Experimental investigations of influence on grain size and cold deformational behavior of AL6063 during the microforming process

Miniature products are requisite to make micro creation widely in electronics and micromechanical products. The microfabrication process is identified to satisfy the production of such miniature products rather than an ordinary manufacturing process. Microextrusion is one of the microforming processes in micromanufacturing. In this present work, an attempt has been made to investigate the influence of grain size and deformation behavior of Al6063 microstepped pin with annealed billets for the cold extrusion process. The methodology on the characterization of the microextrusion of Al6063 includes, annealing of the billet, extrusion testing, microhardness examination and surface roughness analysis. The billet with three different sizes of grains is extruded. The experimental result shows that the deformation load and average microhardness of the AA96 (annealed Al6063 with an obtained grain size of 96 μm) are high compared to AA208 (annealed Al6063 with an obtained grain size of 208 μm). The surface finish has improved using diamond-like carbon (DLC) coated die compared to uncoated and lubricated dies. DLC coating with AA208 billet achieved a maximum pin length of 13.1 mm, and uncoated die with AA96 billet achieved a minimum pin length of 5.5 mm. Thus, the findings of this study contribute to the fundamental understanding of cold microextrusion of aluminium 6063 alloy.


Introduction
Manufacturing micro metallic parts is essential in many industries, from watches and computers to medical devices. Manufacturing industries focus on mass production of micro parts, which are difficult to process using conventional methods due to high tooling costs and a lack of available tools for miniature products. Nanthakumar et al [1] and Rahman et al [2] fabricated a micro-stepped pin using miniature machine tools, and the problem faced with tool-based micromachining is the workpiece deflection. One of the micromanufacturing processes is micro-forming (Joo et al; Chern and Chuang; Rajenthirakumar et al; Razali et al; Nanthakumar et al) [3][4][5][6][7] and its cost-effective process for mass production of micro metallic parts which includes micropin, microgear, microstepped shaft, microbulged pin, micro screw, IC chips, facial implants, clavicle implant, and knee plate. Qin [8] describes the dimension of microparts varying from sub millimeters up to a few millimeters used extensively in electronics and micromechanical products. M Geiger et al [9] recommended that any two dimensions of parts less than 1 mm range is known as micro part. However, the use of metal forming in producing micrometalic parts is still limited. One reason is that traditional knowledge cannot be easily transferred to the micro-scale.
When a macro-scale forming process of metallic parts is scaled down to a micro-scale forming process, the size effect is one of the significant issues in microscaled metallic parts. Vollertsen et al [10] investigate the size effects that are different from intensive or extensive values of process characteristics that occur when scaling the geometrical dimensions of a part. Size effects happen because the ratio among all critical features cannot be held in reserve constant according to the process requirements. Yao et al [11] investigated the size effects in micro/ Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. meso extrusion of A356 aluminium alloy. Srinivasan et al [12] examined the size effect and temperature in microscale deformation of Al6063 gear and mentioned that the grain size and boundary orientation significantly affect the microextrusion process. Liu et al [13] examine the size effect of H62 brass with the uniaxial tension experimentations, and the results show both the grain size effect and the feature size effect. The material behaviour and size effect in microextrusion of pure copper and aluminium with dissimilar grain sizes are investigated by Nandhakumar, and Rajenthirakumar [1]. Vollertsen [14] has classified the size effects into three categories as density effect, shape effect, and microstructure. Cao et al [15] investigate the extrusion process for micropin and specify that surface interactions become a big part of the extrusion of micropins.
The effects of miniaturization on micro metallic parts and their behaviour during forward extrusion are investigated by Rajenthirakumar and Sridhar [16]. The authors investigated the effect of grain size on the selected work material. Metal micro-component forming necessitates careful design to achieve uniform material flow, small-scale tooling, and limited handling capability. Rivas et al [17], considering the high strength and excellent mechanical properties, Al6063 aluminium alloy is very effective in the extruded parts feature. FE simulation was done by Rosochowska1 et al [18] to identify the process design before engaging in expensive experimental trials. Nanthakumar et al [1] and Zhengyi Jiang et al [19] investigated the behaviour of pure copper at elevated temperatures and highlighted the micro-scale deformational behaviour of the selected metals.
AdeoSun et al [20] investigated both Cold and hot extrusion of annealed metallic billet responses by measuring extrudates' extrusion pressure, extrusion ratio, linear strain, and surface hardness. Cao et al [15] describe that Cold microforming is limited by the final extrudate on grain size and orientation. This causes troubles, such as non-uniform deformation and large scatter, resulting in restrictions on product reproducibility. The effect of elevated temperature on extrusion force, force resistance, microhardness, and surface finish of pure copper was investigated by Nandhakuar et al [7]. Bhupatiraju et al [21] investigated the punch pressure in a cold extrusion process. The process depends on the flow stress of the workpiece to be extruded, the degree of the deformation, workpiece geometry, and interfacial friction between billet to die and die design. When billets are subjected to plastic deformation, the extrudate becomes unsupported as it exits the die cavity. Fixing the length of the die cavity with the extrudate can help reduce the bent effect on the extrudate during the microextrusion process. It can be used to create a product with a short length. The extrudate bend can be reduced during the cold extrusion process.
The objective of this research is to investigate the deformation behaviour of the Al6063 micropin at room temperature by varying grain sizes under different forming conditions. A novel extrusion die set assembly has been designed and developed to carry out the experimental test with Al6063 to investigate the size effect of the microextrudate. Removing extruded micropins after extrusion with a commercial die is extremely difficult. It is also not possible to apply an inner coating to the surface of the die cavity. As a result and novelty of this work is , the novel die set assembly i.e. segmented die has been used for ease of removal of extruded billet and die holder for ease of carrying split die instead of regular die. Three different die cavities were used to demonstrate the microforming of a pin, namely a dry die cavity, a lubricated die cavity, and a DLC-coated die cavity. The coating is impossible with a standard die, i.e., a non-segmented die.

