while the average value is 10.37 kN. The hardness is higher in double u joints compared to single u joints. Copyright © 2020. Journal of Mechanical Engineering Science and Technology. All rights reserved.


I. Introduction
Friction Stir Welding (FSW) is a solid-phase joining technique on fabrication industry. Good quality single-sided and double-sided butt, "T", and lap joints. It was invented in 1991 and was originally used to produce butt joints of aluminum alloys [1]. FSW is a method of welding solid metals, capable of connecting two different metals (dissimilar metal) in plastic conditions. The probe is a solid welding aid to produce heat and make the material plastic. This connection occurs because the plastic material is continuously stirring along the joint line. This method was discovered and patented by The Welding Institute (TWI) [1]. FSW combine two ferrous or non-ferrous metals without the metal transfer, under pressure or without pressure. It have applied to metals and does not change its basic characteristics. The FSW is often applied to Aluminum and not post weld heat treatment (PWHT). PWHT is heat-treatment of metal after welding with the aim of reducing residual stresses [2]. FSW has several advantages including, metals low carbons with good weldability, low heat energy, no electrodes, no residual stress, low distortion, good appearance welds at relatively and low cost. The relative motion between the rotating tool and the substrate generates frictional heat that creates a plasticized region around the immersed portion of the tool [3]. The success of the FSW process depends on the design, diameter, and probe material that will be used. The probe consists of a shoulder, arm, and pin, each part of which functions as follows: a heat-producing shoulder, a chuck grip arm on the engine, and a pin as a plastic The FSW has become an efficient option of welding method for the same or dissimilar aluminum alloys, especially those which are difficult or impossible to be welded by the conventional fusion welding without any hot crackings, blow holes or distortions [7]. Lap connection is a connection process by overlapping or stacked, then the probe is rotated on a pile material ( Figure 2). The length of the pin penetrates the two materials, then the connection process is carried out with the probe rotated and shifted on the workpiece surface [8]. Key features on T-Joint FSW optimization are 1000 rpm rotation speed, 15 mm shoulder diameter, 3.9 mm, and 2.5 mm in aluminum thickness [9].  [8] FSW is a current technique in joining aluminum without feeder electrode required. The heat is generated from the rotation of welding tool which results in deformation in the welding area. This joining process is called solid welding [10]. The heat is generated from the spindle rotation and the joining occurs because of the radial force toward the axial  direction. The higher the pressure, the more heat is generated. Such condition will greatly influence the result of the welding and affect the material's mechanic state [11]. FSW is a welding method for two Ferro or non-Ferro metals without liquefication and can be done with or without pressure. FSW does not alter the basic characteristics of the metals and is often performed to aluminum that does not endure post weld heat treatment (PWHT) PWHT is heat-treatment of metal after welding with the aim of reducing residual stresses [12].
The heat in FSW technique is influenced by the probe's diameter. Its size will affect the formation of microstructure, macrostructure, and the tensile strength on the welding of the dissimilar metals between Al 6061 dan AZ 31 Magnesium [13]. The micro hardness value above 52 HVn was formed by all joint variations on 30 mm/min feedrate speed because the temperature distribution with such speed was low, therefore the Magnesium loss was just minimum. The highest tensile test score was 163.7 MPa by C-B 45 joint on 10 mm/min feedrate speed. The temperature distribution reached 420°C, consequently forming plastic state on the work-piece [14]. This study aims to evaluate a new design of fitting model single U and double U for FSW in Aluminum welding.

II. Material and Methods
The aluminum 6061 as the object of this research contain chemical substance as described in Table 1. The probe was made from low carbon steel with hardened EMS 45, simple design and with size is shown in Figure 1, then hardened to 62 HRc  Before welding 6061 aluminum sheets were made in pairs of single U and double U grooves using a CNC machine as shown in Figure 2 and speed and 10 mm/min. feed rate. This study measured the temperature manually using a simple thermocouple measuring instrument that has been calibrated (portable temperature).

