Effect of Friction Stir Processing on Microstructure and Mechanical Properties of In-Situ Composite A356/Al3Ni Fabricated by Stir Casting
Noor Alhuda Baheer*
| Muna Khethier Abbass | Israa Abd Al-Kadir Aziz
© 2024 The authors. This article is published by IIETA and is licensed under the CC BY 4.0 license (http://creativecommons.org/licenses/by/4.0/).
OPEN ACCESS
A new technique for refining grains and creating surface composite materials was created using friction stir processing (FSP), which shows promise for improving the properties of metals. This technology can effectively overcome the limitations of methods that depend on melting. The work employed stir casting to produce in-situ composites, including an Al3Ni reinforcing phase, Al alloy A356, and 15% weight percent pure Ni powder. The development of several phases of Al3Ni, AlNi, and Al3Ni2, which are dispersed throughout the matrix of A356 alloy, was verified by XRD analytical examination. The objective was to determine how FSP impacted the stir-cast A356/Al3Ni in-situ composite's mechanical properties and microstructure. Porosity was successfully reduced, the α-Al dendrites were refined, the main Si phase and Al3Ni were broken and fragmented, and the grain structure was improved by the FSP method. There was a consistent and equal distribution of in-situ Al3Ni in the stir zone (SZ). There were no hazardous phases present when the particle and matrix came into contact. The application of FSP improved the tensile strength of the A356 alloy by 38.35% and the in-situ composite A356/Al3Ni by 69.17%. It was found that the improvement in hardness was 14.47% for A356 alloy after FSP and 13.81% for in-situ composite A356/Al3Ni compared to in-situ composite without FSP.
Al-alloy A356, in-situ composite, Al3Ni; fiction stir processing, stir casting, microstructure, tensile strength
High-strength aluminum alloys are the primary need for the automotive and aerospace sectors. Of the aluminum alloys' documented uses, the most promising ones are found in electronic packaging, aerospace constructions, parts for internal combustion engines and airplanes, power transmission towers, and a range of recreational goods. A356 is a cast alloy made of silicon, magnesium, and aluminum. Along with outstanding casting qualities, high corrosion resistance, and superb fluidity, this alloy is strong and ductile. In order to replace steel components, this alloy is widely used in the automotive, aerospace, and military industries. One of its main disadvantages in use is resistance [1].
In order to improve its quality, aluminum matrix composites, or AMCs, are designed to combine ceramic or hard particle reinforcement with aluminum (the matrix). The matrix has embedded reinforcing material. To enhance the mechanical and microstructure properties of AMCs, a wider range of production processes has been used throughout time. Since the fabrication method will define the final product's qualities, it is one of the most important variables in the production of AMCs. AMC manufacturing approaches may be divided into three main categories: in situ state fabrication, solid state fabrication, and ex-situ (liquid state fabrication) [2].
There are several methods for producing MMCs. For these, the dispersed phase can be introduced as an ex-situ phase or produced in situ. In-situ synthesis is the process of creating the dispersed phase inside the matrix, usually by means of an exothermic chemical reaction. Ex-situ, on the other hand, involves synthesizing the dispersed phase independently prior to its incorporation into the matrix [3].
Improved compatibility, stronger bonding between the reinforcements and the matrix, more thermodynamically stable reinforcements, and a defect-free reinforcement-matrix interface are only a few benefits of in-situ composites [4].
The primary problem in creating in-situ composites with conventional methods is the segregation of the reinforced particles produced in situ [5]. FSP combines the advantages of high temperature to aid the in-situ reaction, intense plastic deformation to promote mixing, and hot consolidation to produce a totally thick solid, making it an effective technique for producing in-situ composites [6, 7]. Because in situ-formed reinforced particles are nanoscale and significantly increase strength, the FSP approach is also appealing [8].
Furthermore, the in-situ produced particles at the interface can be successfully removed by the enormous plastic strain exerted in FSP. Due to their quick removal from the surface, the particles' ability to develop is constrained, resulting in nanoparticles [9]. Superplasticity, homogeneity of aluminum alloys and AMMCs, ex-situ and in-situ composites, metal foam production, and microstructural refinement of cast aluminum alloys are the main applications of FSP for microstructural modification in metallic materials [10, 11]. In the stir-casting process, reinforcement is combined using a mechanical stirrer that creates a vortex in the matrix material. Stir casting's main advantages are its versatility, ease of usage, and capacity for large-scale manufacturing [12].