Materials and methods
The cold extrusion process has been done with the Al6063 billet. The experimental practice includes sample preparation, compression testing, and microextrusion of cylindrical pins, discussed further below.

Material processing
Al6063 is a material used in extrusion testing and is widely used in connector pins, electrical and electronic components, instrumentation, automotive, medical, surgical, and industrial assemblies. It has good corrosion resistance, ductility, strength, and excellent electrical and thermal conductivity. This research also benefits from its excellent extrudability and manufacturability. Spark testing method is employed to examine the chemical composition of the Al6063 and shown in table 1. The billet has a circular cross-section received with a 6 mm diameter and reduced with a constant diameter to a length of 10 mm. The extrusion ratio is an important metric in the extrusion process. It is defined as the ratio between the initial cross-sectional area (A 0 = 28.27 mm 2 ) and the final cross-sectional area of the billet The Al6063 billets are annealed at different temperatures to vary the grain size to demonstrate microextrusion testing, and the details are listed in table 2. Grain expansion occurs during the heat treatment process, and larger grains grow even more significantly with the exposure of smaller grains. As a result, the number of grains per unit volume decreases. Ground with 1200 grit SiC paper, the heat-affected layer in the specimen is removed. The etchant composition is used to expose the grain dimension by the ASTM E407-07 standard. The grain size is revealed by etching the mirror-finished billets with the etchant. The Kroll's regent is used as billet etching and is used as follows: 2 ml hydrofluoric acid, 6 ml nitric acid, and 92 ml distilled water. A cotton swab is used to brush the prepared etchant onto the billet for about 30 s.

