Introduction

Magnesium (Mg) is a common element in alloys made with a number of other metals. AZ31 is 1/3 lighter than aluminium, and 2/3 than steel1,2. The uniformity of applications of Mg alloys in a wide range of applications because of properties such as light weight, deformation properties, higher indentation, high impact resistance as well as fatigue resistance3,4. However, the AZ31 magnesium plate has extensive use across diverse sectors such as automobiles, aerospace, electronic devices, and medical5,6,7,8. Within the aerospace sector, it finds use in the production of helicopters and components for aeroplanes. Within the automobile sector, this technique is used for the manufacturing of engine components, gearboxes, and suspension elements. In the electronics sector, it is used for the production of portable gadgets like laptops and cellphones. Within the medical sector, this material is used for the fabrication of surgical equipment, plates, and implants.

In industry various types of forming9,10, casting/sintering11, advanced high precision welding12,13, and coatings14 are used in industry to manufacture various types of products. Advanced joining processes like advances welding15, wire-arc additive manufacturing16,17, friction stir processing18, and friction stir welding19 are widely used processed to joining and materials development of the dissimilar metals and alloys. However, the development of composites, their processing, and joining are quite tough processes20,21,22,23,24. In earlier years, Christener and Sylva25 had performed Friction stir welding on aluminium 2014AT-6 alloy and investigated mechanical properties on the effect of joint gap through parameter tolerance technique. Darras26 had performed an experimental as well as analytical study on aluminium 5052 alloy and investigated the microstructural and mechanical characteristics. They found that grain size structure, hardness is depends upon rotational and translational speeds and microstructure and properties depends upon temperature achieved during FSP. The microstructure of the materials highly relies on the transformational temperature during the process27,28,29,49,50,51.

Literature survey points towards the direction that the most of studies deals with the friction stir welding and there is lot of work which has necessary to carry out in Friction stir processing. The most of work has been processed on Aluminium by varying reinforcement in FSP but the number of reinforcements was restricted to one or two. So, present study deals with Magnisium composite fabricated by using four different reinforcements. The reinforcements used are Silicon carbide40, Alumina, Chromium and Silicon powder. Experimental procedure is performed on vertical milling machine26. Tool used for FSP is of cylindrical profile18. The FSP is performed at constant transverse speed and varying the rotational speed32. Investigation of mechanical properties including the hardness, impact strength and tensile strength is graphical representation. The microscopic images are obtained at two different magnifications for each specimen of specific reinforcement.

Materials and methods

Materials

In fabrication process, the feedstock powders such as commercially available SiC, Al2O3, Cr and Si (99.9%) powder are used. For the purpose of the experiment, the matrix material that was used was AZ31 magnesium alloy. AZ31 magnesium plate (80 × 80 × 8 mm) in rolling state is used in this study. In order to produce these magnesium ingots, a power hacksaw was used to cut them into little pieces with nominal dimensions of 80 mm by 80 mm by 8 mm, as shown in Fig. 1a.

Figure 1
figure 1

Groove cutting before starting the FSP process.

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Composite fabrication technique

The FSP technique in conjunction with a semi-automatic vertical milling machine is used to assemble the magnesium metal matrix composite. A brief overview of the fabrication process includes the tool design and placement, friction and heating, stirring and plastic deformation, reinforcement incorporation, and fabrication of surface composites18,52,53. The experimental setup of vertical milling machine is shown in Fig. 2. In this process, various parameters like tool design geometry, transverse speed of tool, tool rotation, and downward vertical force and the reinforcements are applied for purpose of significant results on the material flow sample and temperature circulation. These parameters thereby influence the microstructural development of material.

Figure 2
figure 2

Tool profile and geometry.

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Additionally, the tool arbour holding spindle and axis of the vertical milling machine are orientated in a vertical direction, as shown in Fig. 3. This machine is used for FSP of magnesium sample plates. The tool is also held in a vertical position, which is characteristic of the manual chuck. Milling cutters are rotated on the axis of the spindle while they are kept in place. In most cases, the spindle may be extended (or the table can be lifted or lowered, in order to achieve the same effect), which enables plunge cuts and drilling to be performed. Holding the tool in the required axis in the machine is the primary purpose of the tool holder, and the ER-Style Collets tool holder is the one that is employed. The fixture plate used for FSP work consists of rectangular base proportions 200 × 150 × 20 mm plate and two steel bars are bolted to a machined base plate by 2 bolts M15. The condition is given by 2 smaller bolts that press down an assembly of plates, yielding a uniformly distributed force along the work piece or matrix and the base plate is bolted to the machine worktable of the FSP equipment. The fixture system is of square dimension i.e. 81 × 81 mm, as shown in Fig. 1b. The specifications of FSP tool used are certain as below in Table 1. The tool used in this study has been illustrated in Fig. 3.