Balakrishnan et al. [13] investigated the characteristics and microstructure of AMCs in 2019. Al/(0-15 weight percent) Stir casting of pure metal powder into molten aluminum and FSP casting were used to create Al3Fe AMCs. Particle reorganization into a homogeneous distribution following FSP. Defects like porosity in the casting were removed. Al3Fe particles had their sharp edges cut away and were softened into a nearly spherical form. The reinforced particles' severe plastic deformation and pinning action caused the grain size to decrease drastically. Following FSP, the dislocation density dramatically increased. Tensile strength and ductility increased as a result of the microstructural modifications.
Dinaharan et al. [14] examined the microstructure and characteristics of AMCs. Al/(0-15 weight percent) Al2Cu AMCs were created by stir-casting molten aluminum with pure copper powder, then undergoing FSP. Microstructural alterations before and following FSP. Following FSP, the particle dispersion was reorganized into a homogenous distribution. Pores and other casting flaws were removed. The big particles of Al2Cu were broken up into smaller ones. The grain size significantly decreased. Following FSP, the dislocation density significantly increased. Tensile strength and ductility improved as a result of the microstructural modifications. Fotoohi et al. [15] mechanically alloyed Ni, Ti, and C particles to create the reactive powder. The AA1050 alloy's channels were filled with reactive powders. The in-situ composite that was produced using six FSP runs demonstrated phase stability when exposed to high temperatures. Friction stir processed AA1050 without powder addition (30Hv, 90MPa) and Al-Al3Ni/TiC hybrid composites outperformed AA1050 (25Hv, 84MPa) in terms of ultimate tensile strength of 179MPa, and hardness of 70Hv.
Gobalakrishnan et al. [16] used the stir-casting process. The ex-situ composite Al/SiC and in-situ composite Al/TiB2 were produced. There were three different weight percentages of ceramic particles used: 4, 6, and 8%. The mechanical properties significantly improved when the concentration of reinforcing particles was raised from 4 to 8 weight percent. The in-situ composite TiB2 MMC has superior mechanical characteristics over the ex-situ composite SiC and the base metal, including tensile strength, hardness, and 0.2% proof strength. Kumar et al. [17] determined the impact of FSP on the kind of industrial waste particles in cast A356 alloy. Analysis showed that FSP decreased porosity, broke up α-Al dendrites, collapsed and re-distributed Si particles, and remodeled grains. In the SZ, the reinforced particles were uniformly dispersed and had little particle interaction. No destructive phase came into touch with the matrix or the particles. FSP produced microstructural alterations that enhanced the alloy's strength, ductility, and resistance to wear in comparison to the as-cast A356.
Daneshifar et al. [18] created in-situ composites of Al/Mg2Si using FSP. In order to do this, a sample of pure magnesium was added to an Al-Si cast eutectic alloy via Friction Stir Processing (FSP). The Mg2Si production process was investigated using a range of analytical methods. According to the findings, FSP is an appropriate method for creating Al/Mg2Si composites. The resulting composite has a little increase in hardness, around 15% more than the original alloy that was subjected to FSP.
Using ultrasonic therapy with stir assistance, Sujith et al. [19] examined the Al3Ti reinforcing phase's in-situ synthesis. In an Al-7075 alloy matrix, this technique improved the reinforcing particles' thermodynamic stability, uniformity, and wettability. Unlike the dendritic cells seen in the base alloy, the Al3Ti particles stayed in their original location and acted as sites for the creation of more grains, resulting in the development of a non-dendritic globular form. Upon adding 2, 5, and 7 weight percent Al3Ti, the α-Al dendrites' grain size decreased from 160µm to 65, 50, and 40µm, respectively.
Using the technique of stir casting, Chandrashekar et al. [20] created a composite material by combining B4C powders with aluminum alloy A 356. A356 alloy is combined with 2% B4C particles sized 44μm and 105μm in a 1:1 ratio, 4% B4C particles sized in a 3:1 ratio, and 6% B4C particles sized in a 1:3 ratio in three different composite compositions. There was a favorable link between the concentration of B4C particles and the hardness and tensile strength.
