Enhanced energy absorption in heat treated SS304 in FILL/AL6061 composites: novel structural design for offshore applications

ABSTRACT

In-fill aluminum bar is a lightweight product with high energy absorption, weight ratio, and mechanical, chemical, and physical qualities. Al6061 bar in-filled with Stainless Steel Structure (SSS) is mould cast to test its flexural response. The experimental investigation has two stages: the first predicts the binding ability between the Stainless Steel (rod & wire) core and aluminium, and the second estimates flexural, pull-out, and compression properties to ensure fit. Pull-out and SEM tests evaluate SS304 rod and wire reinforced with AL6061 matrix binding. Secondary evaluations of SSS-filled composites use several material characterisation methods. MIG is used to wire and rod-form SS304 in-fill structures. The mechanical properties of wired and rod-in-fill SSS with aluminum bar have been experimentally examined. After heat treatment, the rectangular bar’s energy absorption and flexural strength improved. SEM examination is used for micro and macroscopic analyses to show the in-fill structure’s role in the aluminum matrix. Higher flexural strength of about 1559.5 MPa is attained after heat treatment process for perforated small hole specimen with energy absorption rate of 988.5 J on comparing other specimens. Finally, in-fill structure with aluminum bar outperforms higher than raw Al6061 alloy.

Keywords:
SS304; Al6061; Mechanical Properties; Pull-out test; In-fill structure

1. INTRODUCTION

Due to the rise in material demand to attain better performance on the whole, it becomes huge and diverse day-by-day as no such single material could satisfy the need [¹]. This in turn leads to the resurgence of ancient thought to combine diverse materials in the integral composite material system and it results in unattainable performance by an individual constituent which offers huge benefits [²]. Composites are materials which undergo integration of two phases, typically with a strong interface between them. Aluminium alloys still exist as a serious topic in various intense researches. Also, Aluminium alloy 6061 possesses better machining characteristics, fatigue resistance, and higher strength. It is most widely utilized in aircraft structures, specifically in wing and fuselage structures under tension. At present, there exists a significant demand for the aluminium alloy which has improved mechanical properties [³]. This demand is higher specifically in the aerospace industry, where the weight plays a dominant role in the selection of materials. Moreover, automotive and other industries might too exploit enhanced aluminium alloys where the reduction of weight has more advantages: commonly, for entire moving or rotating components.

An emerging technology for the production of advanced aluminium alloys covers rapid solidification, composite materials, and mechanical alloying. With the use of material technology, about 5 to 30% of hard ceramic phase (i.e., typically either SiC or Al203) could be mixed with the ductile, lightweight aluminium alloy matrix [⁴,⁵,⁶]. The resultant composite is much stiffer and harder compared to the traditional aluminium alloys, however tougher and more ductile than the ceramic. The stiffness and the strength of the added metal matrix composites with the reinforcements reveal relatively higher while comparing the base matrix. The most reliable reinforcements for aluminium matrix might be fiber, wire mesh, continuous rods, and particles. In the event of structural applications, aluminium alloys incorporated reinforcement shows enhanced wear and mechanical properties compared to the base aluminium alloy [⁷]. Most commercially used reinforcements are continuous fiber rods or wires & particles and generally, the addition of ceramic particles as a reinforcement to the aluminium alloys leans towards improving the hardness, tensile, fracture toughness, and wear properties [⁸, ⁹]. In this study, the aluminium matrix has been mainly adopted for the structural applications to perform off-shore piers and the employability of aluminium matrix for typical extruded shape, roofing structure, tower and off-shore platforms is expressed. The aluminium alloy also exhibits maximum yield point than the mild steel in structural applications [¹⁰, ¹¹].

Steel is an alloy of iron with certain carbon percentage which ranges from 0.15 to 1.5% [¹²] and the plain carbon steels are the ones that have 0.1 to 0.25% [¹³]. Steel predominantly has an alloy of iron and carbon, at which other elements are available only in smaller proportions to affect the properties. Other elements of alloys that are allowed in plain-carbon steel are the silicone and manganese. Steel with lower carbon content has similar properties like iron and it is soft however formed easily. With the rise in carbon content, metal becomes stronger and harder but also less ductile and it is more difficult for welding purpose [¹⁴]. There are two main reasons for the popular use of ICMIEE-PI-140160-2 steel: (1) It is abundant in the earth’s crust in the form of Fe2O3 and little energy is required to convert it to Fe. (2) It can exhibit a variety of microstructures and thus a wide range of mechanical properties. Although the number of steel specifications runs into thousands, plain carbon steel accounts more than 90% of the total steel output. The reason for its importance is that it is a tough, ductile and cheap material with reasonable casting, working and machining properties and it is also amenable to simple heat treatments to produce a wide range of properties [¹⁵]. The reason for choosing heat-treated carbon steel is to alter the mechanical properties of the steel, typically hardness, ductility, tensile strength, yield strength, and impact resistance. Steel’s standard strength employed in the structural form is thus prescribed from its yield strength. Most computations of engineering for the structure depend on yield strength. The heat treatment process offers softness and hardness, thereby enhances the mechanical properties (like yield strength, tensile strength, corrosion resistance, ductility, and creep rupture). These processes too aid in enhancing the effect of machining thus making them more versatile and they are most commonly found in applications like beams for the building support structure, railroads, construction of ship [¹⁶,¹⁷,¹⁸], reinforcing rods in concrete, boiler tubes for power plant generation, pipelines of oil and gas, cutting tools, car radiators, and so on. The mild steels are the ones that have lower carbon steel as a major component which is processed by the heat treatment and they contain several characteristics. Typical range of mild steel varies from 0.05% to 0.35% and is regarded as a very versatile and useful material. This could be machined and converted to complex shapes which have lower cost & better mechanical properties. It could be converted to huge bulk of steels and utilized for typical structural fabrication, sheet metal and so on. Usually, bolts and studs are mainly supposed to be formed from mild steel (having 0.25% carbon) with ductility and toughness characteristics. Another method of attaining certain desirable conditions in metals is the heat treatment, a process, or the combination of processes which includes heating & cooling of alloy or metal in a solid state [¹⁹,²⁰,²¹]. Further research has revealed that low carbon steels like mild steel could be strengthened over the process of heat treatment, whereas quenching after the process of heat treatment enhances the mechanical properties of the steel metal [²²,²³,²⁴]. In the most metallic materials, higher-cycle resistance is dominated by its ductility and strength, correspondingly. The structural form of steel, stainless steel, and mild steel are applicable in varied forms by performing reinforcement and they are in-filled [²⁵,²⁶,²⁷] with various range of alloys or materials such as zinc, aluminium, magnesium, and cast-iron. It results in enhanced physical, corrosion, structural, and mechanical properties.