Design of experimental setup
A die set assembly is designed and developed to conduct the experimental test with Al6063 to investigate the deformation behaviour in the microextrusion process. In the micropin extrusion process, the specimen has a circular cross-section of the billet, and the feature size is the diameter of the reduced cross-section. The die and punch are dimensioned correctly based on the geometry output. A segmented die is used to achieve the highest possible extrudate output and facilitate easy removal of the micropin. The creation of flash occurs due to the effect of the segmented die and clearance between punch and die. An electrical discharge machine is used to create the die cavity. The die is slitted into two halves before making the die cavity using a wire-cut electrical discharge machine. The die holder has holes to remove the segmented die at the end of the extrusion process. The microextrusion assembly, depicted in figure 1, consists of a die holder, a punch, a split die, and a punch holder. Figure 2 depicts the geometrical details of the die and punch.

Experimental procedure
The experiments are carried out in three stages, each with a different annealing condition. The test was carried out to achieve the most significant possible punch displacement. The parameter used to produce micro extrudate is billet size, output geometry, extrusion ratio, temperature, friction condition, billet material, and die material. The extrusion force is recorded for every 0.5 mm punch displacement up to 8 mm during the micro forming testing process. The testing process is stopped before the interference occurs between punch surface to die surface and noted the extrusion force and length of the extruded at each trial. In the first phase, the specimen is tested without any lubrication; in the second phase of the work conducted under lubrication conditions, the type of lubricant used in this experiment is servo 68 oil type lubricant. In the third phase, the dies are coated with  diamond-like carbon (DLC) with silicon to condense the effect of frictional force. It has the unique properties of natural diamond, high hardness, less friction, and good corrosion resistance. DLC is a nanocomposite coating, this kind of coating is done by a plasma-assisted chemical vapor deposition method. The forward extrusion testing was carried out using a computerized universal testing machine (UTM). Figure 3 depicts a micro extrusion testing platform outfitted with a load cell (600 kN) and a Linear Variable Differential Transformer (LVDT: 0-1000 mm). Ten trials with different annealing conditions were taken from each die, and the values were averaged. The tests are carried out at room temperature. The billet is held inside the uncoated die with a rough surface finish, and the segmented die is held in the die holder. After attaching the punch to the punch holder, the extrusion setup is assembled and placed in the extrusion testing platform. The billet is gradually loaded to ensure positive displacement of the extrudate through the punch. Force and corresponding displacement are recorded at every 0.5 mm movement of punch travel. Forces are applied to the billet at the possible geometry output. Similarly, testing is done with a lubricated and coated die for the required output.

Characterization
The microhardness of the extrudate has been measured using a Mitutoyo microhardness testing machine with a load of 100 gf and 15 s. Next, the extrudate's surface roughness is measured using a Taylor Mitutoyo surface roughness tester, and the results are averaged. Next, the extrudate's microstructure is examined using Sigma  FESEM equipment. Next, specimens are mounted using bakelite powder and hot embossed for microstructural analysis. Finally, the line intercepts in the grain boundary are utilized to compute the average grain size.

Results and discussion
The microextrusion of the Al6063 microstepped pin is extruded successfully under cold extrusion by segmented die. The following sections discuss the results of force-displacement response during extrusion testing, microhardness surface finish, and numerical investigation of microextrudate.