A. Macrostructure
Visually, the material produced by the friction stir welding process with the design single U and double U, as shown by Figure 5 and Figure 6. The texture of the nuggets looks rough. This is due to the low heat energy of the probe below 0.8 TM (Temperature Melting) ( Figure 7A); it also affects the hardness of the heat generated. The resulting texture is smooth due to high heat energy or reaches 0.8 TM ( Figure  7B). This will affect the hardness value and tensile test. The design of this welded joint does not affect the tensile strength that occurs; it only functions as a lock. The double U joint design in Figure 8a and Figure 8b is a pairing of the FSW process with good texture results. This will increase the locking strength of the joint. In Figure 8c, the texture of the image looks very rough. The chips appear on the advancing and retreating side due to the high probe pressure and high heat. Double U joint design fits well for FSW processing. The double U tensile strength is better than the U single due to the two protrusions as anchors. The FSW welding itself only functions as a lock so that it does not get separated from the welding pair.

B. Temperature Measurement
Temperature measurement is done manually, using a thermocouple which is attached to the workpiece. Recorded manually in several sections on a single U or double U connection and graphed. Results of temperature evolution is shown in Figure 9.  Figure 9 show a low temperature below 200° on both single U and double U joints. At this temperature, a coarse microstructure will be produced, which will result in high Vickers hardness, but the resulting shear strength is high, also has not reached perfect plastic. As a result the mechanical properties that occur are not as expected, so as a result the microstructure contains little oxide and low hardness.
C. Microstructure Figure 10 shows a lot of black spots are oxides trapped in the nugget area due to the position of the probe pin directly stirring the plastic material. The heat that occurs is high and the cold quickly due to the heat loose that results in an oxide layer on aluminum 6061. This oxide will reduce the mechanical properties of aluminum 6061. The microstructure shown in the double U joint is slightly different. The microstructure in the nuggets is mostly formed by oxides, this is due to the low temperature achieved or a very rapid decrease in temperature resulting in decreased mechanical properties. In TMAZ the microstructure that occurs is dominated by small oxides that are evenly distributed, this is due to the low process temperature.

D. Micro Hardness Vickers Tests
Hardness tested against aluminum 6061 with single U and double U designs using the Vickers method with a loading of 300 gf or 0.3 kgf. It can be seen that the double U design has a higher hardness than single U. This is because the heat energy at low FSW does not change the basic structure of 6061 aluminum, especially the magnesium (Mg) content. The trapped oxide in the nugget will slightly increase the mechanical properties, especially hardness ( Figure 11). Silica (Si) alloys have no effect on improving mechanical properties. Silica in aluminum alloy 6061 will increase the flowability of the casting material. The blue color is the hardness on the upper side is 52 -69 VHn. Red color decreased Vickers microhardness, namely 45 -50 VHn. This is because the high heat energy (0.8 TM) will cause a decrease in the hardness value, also the oxide trapped in the nugget will reduce its mechanical properties, especially hardness ( Figure 12).  Figure 13 shows tensile test results. The highest tensile strength at the double U joint is 13.5 kN, the lowest is 10 kN, while the average tensile strength is 12 kN. ISSN 2580-0817 Vol. 4, No. 2, November 2020, pp. 135-143 FSW welded joint only functions as a lock, while the tensile strength is focused on the joint design. The double U joint design has a major influence on the tensile strength of the test. Single U joint design, the highest tensile strength is 12.9 kN, the lowest is 7.35 kN, while the average tensile strength is 10.37 kN. This tensile test is high; this is due to the joint design as a strength enhancer. Nugget, TMAZ, and HAZ on the weld have functioned as fastening only.

IV. Conclusions
FSW for 6061 aluminum using single U and double U joint was evaluated. The joint design studied has good tensile strength and will provide information about the newly designed joints in the FSW welding method. The nugget, TMAZ, and HAZ only function as locks, while their shear strength lies in the joint design.