An effect of stretching during annealing and decarbonizing treatment was carried out on magnetic properties of grain-based electrical steels and is studied in the work [²⁸]. The outcome indicates the significance of controlling stretching during annealing for optimizing magnetic performance for its application transformers and generators. An enhanced mechanical and wear characteristics on composites were observed over stir casting approach and RSM modeling in the work [²⁹, ³⁰]. The model predicted the wear rate with 95% confidence level by confirming overall validity using variance analysis. The author in the work [³¹] stated that reinforcing metals such as magnesium, aluminium, and titanium with ceramic particles leads to blending of properties from ductile metal matrix and higher strength higher module reinforcement.

More and more existing concrete structures need to be reinforced to meet the durability requirements. At present, the reinforcement methods for concrete members mainly include increasing member section, replacing concrete, wrapping section steel or steel plate, pasting fiber-reinforced materials, etc. Some scholars have adhered fiber-reinforced polymer (FRP)plates to the surface of reinforced concrete beams to improve their bending performance [⁶, ⁷] More and more existing concrete structures need to be reinforced to meet the durability requirements. At present, the reinforcement methods for concrete members mainly include increasing member section, replacing concrete, wrapping section steel or steel plate, pasting fiber-reinforced materials, etc. Some scholars have adhered fiber-reinforced polymer (FRP) plates to the surface of reinforced concrete beams to improve their bending performance [⁶, ⁷] More and more existing concrete structures need to be reinforced to meet the durability requirements. At present, the reinforcement methods for concrete members mainly include increasing member section, replacing concrete, wrapping section steel or steel plate, pasting fiber-reinforced materials, etc. Some scholars have adhered fiber-reinforced polymer (FRP) plates to the surface of reinforced concrete beams to improve their bending performance [⁶, ⁷]. There is a need to reinforce more concrete structures to meet the requirement of durability. The reinforcement of material is made to enhance the bending performance, flexural [³²] and compression strength. Both the materials have huge advantages, however, there exists some drawbacks like reduced mechanical properties while it is used individually. Hence, reinforcing material will make them durable, stronger and versatile for a wide range of industrial applications [³³,³⁴,³⁵,³⁶]. On comparing steel, Al alloys have several benefits and subsequently, they are employed widely in aerospace, machineries, light industrial construction materials, electronics, and some other areas. Despite an extensive review on aluminium matrix composites and its reinforcements with metallic and ceramic elements, there exists some notable gap for understanding the synergistic effects of SS304 rod and wire in-fill structures within Al6061 matrix, specifically under the heat treatment process. Existing studies primarily focus on ceramic reinforcements or single-phase metallic additions, having limited quantitative analysis of mechanical and energy absorption improvement over complex in-fill structures and post-processing treatments. Moreover, desired impact of heat treatment protocols encompasses solutioning, quenching, and artificial aging of interfacial bonding, flexural strength, and energy absorption of SS304/Al6061 composites were not quantified systematically. This work intends at filling this knowledge gap on offering comprehensive experimental evaluation of mechanical properties and energy absorption behaviour of the heat-treated SS304-in filled Al6061 composites, thus informing the design of advanced materials for the off-shore structural applications. Various researches have been carried so far on the mechanical properties of Al alloys and thus, manual and specifications are shown on these alloys’ structural design. Since Al alloy material possesses superior performance compared to ordinary steel bars, it has huge potential to be employed as a structural reinforcement material [³⁷, ³⁸]. Also, due to the characteristics of Al6061 like higher strength, better corrosion resistance, and excellent weldability, the selection of Al6061 material is a versatile option for several applications in industries and it has been considered as a reinforcement material. For this purpose, the research work aims at reinforcing Al6061 with SS304.