Figure 3
figure 3

Tool profile and geometry.

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Table 1 Parameters of machine tool.
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Machining process

Before starting the machining process, a groove has been cut with 2 mm width and 5 mm depth on the Mg-matrix composite and channel or groove is placed at the edge of pin in the advancing side and center of the matrix plate as shown in Fig. 1a. To keep the various components from falling out of the groove, a cylindrical tool with a shoulder is used to seal it. The groove is then filled with SiC, Al2O3, Cr, and Si-powder particles. Kerosene oil is added to the particle powder so that it cannot be removed from the plate by air during operation and proper matrix mixing. With a single pass run, the following FSP settings are used: a constant tool travelling speed of 40 mm/min, a rotation speed of 1000 rpm, and a rotation speed of 1200 rpm. The double pass run in FSP was followed at these speeds without using feedstock. In this manner, a total of 12 plates undergo to FSP passes with different feedstocks and speed. Figure 4a and b illustrates the transverse movement of tool and the rotational movement of tool respectively.

Figure 4
figure 4

Schematic diagram of FSP showing (a) top view of transverse movement of tool, (b) front view of rotational movement of tool.

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Materials characterization

Materials characterization includes the mechincal and microstructural analysis. This investigation used a Rockwell hardness machine to test the hardness of AZ31 and 12 FSPed plate samples. Each composite undergoes to 3 trails during hardness test and the mean of these values is taken for accuracy purpose. Impact strength is tested on Charpy impact testing machine. For Charpy test, the dimensions of specimen are 55 × 10 × 7 mm. The specimens are V- notched at angle 45˚ and depth of V is 2 mm. The specimen geometry for impact strength testing is shown in Fig. 5. For tensile test, the sample is fixed in the grips of universal machine in such a manner that the pull is applied axially. The cross section of specimen is rectangular shaped with dimensions of 70 × 12 × 7 mm as shown in Fig. 6.

Figure 5
figure 5

Model of specimen for impact test.

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Figure 6
figure 6

Model of specimen for tensile test.

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The microscopic results contain Scanning electron microscopy and Optical microscopy based analysis. The Scanning electron microscopic analysis for the Al2O3, and Cr reinforced composites is performed at magnification of × 100 and × 1000, for SiC and Si-powder reinforced at magnification of × 100 and × 500 fabricated at speed of 1000 rpm. Optical Microscopy is conducted at magnification of × 500 for AZ31 and SiC reinforced composite fabricated at 1000 rpm.

Results and discussion

The mechanical and microstructural properties have been investigated. Mechanical investigation includes Hardness, tensile strength and impact strength tests. Microstructural investigation includes the Scanning electron microscopy (SEM) and Optical microscopy (OM). The results from the analysis are described below in details.

Visual analysis

Sample specimens are made by single pass as well as double pass FSP. Figure 7 shows the FSP processed plates containing reinforced region of different reinforcements after single pass run. The base plate fabricated through single pass with reinforcements SiC, Al2O3, Cr and Silicon powder are shown in Fig. 7. It was perceived that the samples were embedded together and seems to be bonded well visually. Figure 8 shows the FSP processed plates without reinforcements after single and double pass run. It can be seen that the FSP plate was joined together smoothly without any imperfection here, the friction stir welding phenomenon comes into play.

Figure 7
figure 7

FSPed plates of (a) SiC (b) Al2O3 (c) Cr (d) Si-Powder; at 1000 rpm, 1200 rpm and 1400 rpm speeds after single pass run.

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Figure 8
figure 8

FSPed Mg plates of at 1000 rpm, 1200 rpm and 1400 rpm speeds after (a) single pass and (b) double pass run without using reinforcements, experiencing friction stir welding.

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Hardness

The average hardness of magnesium alloy was determined to be 32 HRB in the hardness test. Because the treated plate has a fine grain microstructure, which increases hardness compared to unprocessed base plate18, FSPed reinforced sample plates exhibit superior hardness to base metal18. Figure 9 shows that the lower value of hardness in the stirred region at lowest Speed i.e. 1400 rpm as compare to higher speed for SiC, Al2O3, Cr and Si Powder. The order of hardness was found as follows: SiC > Al2O3 > Cr > Si. The FSPed area now has a hardness of 65 HRB, which is 50% higher than the base material, and is an improvement over it. This is due to higher speed enhance the temperature of internal section54,55. In a previous study56, it was already found that the alumina is a better reinforcement for the AA6101-T6 and AA1350 alloys.