Kumar et al. [21] created surface composites using in situ AA5083-H111/Al-Fe. A two-pass friction stir process was applied to the AA5083 substrate, with the tool's movement direction being changed throughout the manufacturing process. After the second cycle, the tensile strength rose from 225.8MPa to 253.6MPa. Furthermore, during the first and second passes, the microhardness measurements revealed values of 123.3 and 128.3Hv, respectively.
Padmanaban et al. [22] investigated the friction stir welded process parameters on the tensile strength of dissimilar aluminum joints. They concluded that the tensile strength of the joint increases with the increase in tool rotational speed up to a limit of 1050rpm and decreases after that. The tensile strength of the joints is also increasing with the welding speed up to 15 mm/min; while further increase in speed caused a reduction of the tensile strength of the joints.
The goal of this research is to study the effect of friction stir processing on microstructure and mechanical properties of in-situ composite A356/Al3Ni fabricated by stir casting. For that, an in-situ composite of A356/Al3Ni has been created using stir casting to accomplish a reaction between molten aluminum and nickel powder. Furthermore, using FSP technology, the research seeks to improve the tensile properties, microhardness, and microstructure of the composite material.
2.1 Materials used and specimen preparation
Aluminum alloy A356, which was made from commercially pure aluminum and an Al-11% Si master alloy, was the alloy utilized in the presented work. An electric resistance furnace was utilized to melt every material that was utilized. The furnace's temperature was predetermined at 750°C. After adjusting its chemical composition, the alloy was heated and then poured into the metallic mold. Utilizing a Thermo ARL 3460-type optical emission spectrometer, the chemical makeup of the generated A356 aluminum alloy was investigated. The Al alloy A356's chemical composition is listed in Table 1. Using aluminum alloy A356 as the matrix and 1.0175-µm-sized micro-Ni particles as reinforcement, Al3Ni in-situ composites with Ni 15wt% have been created.
Al alloy A356 was placed in an alumina crucible in an electric furnace at 750°C, which is higher than the liquidus temperature. After holding the melt at this temperature for around 15 minutes to homogenize, a flux of 1% CaF2 was added to the melt to remove the impurities (as slag) from the melt. Ni particulates have been added, warped in an Al foil, and pre-heated to 350°C for 1hr for the purpose of removing moisture and added gradually to the molten alloy along with an electric stirrer at 500rpm to achieve good dispersion of reinforcing within melt and degassing using an inert gas (argon), as illustrated in Figure 1, After that the melt is poured into the cavity of the pre-heated metallic rectangular mold at 250℃, which has dimensions of 10×100×150mm.
Table 1. Chemical composition of prepared A356 alloy
|
Element (wt%) |
Actual Value Measured |
Standard Value |
|
Si |
7.38 |
6.6-7.5 |
|
Fe |
0.5 |
0.6 |
|
Cu |
0.2 |
0.25 |
|
Mn |
0.1 |
0.35 |
|
Mg |
0.4 |
0.20-0.45 |
|
Zn |
0.3 |
0.35 |
|
Ti |
0.02 |
0.25 |
|
Al |
balance |
balance |
Figure 1. Stir stir-casting technique used in this work
The FSP was carried out on a CNC vertical machining center that was computer numerically controlled model Knuth Werkzeugmaschinen GmbH, Germany. Following a series of tests, the FSP's fixed rotation speed of 1250rpm, travel speed of 75mm/min, and plunge depth of 3.1mm were adopted. All experiments were performed at room temperature. Each FSP experiment utilized a single pass. High-speed steel (HSS) was used to develop and build the FSP tool. A 6 mm diameter pin that is 3mm deep and a 16mm shoulder make up the geometric shape.
The specimens obtained from the castings and friction stir-treated plates were examined using an optical microscope for microstructural characterization. The wet grinding process included the use of water and SiC emery paper with different grit sizes, including 320, 500, 600, 800, 1000, and 1200. Lubricant, a special polishing cloth, and 0.5μm diamond paste were used to polish the specimen. Using Keller's reagent, a solution consisting of 95ml H2O, 2.5ml HNO3, 1.5ml HCl, and 1.0ml HF, the specimens were etched. Following a cleaning procedure, the objects were exposed to a moisture removal process.