Thus, the aim of this present study is to produce improved structural stability of stainless-steel rod in-filled Al6061 bar. The Al6061 material is reinforced with 3 mm stainless-steel rod & wire core in the primary stage of work. This fabricated structure is subjected to pull out, bending and compression test. This fabrication done with the use of steel wire core fails to withstand bending and compression test, whereas the bending and compression ability is better in SS rod. Due to this, fabrication of metal matrix or in-fill steel structure has been carried and reinforced with Aluminium bar at the secondary stage of research work. At which, the metal matrix formed is reinforced with Aluminium bar. Both the core reinforcement and the in-fill stainless steel structure SS304 composites are thus fabricated through the mould casting technique. The fabricated specimen consists of SS304 and Al6061 alloys. The binding ability between SS304 and Al6061 is evident through pull-out test and micro and macroscopic analyses. Also, the structural stability analysis is performed using a three-point bending test, compression, and flexural strength to employ in off-shore applications as aluminium piers.

2. MATERIALS AND METHODS

The materials chosen are aluminium Al6061 and Stainless steel SS304 both purchased from Hindalco industries, India. In this research methodology, Aluminium material is reinforced with the 3 mm stainless-steel rod and wire core in the primary stage of work. This fabricated structure is subjected to pull out, bending and compression test. This fabrication using steel wire core fails to withstand bending and compression test, whereas the bending and compression ability is better in SS rod. For this reason, fabrication of metal matrix or in-fill steel structure is made and reinforced with the Aluminium bar in the secondary stage of research work. In the secondary stage, the metal matrix formed is reinforced with Aluminium bar. At this stage, raw aluminium is heated and the molten form of this Al6061 is poured into the in-fill steel structure by means of perforated sheet so as to get fabricated specimen. Further, the behaviour of this reinforcement is subjected to various testing processes like flexural strength, bending test, and compression test analysed from Instron 4000KN made in United states. Both the core reinforcement and the in-fill stainless steel structure SSS composites are thus fabricated through the mould Squeeze casting technique with the help of local tool maker, Coimbatore. Thus, the fabricated specimen consists of SS304 and Al6061 alloys. The wrought alloy of Al6061 is chosen as a metal matrix brought in ingot form. The major intention of selecting this kind of Al 6061 material is its mechanical properties like good energy absorption, flexural strength and easy availability. A good energy absorption for the designed specimen is estimated and enhancement is carried by employing heat treatment process. In view of the application, Al6061 is thus chosen as reinforcement and its chemical composition is provided in Table 1. These chemical compositions have been attained from the optical emission spectroscopy analysis. Likewise, the density nature of AL6061 is 2649 kg/m³. A quasi-static matrix material property is represented in Table 1. Also, the reinforcing element’s quasi-static properties could be taken from Table 2. No such variation seen in the mechanical properties due to the influence of heat treatment during the extrusion of composite and additional heat treatment must be observed.

Table 1
Chemical composition of Al6061.

ELEMENTS	Si	Fe	Cu	Mn	Mg	Cr	Zn	Ti	Al
Weight (%)	0.67	0.7	0.22	0.15	1.1	0.1	0.20	0.1	Remains

Table 2
Chemical composition of SS304.

ELEMENTS	C	Mn	Cr	Ni	P	S	Si	Mb	Fe
Weight (%)	0.08	2.0	18.6	8.1	0.045	0.035	0.75	1.86	Remains

In the present investigational profiles having cross-section of size 40 × 10 mm² are produced through the composite extrusion having pressing ratio of about 42:1. The temperature of block was set as 5500 C & 4200 C made from Technotherma (India) Pvt limited, New Delhi. Since the block temperature is at the range of annealing temperature solution of the matrix material, heat treatment might be directly carried by means of fabrication. Thus, the quenching process is carried by integrated system for the quenching with moving air correspondingly by setup for direct water quenching after extrusion. This setup thus allows for quenching having distance of nearly 300 mm in front of extrusion process’s outlet port. However, the heat treatment was investigated to improve the material overall performance.

The composite profile was reinforced with 5 symmetrically arranged wires with a diameter of 1 mm resulting in a volume fraction of the profile of 0.98 vol.%. The unreinforced profiles where produced on the same run by cutting the reinforcing wires after producing the composite profiles. The cross-sections of the fabricated profiles in heat treatment state T4(F) exhibit an interface between wire and matrix that appears to be free of macroscopic defects and shows a good optical bonding. The specimen shows a fine grain formation with an even finer microstructure around the steel wire and along the longitudinal welding seam (LWS) due to the higher local degree of deformation during the composite extrusion process. These illustrated conditions could be observed for all investigated heat treatment states and no significant change in grain size distribution was observed using light microscopy.

As a part of reinforcement for the in-fill structure, the Stainless steel (SS304) in the form of rod and wire has been selected. The SS304 rods have a smoother surface while the wires have rough surface with a diameter of 3 mm and are employed as a reinforcement (in-fill) material. The evaluation is carried out for an aluminium matrix with rod and wired form in terms of binding ability. While comparing other reinforcements, this SS304 has better corrosion resistance. The chemical composition of this SS304 material is given in Table 2.