Effect of friction at micro-scale deformation (Force Displacement response)
The experiment's first phase was conducted with an uncoated die at room temperature. Ten experimental trials are conducted for each parameter with Al6063 to confirm process repeatability, and the force-displacement curve after the cold extrusion process is shown in figure 4. According to the experimental results, the extrusion force for maximum punch displacement of AA96, AA144, and AA208 is 52 kN, 41 kN, and 35 kN, respectively.
The second testing phase was done with a lubricated (servo-based 68) die under cold extrusion. The forcedisplacement responses of Al6063 microextrudate are shown in figure 5. Extrusion forces for AA96, AA144, and AA208 are 47 kN, 34 kN, and 30 kN, respectively.
In the third phase, the experiment was carried out with a DLC-coated die at room temperature. Figure 6 depicts the force-displacement curve of the forward microextrusion process. Extrusion forces for AA96, AA144, and AA208 are 42 kN, 30 kN, and 25 kN, respectively.   According to the force-displacement curve, the AA96 requires a high extrusion force for extrusion because it has a fine grain structure with a more significant number of grain boundaries. These grain boundaries act as a barrier to slip transfer, causing dislocation movement to be disrupted. However, in terms of forming force, the extrusion force with coarse grains (AA144, AA208) is generally lower than that with fine grains (AA96) for Al6063. Figure 8 depicts a sample output with Al6063 micro stepped pins of varying lengths obtained under various forming conditions. Figures 4-6 show that the increase in force during the extrusion process is strongly related to the grain size and surface condition of the die cavity. Extrusion force is reduced as grain size increases, while extrusion force increases as grain size decrease. Similarly, different surface conditions of the die cavity can minimize friction between die and billet, resulting in a lower extrusion force of the microextrusion process. Figure 9(a) depicts the specimen with Al6063 samples (AA144). The Al6063 microextrudate of AA144 is depicted in figure 9(b). Fine-grained arrangements restrict metal flow through the die cavity due to more grains and fewer interstitial sites. In contrast, there will be fewer resisting areas because there are fewer grains and more interstitial sites in a coarse-grained structure. Many grain boundaries necessitate a higher deformation load and flow stress, resulting in higher hardness. The grain size is smaller at a significant number of grain borders [1,5,12,22].
Since a few grains are present in the billet and even allocation of different grains no longer exists, the anisotropic properties of each grain can be considerable to the deformation behavior and lead to the inhomogeneous deformation. The non-uniform deformation character of the grain is denoted as inhomogeneous deformation [6,23]. In this attempt, the testing results revealed that all grain structures AA96, AA144, and AA208 increase extrusion force during the extrusion process. It was also discovered that an uncoated AA96 die required more extrusion force, whereas a DLC coated with an AA208 die required less deformation. Because the uncoated die had a higher interfacial friction effect on the billet than the DLC coated  die, servo lubricated dies (servo 68) will reduce friction between die and billet, but not as much as DLC coated dies.
The forming force and pin length were compared (refer to figure 10) with different grain sizes to conduct a thorough investigation of friction at cold forming with an uncoated die (refer to table 3). Pin lengths are compared based on the output of microextrusion testing with different testing conditions to determine the maximum and minimum length of the formed specimen. The highest length of the extrudate is taken as the maximum pin length, and the lowest length is taken as the minimum pin length. The experiment stops before interference occurs between the punch and die surfaces. Up to this, the billet is extruded and noted the extrusion force. In addition, at the end of each trial, the extrudate length is measured, and the values are averaged. It will   allow us to compare the length of the extrudate of annealed billets with different friction conditions. From the figures 7, 10, the minimum force (35.16 kN), maximum pin length (8.7 mm), and maximum average pin length (8.4 mm) for the AA208 billet are noted. A longer pin length indicates that the workpiece contains a larger grain size. The maximum force (52.25 kN), minimum pin length (5.5 mm), and average pin length (5.9 mm) were recorded for AA96.
When the die is lubricated with servo-based 68, an overall reduction in forming force is observed in all three types of billet grain size. Furthermore, the pin length increases when the die is lubricated, and the maximum pin length (9.2 mm) is observed. The maximum pin length of AA208 (13.1 mm) is extruded with an average pin length of 12.9 mm after forming a DLC coated die. So, it can be demonstrated that when the billet is cold extruded, there is a restriction in the flow of material inside the die orifice, resulting in poor replication of die dimensions. Further, friction increases significantly due to the non-appearance of lubricant and surface coating, which results in the lowest pin length. In the case of servo lubricant being used, it could not be retained inside the die cavity as it is easily squeezed out from the die wall surface [7]. But when the extrusion die is coated with diamond-like carbon (DLC), the flow of material inside the die cavity is more uniform with minimum resistance resulting in the most significant pin length.
To reduce the extrusion force, extrusion at elevated temperatures can experiment with, leading to lesser extrusion force and better forming. It is due to the dislocation of thermally activated grains at elevated temperatures [1]. In this view, further research actions are needed, leading to better results and understanding.