Figure 9
figure 9

Effect of N on hardness.

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A larger value of hardness is being shown in the welded area as a result of the action of double pass, as shown in Fig. 10. The hardness of AZ31 FSPed composite was found ~ 4.4 to 3.05 times in double pass as compared to the single pass for the increase in rotational speed. There is no difference in the rating of hardness between single pass and double pass FSPed plates when it comes to chromium particle FSPed plates. When compared to the single pass FSPed area, the double pass with greater grain refinement yields superior results. This is something that we can claim with certainty. On the other hand, Si powder exhibits a lower value of hardness simply due to the fact that it has a lower level of hardness in comparison to other reinforced particles. Kumar et al.57 also revealed that the FSPed properties of the AA6101-T6 and AA1350 were achieved superior during the dual pass.

Figure 10
figure 10

Effect of FSP passes on hardness (without reinforcement).

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Impact strength

Investigating the impact strength deviation of both the reinforced and base matrix FSPed specimens revealed that the material bends, deforms, and does not completely break down, indicating that it is delicate. At the stirred region zone, which is defined by the notch in the processed zone and the base plate, the impact strength of FSPed samples is measured. The impact strength varies material to material as the speed increases shown in Fig. 11. A base plate with impact strength of 2 J is inferior than the FSPed zone, which has an impact strength of 11.5 J. The impact strength of the Al2O3 FSPed area is much higher than that of silicon carbide particles. By incorporating various reinforcements into the matrix material, the composite's impact strength is enhanced. Without reinforcement, when single and double passes are made, the impact strength of the composite material is significantly affected, as shown in Fig. 12. The impact steength of AZ31 FSPed composite was increased by ~ 3.25 times in double pass as compared to the single pass for the increase in rotational speed. Because of the greater grain refinement achieved by the double passes FSPed plates in comparison to the single passes in the matrix metal, the impact strength of the double passes FSPed plates is superior to that of the single passes FSPed plates. To a considerable degree, the impact strength is not much affected by speed.

Figure 11
figure 11

Effect of N on impact strength.

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Figure 12
figure 12

Effect of FSP passes on impact strength (without reinforcement).

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Tensile strength results

Wire electrical discharge machining (EDM) is used to cut the tensile samples, which determines the section size. To ensure that the tensile section gauge segment is in the middle of the three sections, the specifications are preferable. This is a standard mechanical test that measures the practical load by loading a carefully regulated set in a controlled environment58. The primary factors influencing the composite's tensile strength are the speed and the particle of reinforcements. For tensile strengths between 148 and 205 MPa, see Fig. 13. A tensile strength of 125 MPa was recorded for the base material. The increase in number of passes resulted in an increase in the composite's tensile strength. According to the results, the tensile strength of the two passes FSPed plates is much higher than that of the single section without any reinforcing, as revealed in previous study also20. When compared to speeds of 1200 and 1400 revolutions per minute, Fig. 14 demonstrate that double pass and single pass have a higher tensile strength at a speed of 1000 revolutions per minute. In litertaure, Alumina is found as a better reinforcement in terms of improving the tensile strength of the AA6101-T6, AA1350, and AA5052 alloys59,60.

Figure 13
figure 13

Effect of N on tensile strength.

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Figure 14
figure 14

Effect of FSP passes on tensile strength (without reinforcement).

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Microstructure of FSPed samples

The microstructure of Al2O3 reinforced composite is shown in Fig. 15. The parameters are optimum as N = 1000 rpm and r = 40 mm/min. A very fine homogeneous microstructure of Al2O3 particles is observed at × 100 magnification, as shown in Fig. 15a. In Fig. 15b, as the magnification increases to × 1000 the mixing of Al2O3 particles with Mg in stirred region is observed. The white region shows the Al2O3 particles dispersed in black Mg particles. Because of higher melting point of Al2O3, Mg melts earlier in FSP process which is results in squeezed Al2O3 white particles between layers of Mg grains.

Figure 15
figure 15

SEM photograph of Al2O3 reinforced composite at speed 1000 rpm in (a) × 100 magnification, (b) × 1000 magnification.