2.1. Primary processing and testing

The composite specimens are fabricated in two varied stages; in this primary stage, a 3 mm SS304 rod and wire core is reinforced with Al6061. The fabricated composites have a dimensional length of 25 mm according to the testing requirements. For the mould casting technique, two halves of 25 mm inner diameter mould are prepared. At first, aluminium alloy 6061 is being charged with an electrical resistance furnace and is then heated till 850°C. This melt is stirred manually for about 60 minutes at regular time intervals of about 10 minutes to get a better grain structure [³⁹,⁴⁰,⁴¹,⁴²,⁴³]. Likewise, the SS304 rod is preheated in a heat treatment furnace. Once the stainless-steel rod reaches 400°C, it will be placed at the centre of a circular-shaped mould having 25 mm diameter and after which, the molten Al6061 is poured into a mould cavity comprising of reinforcement rod that is then allowed to solidify for attaining final composites as depicted in Figure 1. This process is also followed for 3 mm SS304 wire to get composite fabrication.

Figure 1
(a) Raw aluminium (b) fabrication of aluminium rod with steel core (pull out test).

In this primary stage, the suitability of SS304 rod and wire core with that of AL6061 is analysed by means of different experimental testing procedures. The fabricated core-reinforced two composite specimens are subjected to pull-out, flexural strength, compression test, Energy absorption (EA) and SEM analysis. The mechanical properties were identified by 50 KN UTM machine made from Fine Testing instruments company, Indian made. The energy absorption of the specimen is carried and the outcomes are compared before and after applying heat treatment process. As a part of the pull-out test, binding ability is measured by means of a Universal Testing Machine (UTM). This pull-out test is carried out at which the free-end stainless steel rod gets clamped with the upper gripper and the composite end is thus clamped to the lower end. The load will be applied at room temperature thus maintaining a constant displacement of about 0.02 mm/min and the testing process is continued till the entire part of the rod is pulled out. This pull-out displacement is carried from Linear Variable Differential Transformers (LVDT) that are built into the system. Also, the Scanning Electron Microscopy (SEM) analysis procedure is carried out to ensure the binding ability of SS304 and Al6061 graphically. Likewise, an axial compression test is adopted for measuring the effect of stainless steel with aluminium reinforcement on the compression strength. This compression test is carried by means of hydraulic-operated compression testing machine possessing 50 kN capacity. The formed phases of SS 304 and Al6061 are determined by Quanta FEG 250 SEM analyser. For the SEM analysis we used the cross-section of 10-min preheated samples. The tested specimen’s EA ability is also computed by incorporating the area under the experimental load-deflection curves. The test specimen is prepared with a diameter of 25 mm having 50 mm height. For flexural testing, it is prepared with 25 mm diameter and 350 mm length. The test specimen is loaded under a three-point bending condition with a gauge length of about 250 mm and the load will be given at 0.05 mm/min displacement. The testing process carried out on the fabricated specimen is shown in Figure 1.

2.2. Secondary processing and testing

In the secondary process, the SS304 rod is considered for reinforcement with Al6061 metal. This stage involves making an in-fill structure by SS304 rod and the reinforcement of this is made to Al6061 matrix by mould casting technique. At first, the SS304 rod is sized and cut using the steel bar cutting electric machine after which the dimensions are ensured once again for enabling better joining of elements with the use of a surface grinding machine thereby removing unwanted burrs on the cutting edges. The fabrication of the in-fill structure is made with the use of Metal Inert Gas (MIG) Welding process which manufactured by ESAB India Pvt Limited. This in-fill structure’s length, breadth, and height are 300 mm*40 mm and *50 mm correspondingly. This defect-free welding is ensured in the entire rod and in-fill structure by the MIG welding process. The mould is prepared to have dimensions of length, breadth, and height as 320 mm *50 mm and *60 mm correspondingly. As a part of the casting process, two specimens are fabricated and termed as raw aluminium bar, and SS304 rod in-fill aluminium bar. A required quantity of raw Al6061 is kept in an electric furnace and is heated till 850° C after which the molten Al6061 is poured into the mould cavity to get the fabricated structure. Likewise, the SS304 rod in-fill aluminium bar is cast before which the in-fills are heated till 400°C to get better wet ability. At this stage, a molten form of Al6061 is poured into the in-fill steel structure with a perforated hole so as to get a fabricated specimen. The in-fill SS304 structure with the fabricated specimen is shown in Figure 2. In Figure 2, the images of raw aluminium, steel rod, steel structure fabrication process, and the Aluminium bar in-filled with SS304 structure are shown.

Figure 2
(a) Raw aluminium (b) steel rod (c-d) fabrication of steel structures (e-f) aluminium bar with in-fill SS304 structure fabrication.

The fabricated test specimen has been prepared and it is SS304 in-fill aluminium bar structure which is then subjected to different experimental testing like compression test, Energy absorption (EA), SEM, EDX analysis and three-point bending test. The structural ability of the structure is measured by means of UTM, at which the test is carried by Instron using 4000 kN, capacity machine made from United States. Compression ability is observed intensely from major affecting parameters like load and its displacement. The FEG-SEM and EDS images acquired from Quanta FEG 250 manufactured by Thermo Fisher Scientifc, American company. The fractography analysis is carried out for primary processed specimens of size 20*20 mm in terms of microstructure analysis at which the role of SS304 within testing specimens is evidenced graphically. The diagram representing this fabricated specimen and testing process is shown in Figure 2.

3. RESULTS AND DISCUSSION

An Experimental study has been carried for this research design and the outcomes attained are projected in this section.