A scanning electron microscope (SEM) device, namely the VEGA3LMe model, with a high-resolution mode and Oxford MAX3 model-based energy dispersive spectroscopy (EDS), was employed in this study. To identify the various phases contained in the microstructures, the specimens' X-ray diffraction (XRD) patterns were generated using a diffractometer (Bruker D8) using Cu-K radiation.
The digital micro-hardness tester Laryee HVS-1000 was used to conduct the Vickers hardness test. An applied force of 200 grams was used for 15 seconds. The tensile specimens were manufactured in accordance with ASTM standard E8/E8M-09 and evaluated at room temperature using a computer-controlled universal testing apparatus (INSTRON, 8810). The speed throughout testing was 1.0mm per minute. The broken tensile specimens' shattered surfaces were examined using a scanning electron microscope.
3.1 Microstructure
A micrograph of the A356 Al alloy sections as cast following FSP is shown in Figure 2 (a). Four different zones are clearly seen in the FSP specimen. The SZ, a thermomechanically treated area, is distinguished by its small grain size and equiaxed or homogenized grain structure. There are three zones present: the thermomechanically affected zone (TMAZ), the heat-affected zone (HAZ), and the friction stir-treated zone. The expanded grain in the TMAZ is due to thermomechanical deformation. The HAZ's grain structure is analogous to that of the base metal (BM).
The area that the FSP process does not affect is the base metal (BM). The A356 Al alloy features a main α-Al phase in the form of a dendritic structure, which appears as white areas. It also has an Al-Si eutectic, which appears as black regions. In Figure 2 (b), the eutectic phase is seen as a continuous phase encircling the α-Al dendrites, whereas the Si particles are fractured into irregular fragments inside the base metal. Following the process of FSP, the microstructure undergoes a full transformation, resulting in a changed structure that is desired. As seen in Figure 2 (c), the cast matrix alloy's dendritic microstructure experienced considerable severe plastic deformation before being refined. Silicon was altered, and grains were refined using Friction Stir Processing (FSP). Subtle microstructure alterations were seen in the SZ, which had a globular shape with small Si particles. In the stir zone, these particles were notably more uniformly dispersed. At high temperatures, Friction Stir Processing (FSP) causes considerable plastic deformation of the material in the stir zone. When the material was stressed at high temperatures, dynamic recrystallization took place, creating nucleation sites [23].
Figure 2. Optical micrographs showing the microstructure of the base A356 after FSP, showing (a) base metal, HAZ, TMAZ, and SZ at 100 x, (b) Base metal at 400 x, and (c) SZ at 400 x
Following FSP, optical micrographs of the in-situ composites A356/Al3Ni are displayed in Figure 3 (a). The small Al3Ni particles in the matrix alloy are distributed uniformly. Figure 3 (b) shows that following FSP, the Al3Ni was generated in situ and evenly dispersed throughout the Al matrix. Along with Al3Ni, intermetallic compounds of various sizes and shapes were also seen in the composites [19, 24].
Figures 4 (a) and 4 (b) show SEM micrographs of the microstructures of the basic alloy A356 and in situ composites containing (15%) Ni. The primary α-Al phase and Al-Si eutectic make up the alloy A356 initial microstructure, as can be seen from Figure 4 (b) Al3Ni intermetallic compound various shapes of agglomeration of Al3Ni intermetallic compound have been observed in the 15wt% Ni. These findings are consistent with those of Abbass et al. [25-27].
Figure 3. Optical micrographs showing the microstructure of In- situ composite A356/Al3Ni after FSP (a) base, HAZ, TMAZ, and SZ at 100 x (b) Base metal at 400 x (c) SZ at 400 x
Figure 4. SEM micrographs of A 356 as cast before and after FSP. (a) A356 as cast, (b) A356/Al3Ni as cast, (c) A356 after friction stir processed, and (d) A356/Al3Ni after friction stir processed
Figure 4 (c) shows scanning electron microscopy (SEM) pictures of the microstructure of A356 cast after friction stir processing. This process resulted in the refinement of aluminum dendrites, the repositioning of small and equiaxed silicon particles within the aluminum matrix, and the visible disintegration of fibrous silicon particles [28].