Microhardness
A series of Microindentation hardness testing (MHT) on microextrudate with various radial locations compares the deformation of fine and coarse grain size materials. MHT is primarily used to examine fine-scale changes in hardness. The Vickers test is the most commonly used microindentation test. The micro viker hardness tester (Shimadzu load range 10 g-2 kg) is used to take the measurements. Samples must be prepared for MHT testing to provide a specimen to fit into the tester ( figure 11). For the microhardness test procedure, ASTM E384-17 is followed, and the values are averaged [24]. In this test, the load is applied smoothly without impacting the microextrudate and held in place for 10 to 15 s.
The microindentation test revealed that the hardness of the microextrudate increased significantly as the die condition changed from DLC coated to uncoated die, and grain size changed from larger to smaller. Figures 12,13 and 14 show the hardness of microextrudates with grain sizes of 96 μm, 144 μm and 208 μm under three different friction conditions. The friction conditions, in this case, are DLC coated die, lubricated die, and uncoated die. Furthermore, fine-grained extrudate has a more hardness after the microextrusion process than  coarse-grained extrudate. It is stable with the Hall-Petch relation, which states that the coarse grain size material's strength should be lesser than that of finer grain size material. Yooseob Song et al [25] investigated experimentally and numerically aluminium with various grain sizes and found that the hardness of aluminium becomes stronger with decreasing the average size of the grain and follows the Hall-Petch relationship. Grain   boundary strengthening is based on these observations. The hardness distribution along the pin's extruded from fine grain was more consistent than pins fabricated from coarse-grained material [26].
Further fine-grained material deformation confirmed a consistent pattern along the length of the pin. In contrast, many individual deformed grains were observed in the coarse-grained structure, resulting in a less consistent pattern along the length.

Surface roughness
Surface roughness (table 4) is measured using a portable roughness tester (Mitutoyo SJ210). The specimen is held with a proper clamp to prevent movement of the testing samples. According to the IS1997 standard, the test is performed with a constant velocity of 0.25 mm s −1 and a sampling length of 5 mm. Measurements are taken for all Al6063 microextrudates. The results show that the DLC-coated die has a better surface finish than the uncoated die. Furthermore, the servo-68-based die has a better surface finish than the uncoated die. But not as good as a DLC-coated die.

Conclusions
In this experimental investigation, material behavior on forward microextrusion is presented. The extrusion of the microstepped pin has done successfully. On the experimental side, a novel extrusion die has been designed, and microextrusion testing has been conducted with various grain sizes at room temperature.
The microplastic deformation process shows that the deformation load for the fine-grained billet is high (52.25 kN) due to the presence of high grains count compared with larger grain (24.79 kN) specimens, resulting in the extrusion force being inversely proportional to the grain size of the billet. The coarse grains with DLC coating can extrude the Al6063 billet with maximum pin length (13.1 mm), which is least for the fine-grained billet of uncoated die (5.5 mm).
Microhardness values were increased with fine-grained microextrudate than with coarse-grained AA208 extrudate since grain size and distribution of the billet changed after the extrusion process. It is due to grains present in the billet being highly compressed with a higher extrusion ratio (r x = 44) in the testing. Thus, the hardness of the micro-stepped pin can be strengthened by decreasing the average grain size.
The surface finish of the microextrudate with DLC coated die (0.398 μm) is better than both uncoated die (0.539 μm) and lubricated die (0.512 μm). It is due to the surface of the die and interfacial friction between die and extrudate. Therefore, DLC coated die is suitable for the microextrusion of the Al6063 billet rather than the uncoated and lubricated die.
The results will facilitate introducing the microextrusion for producing micro parts in the aluminium-based manufacturing industry. Furthermore, the findings of the experiments will be beneficial for benchmarking theoretical models. Scope of the future work The annealing temperature of may be increased to approximately upto 550°C (Max. solubility limit) so that still larger grains and max. plasticity could have been achieved. The research will be conducted with elevated temperature which could reduce the extrusion force. The different type of lubrication may be used to examine the friction behaviour of extrudate.

Funding
This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.