3.1. Initial pull-out test

Initially, the 3 mm diameter of SS304 rod and wire reinforced Al6061 composite specimen is subjected to pull-out testing by UTM. The fabricated specimen is shown in Figure 3 which represents the pull-out test done. The bottom end is fixed at lower jaw and the top end is fixed with upper jaw of UTM gripper. The tension load is actuated by upper jaw to perform pull out test. The maximum tension load about 207.999 N/mm² is observed for Aluminum rectangular Bar Reinforced with SS304 (Perforated big hole) and 110 kN for a circular rod with wire reinforcement. Consequently, it is concluded, that the maximum pull-out load has been observed for SS304 wire. The difference in pull-out test is only due to the surface roughness of rod and wire. Moreover, the wire rope comprises number of twisted wires and there by the surface has become rough in nature. In part of SS304 rod, the fine surface is due to extrusion of its process.

Figure 3
Pull-out test of fabricated specimen.

3.2. Optical microscopic imaging analysis of pull out

An optical microscopic imaging analysis of pull out is detailed here. The lower and higher magnification Secondary Electron Images (SEI) using FEG-SEM are analysed through 30 min pre-heated samples and are shown in Figure 4. Figure 4(a) is the representation of FEG-SEM of fabricated Aluminium rod with steel core and Figure 4(b) denotes the FEG-SEM images of fabricated structure with the in-fill composite specimen. The higher magnification includes 3 regions at which EDS analysis has been considered. The entire morphology of the manufactured composites is presented in Figure 4. Though some casting defects (river like pattern) have occurred at some interfaces, it is visible clearly. While considering the entire composite structure, it is obvious that better metallurgical and mechanical bonding occurs. The darker region indicates Al6061 alloy, whereas the lighter region indicates SS304 reinforcement. The light tone on Al region indicates the chemical composition variations. Since the heavier elements appear in lighter part in SEI, regions which contain Al seem to be lighter in colour. The Al6061 alloy microstructure involves intrinsically α-Al and eutectic (α-Al + Si) stages. This could be concluded that the light tones on Al region resemble eutectic structure. When contrasting with FEG-SEM images, light regions indicate Al zone whereas darker regions indicate SS area.

Figure 4
FEG-SEM images of fabricated (a) aluminium rod with steel core and (b) fabricated structure with in-fill composite specimen.

3.3. Mechanical properties of infill structure aluminium composite

The flexural properties of three-point bending test are carried out with 3 mm diameter and 200 mm length of specimens. Three specimens have been prepared for flexural testing, the specimen 1 as cast Al6061 rod of diameter 25 mm, specimen 2 is 25 mm diameter Al6061reinforced with 3 mm diameter SS rod along the centre axis and specimen 3 is 25 mm diameter Al6061 rod reinforced with twisted SS wire of diameter 3 mm alone the centre axis. All the specimens are prepared for the length of 200 mm for three-point bending analysis. Table 3 shows the performance estimation made on flexural three-point bending test. From the results attained, it is clear that the specimen Infill structure with perforated big hole (SS304) possesses higher strength values on comparing other specimens. This is due to the fact, that the material used for infill structure has better mechanical properties.

Table 3
Performance evaluation on flexural three-point bending test.

S.No	SPECIMEN	DIMENSION (mm)	MAXIMUM FORCE (kN)	DISPLACEMENT @ MAX FORCE (mm)	FLEXURAL STRENGTH (N/mm2)	ENERGY ABSORPTION VALUES (J)
1.	Pure Aluminium circular rod	L = 500 Dia = 25	35.04	28.68	34.3	502.4736
2.	Pure Aluminium circular rod with core reinforced	L = 500 Dia = 25	38.01	2268	37.2	43103.34
3.	Pure Aluminium rect. Bar	L = 500 H = 50 B = 40	159.09	9.17	11.28	729.42
4.	Infill structure (Weld mesh big)	L = 500 H = 50 B = 40	1.67	3.6	30.8	3.006
5.	Infill structure (Weld mesh small)	L = 500 H = 50 B = 40	0.98	2.7	30.8	1.323
6.	Infill structure (Perforated small hole)	L = 500 H = 50 B = 40	1.86	9.87	34.3	9.1791
7.	Infill structure (Perforated big hole)	L = 500 H = 50 B = 40	2.181	12.75	37.6	13.90
8.	Aluminium rect. Bar Reinforced with SS304 (Perforated big hole)	L = 500 H = 50 B = 40	207.977	8.93	211.03	928.617
9.	Aluminium rect. Bar Reinforced with SS304 (Perforated small hole)	L = 500 H = 50 B = 40	191.32	7.32	199.88	700.231
10.	Aluminium rect. Bar Reinforced with SS304 (Weld mesh big)	L = 500 H = 50 B = 40	179.57	5.6	237.32	502.796
11.	Aluminium rect. Bar Reinforced with SS304 (Weld mesh small)	L = 500 H = 50 B = 40	165.63	5.9	178.34	488.608

Figure 5 shows the schematic of cross section of specimen three-point loading condition used to ­measure the flexural strength of the fabricated specimen. Initially, the specimen 1 is placed on the bearing block with a span length of 200 mm as shown in Figure 5. The fabricated rod with steel core bending test is shown in Figure 5(a), and the fabricated in-fill Aluminium bar with steel structure vending test is represented in Figure 5(b).

Figure 5
(a) Fabricated aluminium rod with steel core bending test (b) fabricated in-fill aluminium bar with steel structure bending test.