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In Fig. 16a, a fine but layered microstructure of Cr particles is observed at a × 100 magnification. At the magnification of × 1000 of Cr particles mixed with Mg particles shows large white–gray particles and black layers of Mg, from Fig. 16b. It observed that breaking of some particles in stirred zone and this is due to the stirring action generated in pass by the rotated probe in the matrix. The microstructure after dispersion is tends to more homogeneous. The fluctuation in heat produced during the stir processing affects the proportion of growth, dissolution, and reprecipitation of the precipitate1,2,28,61,62.

Figure 16
figure 16

SEM photograph of Cr reinforced composite at speed 1000 rpm in (a) × 100 magnification and (b) × 1000 magnification.

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From Fig. 17a, a more fine and homogeneous microstructure of SiC particles is observed as compared to Cr, at a × 100 magnification. As the magnification increases to × 500, SiC particles seen as the white dots which represents the high volume fraction of SiC particles in contrast Mg particles, observed in Fig. 17b. SEM photographs are necessary to reveal the reinforcement particles that may appearance after the FSPed process. Since the reinforced particles are small and it is rigid to observe with optical microscopy. Similar was found by Li et al.63. Consequently, the FSP area has achieved finer grains with a uniform distribution, hence promoting the creation of FSPed joints without any defects64,65.

Figure 17
figure 17

SEM photograph of SiC reinforced composite at speed 1000 rpm in (a) × 100 magnification and (b) × 500 magnification.

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Silicon carbide FSPed region at optimum speed of 1000 rpm have even distribution of reinforcement and this distribution can be seen in × 500 magnified optical microscopy photographs as shown in Fig. 18. Magnesium matrices have grains with different sizes. This is due to the effect of cooling during FSP process. The regions that appear grey in color are composed of new fine grains with Nano-SiC particles. It is evidently seen that refine grains are developed in stirred zone comparing with original grains in the matrix. It is possible that this indicates that the formation of these tiny and equiaxed grains occurs during the FSP process as a result of the action of dynamic recrystallization. Because no fractures nor exfoliations were found on the interface, it may be concluded that the fine-grained layers and the coarse-grained matrix are successfully bonded to one another. The agitated zone included SiC particles that were distributed in a rather regular manner.

Figure 18
figure 18

Microstructure images at × 500 magnification of (a) base matrix (AZ31) (b) after FSPed stir region with SiC reinforced composite at speed 1000 rpm.

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In Fig. 19a, Si powder dispersion in Mg particles shows much more fine structure as compare to Al2O3, SiC and Cr. At magnification of × 500, Fig. 19b shows the highly dispersed homogeneity of Si powder in Mg particles without any imperfection.

Figure 19
figure 19

SEM photograph of Si Powder reinforced composite at speed 1000 rpm in (a) × 100 magnification and (b) × 500 magnification.

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Conclusions

FSP of Magnesium based composite is performed and effects on microstructural and mechanical properties is evaluated. The reinforcement particles SiC, Al2O3, Cr and Si powder were dispersed in AZ31 and composites were successfully produced. It is concluded that:

  • Magnesium composites are successfully fabricated at various speeds 1000, 1200, 1400 rpm. From the macroscopic approach i.e. by naked eye, it is observed that as the rotation speed of tool increases the smoothness of surface increases.

  • The peak values of mechanical strength were observed in FSPed region at tool rotation speed of 1000 rpm and constant transverse speed 40 mm/min.

  • Addition of reinforcement results in better microstructural and mechanical properties as compared to the AZ31 magnesium alloy.

  • The mean hardness of base plate of magnesium is found 32 HRB. The order of hardness was found as follows: SiC > Al2O3 > Cr > Si.

  • Mechanical testing revealed an increase in hardness, impact strength and tensile strength of AZ31 alloy with the addition of reinforcements. The hardness of FSPed region is increased up to 50% of base material.

  • The hardness of AZ31 FSPed composite was found ~ 4.4 to 3.05 times in double pass as compared to the single pass for the increase in rotational speed.

  • The impact steength of AZ31 FSPed composite was increased by ~ 3.25 times in double pass as compared to the single pass for the increase in rotational speed.

  • The microstructure shows that reinforced particles were uniform dispersed into FSPed region and agglomerated with Mg matrix.

  • Si powder produces finer microstructure as compare to SiC, Al2O3, Cr. FSP decreases the grain size of processed material.

  • The tensile strength of the two passes FSPed plates is much higher than that of the single section without any reinforcement.