Then, the load applying block is moved down to make a third point contact at the centre of the specimen for maintaining firm finger pressure. Once the third contact is made, the load is applied continuously at a rate of .3 mm/s until rupture occurs. The point where rupture occurs, the flexural strength of the specimen is calculated. Similar procedure is followed for testing of specimens 2 and 3 and Table 3 shows the flexural responses of all three specimens. From the result, it is found that the flexural strength of specimen 3 i.e. A6061 reinforced with 3 mm SS rod has higher flexural strength than A6061 alloy and happens due to the usage of high strength material as reinforcement. The flexural strength of Al 6061/SS rod is higher than A6061/SS wire. It has happened because of high stiffness offered by SS rod whereas high flexibility is observed in SS wire and it leads to lesser rigidity to resist bending while comparing with SS rod. Compression test has been carried out using hydraulic compression machine tool. Three cylindrical specimens are prepared and each specimen is fabricated with a height of 40 mm and 20 mm dia. The top and bottom surfaces are grinded to get flatness. Compressive test is carried out by placing the specimen at the centre of the loading area. Then, the middle jaw is activated to apply load at the rate of 3 mm/s. The compression test results are listed in Table 4. From the result, it is found that A6061 reinforced with 3 mm SS rod has higher strength. The compression test images are given in Figure 6 at which fabricated aluminium rod with steel core compression is given as (a), and fabricated in-fill structure compression is given as (b).

Table 4
Performance evaluation on compression test.

S.NO	SPECIMEN	FORCE (kN)	DISPLACEMENT (mm)	COMPRESSION STRENGTH (N/mm2)	ENERGY ABSORPTION (J)
1.	Pure Aluminium circular rod	110.0	1.8	223.9	99.0
2.	Pure Aluminium circular rod with core reinforced	110.0	1.343	235.0	73.865
3.	Pure Aluminium rectangular bar	110.0	1.23	55.5	67.65
4.	Infill structure (Weld mesh big)	3.3	4.33	1.65	7.144
5.	Infill structure (Weld mesh small)	4.96	2.45	2.48	6.076
6.	Infill structure (Perforated small hole)	5.94	3.34	2.97	9.919
7.	Infill structure (Perforated big hole)	7.43	4.21	3.715	15.640
8.	Aluminium rectangular bar Reinforced with SS304 (Perforated big hole)	110.0	0.324	224.0	17.82
9.	Aluminium rectangular bar Reinforced with SS304 (Perforated small hole)	110.0	0.49	229.0	26.95
10.	Aluminium rectangular bar Reinforced with SS304 (Weld mesh big)	110.0	0.732	226.0	40.26
11.	Aluminium rectangular bar Reinforced with SS304 (Weld mesh small)	110.0	0.98	251.0	53.9

Figure 6
(a) Fabricated aluminium rod with steel core compression (b) fabricated in-fill structure compression.

During the compression test, the specimen 1 exhibits barrelling effect and gets continuously deformed without buckling because of high ductile property. Whereas in SS wire reinforced A6061, limited barrelling effect takes place followed by buckling due to the Stainless-Steel wire but in SS rod reinforced A6061, the buckling takes place with barrelling effect. This buckling is characterised by the separation of SS rod from the A6061 matrix.

3.4. Analysis of load and deflection

A performance assessed on material combinations like Al6061 reinforced with SS304 core and rod in terms of load and deflection depends on several factors like method used for reinforcement, volume fraction, environmental condition, and orientation. Usually, if one material is reinforced with other one, their mechanical properties like strength, stiffness, and toughness are enhanced. Al6061 material is known for its good mechanical properties and weldability. Likewise, SS304 is known for its strength and corrosion resistance. While reinforcing these two materials in the composite form, there exists an improvement in performance. In case of load bearing capacity application, the addition of steel structure might enhance the Aluminium matrix’s strength, thus enhancing the load bearing ability potentially before failure. Also in case of deflection, addition of SS304 core wire or rod might enhance the stiffness of Al6061 matrix, thus reducing the deflection under load. Thus, the reinforcement made provides better load bearing ability and deflection reduction.

3.5. Heat treatment analysis

Once the energy absorption is estimated for designed four types of aluminium rectangular bar (perforated small hole, weld mesh big, weld mesh small, perforated big hole). The energy absorption improvement is carried further by means of performing heat treatment analysis process. In the process of analysing heat treatment, three varieties (Solutioning, Quenching, and ageing time) are used which in turn enhances the energy absorption of 4 types of aluminium rectangular bar after the heat treatment process. From the analysis of heat treatment process shown in table given below, it is evident that the energy absorption of all kind of rectangular bars are improved highly on comparing the outcome estimated before heat treatment. Though, the deflection is high, energy absorption shows much more improvement. The reason for increased displacement is due to following reasons:

• Longer ageing times which could increase the displacement is the material becomes over-aged thus leading to reduced strength.

• The reinforcement kind and size may affect load-bearing capacity, and stiffness probably allowing more displacement in the mesh-reinforced specimens.

• High loads applied to the specimen may enhance the displacement.

• The ductility of material which is influenced by heat treatment process might cause the material to deform highly under stress. The Table 5 represented below shows the outcome attained after the process of heat treatment.

Table 5
Outcomes after heat treatment process.

HEAT TREATMENT STATE	SOLUTIONING	QUENCHING	AGEING- TIME	SIZE OF THE COMPOSITE	LOAD	DISPLACEMENT	FLEXURAL STRENGTH	ENERGY ABSORPTION (J)
T4(F) Perforated small hole	Solution Annealing before composite forming	Quenching with air directly after the composite forming process.	Artificial Ageing for 10 hours at 120° Celsius, 45 min after the forming process.	L = 500 H = 50 B = 40	207.9	9.51	1559.5 MPa	988.5
T4(F) Weld mesh big	Solution Annealing before composite forming	Quenching with air directly after the composite forming process.	Artificial ageing for more than 15 hours at 120 º Celsius, 45 min after the forming Process.	L = 500 H = 50 B = 40	191.3	8.19	1434.5MPa	783.5
T4(F) Weld mesh small	Solution Annealing before composite forming	Quenching with air directly after the composite forming process.	Artificial ageing for more than 20 hours at 120 º Celsius, 45 min after the forming Process.	L = 500 H = 50 B = 40	179.5	6.12	1346.8MPa	549.27
T4(F) Perforated big hole	Solution Annealing before composite forming	Quenching with air directly after the composite forming process.	Artificial Ageing for 25 hours at 120° Celsius, 45 min after the forming process.	L = 500 H = 50 B = 40	165.6	5.81	1242.5MPa	481.06

From Table 5, it is observed that Heat treatment state is carried for specimens like T4(F) a, aluminium rectangular bar reinforced with SS304 (perforated small hole); T4(F) a, aluminium rectangular bar reinforced with SS304 (weld mesh big); T4(F) a, aluminium rectangular bar reinforced with SS304 (weld mesh small); T4(F) a, aluminium rectangular bar reinforced with SS304 (perforated big hole). The solutioning, quenching, ageing time varieties are analyzed by evaluating the flexural strength and energy absorption rate for all three varieties. Higher flexural strength of about 1559.5 MPa is attained for perforated small hole specimen with energy absorption rate of 988.5 J on comparing other specimens.

3.5.1. Comparative analysis of energy absorption

An evaluation is carried to analyse the behaviour of energy absorption of the fabricated design. This includes subjecting the specimens to the controlled loading conditions thus measuring the parameters like force, strain, and displacement. Energy absorption denotes the material or structure’s ability for dissipating the kinetic energy at the time of deformation or impact occurrence, thereby reducing impact severity or preventing catastrophic faults. This is a crucial consideration to design the components and systems that are subjected to dynamic loading conditions. The mechanism of energy absorption could vary based on the material and the desired loading conditions but most often, it includes processes like deformation, fracture and material damping. The intention of energy absorption is to mitigate sudden loading effects caused by the conversion of kinetic energy to other forms like heat or work, thus preventing excessive injury or damage. The energy absorption of this fabricated specimen is computed by using Trapezoidal Rule formula as shown below.

The area under curve between two points x_i and x_i+1 by using trapezoidal rule is computed by:

$${\text{Area}}_{\text{1}}={\text{F(X}}_{\text{i}}\text{)}\text{+}{\text{F(X}}_{\text{i}}\text{+1)/2}\text{*}{\text{(X}}_{\text{1+1}}-{\text{X}}_{\text{i}}\text{)}$$

It is applied to each interval in a step wise manner, the energy absorption values are computed for both flexural and compression strengths. By means of this estimation, energy absorption is computed for flexural strength and compression strength. The energy absorption attained for flexural strength before heat treatment is shown in Table 3. And their values after heat treatment process is shown in Table 5. Before applying heat treatment process, a highest energy absorption value of 43103.34 J is attained for Pure Aluminium circular rod with core reinforced structure estimated in terms of flexural strength. Likewise, the energy absorption for compression test is mentioned in Table 4. It is obvious from the table that the highest value of 99 J is attained for pure aluminium circular rod specimen estimated for compression test values. Also, the remaining specimens have better values of energy absorption in terms of both flexural strength & compression tests and it denotes that the fabricated material is a better absorbent thus offering good flexural & compression ability. After the process of heat treatment, a highest energy absorption value of 988.5 J with flexural strength 1559.5 MPa is attained for Aluminium rect. Bar Reinforced with SS304 (Perforated small hole) specimen which seems to be higher than other specimens. Also, entire specimen shows improvement in flexural strength and energy absorption rate after applying heat treatment process which denotes that after applying heat treatment, fabricated material offers good energy absorption & flexural strength ability. The comparative analysis showing this is given in Table 6 and Table 7. Provide the current finding with other existing studies. Figure 7 comparison graph of before and after heat treatment process in terms of flexural strength which showed that heat treated samples makes the flexural strength much better. Similarly, Figure 8 shows the difference in energy absorption before and after the heat treatment method. It shows that the performance is better after treatment.

Table 6
Comparison of energy absorption and flexural strength before and after heat treatment process.

SPECIMENS	BEFORE HEAT TREATMENT PROCESS		AFTER HEAT TREATMENT PROCESS
	FLEXURAL STRENGTH (MPa)	ENERGY ABSORPTION (J)	FLEXURAL STRENGTH (MPa)	ENERGY ABSORPTION (J)
T4(F) Perforated small hole	199.88	700.231	1559.5	988.5
T4(F) Weld mesh big	237.32	502.796	1434.5	783.5
T4(F) Weld mesh small	178.34	488.608	1346.8	549.27
T4(F) Perforated big hole	211.03	928.617	1242.5	481.06

Table 7
Comparison the finding with other existing studies.

STUDY/MATERIAL	REINFORCEMENT TYPE	KEY FINDINGS/PROPERTIES	REFERENCE
Al6061 + SiC (5–15 wt%)	Ceramic	Hardness upto 135 HB, improved wear resistance, but decreased ductility at high wt %	[44]
Al6061 + BN	Ceramic	37% increased hardness, 32% increased, tensile strength, 40% decreased ductility	[45]
Al-matrix + SS304 wire mesh	Metallic	Upto 28% increased tensile strength, strong interface, preserved ductility	[46]
SS clad rebar (hot rolled)	Metallic	423 MPa yield, 602 Mpa, UTS, no cracks after bending	[47]
Al6061 + graphite/SiC (aged, furnace cooled)	Hybrid	28% increased impact energy absorption over base alloy	[48]
Proposed (Al6061 + SS304) heat treated	Metallic	1559.5 MPa flexural strength, 988.5 J energy absorption and strong interface, preserved ductility	–

Figure 7
Comparison graph of before and after heat treatment process in terms of flexural strength.

Figure 8
Comparison graph of before and after heat treatment process in terms of energy absorption.

3.6. EDX analysis

Figure 9 is the SEM image of Al6064 and SS304 composite interface. It can be seen from Figure 9(a–f ), that there is no such major gap between the steel and the aluminium matrix and a diffusion layer is formed. It indicates that the reinforcement of composite material is well combined with that of the matrix and better metallurgical bonding is created in interface. It is a clear boundary between two metals. The interface bonding study is needed in bimetallic composites, and the interface layer thickness study will be significant. The interface bonding layer thickness of dissimilar metal composite could be seen directly from the SEM image and it is primarily due to varied major components at both the sides of interface layer in unrelated metal material. By means of mapping analysis, EDS line scanning of Al/SS composite interface, it could be seen that after the process of extrusion of Al/SS composites billet, at bonding interface, the composite content shows gradual increase and decreases after some point gradually. The transition layer of intermetallic compound formed between the matrix is about 10 μm thick. The EDS helps to identify the Al and SS element which diffused out and form intrametric region. Figure 9(g) showed the presence of Fe, Cr, Si, Ni and Mn. The point 1 showed the presence of Fe, Al, Cr, Ni, Si, Mn element, The point 2 Showed the Al, Cr, Ni, Si element present and the Point 3 showed the Fe, Al, Cr, Ni, Si, Mn. The EDS point 1,2,3 showed solid solution of elements and their Interface region.

Figure 9
(a–f) SS304/Al6061 composites interface microstructure (g) results of point analysis by EDX.

4. CONCLUSION

The present research methodology focuses mainly on improving the structural stability of the stainless-steel rod in-filled Al6061 bar.

• The material Al6061 is reinforced with 3 mm SS rod and wire core at initial phase of work. The fabricated structure has been analysed by performing pull-out, bending, and compression test. From the analysis, it is obvious that the use of steel wire core fails to withstand compression and bending tests, whereas, the bending and compression abilities of SS rod seem to be better.

• Subsequently, the fabrication of metal matrix or in-fill SS is carried and is thus reinforced with Al6061 bar in the secondary stage of work. These core reinforcement and in-fill SS304 composites are fabricated by means of mould casting technique. Hence, the specimen fabricated comprises SS304 and Al6061 alloys.

• The binding ability of these two materials has been analysed over pull-out test, and micro-and macroscopic analyses. Similarly, the examination of structural stability has been carried using three-point bending test, pull-out, compression, and flexural strength.

• The experimental evaluation shows that A6061 reinforced with 3 mm SS rod has higher flexural strength than the A6061 alloy and this happens due to the usage of high strength material as reinforcement.

• As a result, the SS rod shows high stiffness on comparing wire which is more flexible thereby reduces rigidity by resisting bending than SS rod. Also, the outcomes attained have revealed that the fabricated specimens possess better values of energy absorption in terms of both flexural strength and compression tests which denote that the material fabricated is a better absorbent thus offering good flexural and compression ability. Among the fabricated specimens, the Infill structure with perforated big hole (SS304) specimen shows higher strength values.

• On employing heat treatment process for specimens like perforated small hole; weld mesh big; weld mesh small; and perforated big hole. The solutioning, quenching, and ageing time is analyzed to evaluate the flexural strength and energy absorption.

• Higher flexural strength of about 1559.5 MPa is attained after heat treatment process for perforated small hole specimen with energy absorption rate of 988.5 J on comparing other specimens. This is due to the fact, that the material used for infill structure has better mechanical properties. Henceforth, it is concluded that the reinforcement made provides better load bearing ability and deflection reduction thus making it viable option for employing them in off-shore applications as aluminium piers.

5. FUTURE RESEARCH DIRECTION

• In future, this work can be extended by employing interface optimization which focus on optimizing thickness and compositions of diffusion layer at SS304/Al6061 interface for maximizing mechanical performance and corrosion resistance.

• Also, it can be employed with long-term durability studies, alternatives reinforcements, advanced characterization, and process scale up. By addressing these areas, future work could be built on present study’s foundation, leading to more robut, effective, and application-ready composites for offshore and other critical engineering sectors.

DATA AVAILABILITY

All data during this study are included in this published article.

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Publication Dates

• Publication in this collection
  18 July 2025
• Date of issue
  2025

History

• Received
  05 Feb 2025
• Accepted
  09 June 2025