Figure 4 (d) depicts the microstructure of the friction-stir-processed zone of A356/Al3Ni AMC. Uniformizes Ni particle distribution in A356/Al3Ni in situ composites. Friction stir processing (FSP) has been shown to achieve complete homogeneity. The rotating instrument produces enough heat and circumferential force to uniformly distribute the nickel (Ni) particles throughout the larger region. Regarding the parameters of metal flow during friction stir processing of A356/Al3Ni in situ composites in SZ. A single FSP round was enough to improve the distribution and break up particle segregation at grain boundaries [29].
3.2 X-ray diffraction analyses
Figures 5 and 6 show the XRD patterns of cast base Al alloy A356 specimens and in situ composite specimens (15wt% Ni). In the XRD pattern of cast Al alloy A356, it was discovered that the presence of Al phases is evident and greatest, while the intensity of Si peaks is lesser. Additionally, the emergence of peaks of the intermetallic compound Al3Ni, Al Ni, and Al3Ni2 phase is exhibited in the XRD pattern of in-situ composite of A356- 15wt% Ni, as observed by Manjunatha et al. [30].
Figure 5. XRD pattern of cast A356 Al alloy
Figure 6. XRD pattern of in-situ composite (A356-15%Ni)
3.3 Microhardness
The treated zone's cross-section was measured using the axis of the hardness profile. It seems that FSP might change the distribution of hardness on any particular specimen in light of the earlier microstructural discoveries. FSPed specimens' microstructure is made up of four main areas: The BM, HAZ, SZ, and TMAZ. Figure 7 shows the hardness profile of the specimens (A356 and A356/Al3Ni) before and after the friction stir operation. We measured the microhardness of the matrix of the in-situ composites that were cast and friction stir processed (FSPed). Each hardness measurement is calculated as the average of at least three readings. The hardness increases considerably after FSP as a result of the refining grain and dynamically recrystallized zone. The A356/Al3Ni has a hardness of 152 and 173 HV, whereas the as-cast A356 has a hardness of 76 and 87 HV before and after the first FSP pass, respectively. Dispersion hardening is more successful after FSP because of the improved uniformity of particle distribution. The hardness is also improved by the high dislocation density and particle refinement [31, 32].
Figure 7. Hardness values of as-cast A356 alloy and in-situ composites
3.4 Tensile properties
Table 2 summarizes the tensile characteristics of the as-cast and FSPed in situ composites, and Figure 8 depicts the connection between stress and strain. After doing FSP, stamina shows a discernible improvement. The tensile strength is also significantly enhanced following FSP, as illustrated in Figure 9. The strength of the A356 as cast and in-situ composite A356/Al3Ni after FSP is high compared to A356 as cast before FSP, as evidenced by Table 2. Numerous strengthening mechanisms occurred in base A356 and in situ composite A356/Al3Ni after FSP, including precipitation hardening that is dispersed throughout the Al-matrix. This is due to the following variables, in addition to the dispersion strength produced by the distribution of Al3Ni, AlNi, and Al3Ni2, which hinder the movement of dislocations and boost the mechanical characteristics.
Figure 8. Tensile strength as-cast of A356 alloy and composites
Figure 9. Stress-strain curve for A356 as cast and in-situ composite A356/Al3Ni before and after FSP
Table 2. Tensile test results for the specimen of base alloy A356 and in-situ composites before and after FSP
|
Specimen |
Tensile Strength (MPa) |
Yield Strength (MPa) |
Elongation (%) |
Improvement% |
|
A356 as cast |
146 |
70 |
10.32% |
- |
|
A356+FSP |
202 |
130 |
17.78% |
38.35% |
|
A356/Al3Ni |
180 |
90 |
9.98% |
23.28% |
|
A356/Al3Ni+FSP |
247 |
150 |
9.24% |
69.17% |
Additionally, the mechanics of grain boundaries were strengthened in the agitation zone and base metal, resulting in improved dispersion strengthening. As per the Hall–Petch relationship, the strength is further enhanced as a result of the finer particle size that is obtained after FSP. These results agree with the studies [33, 34], which carried out FSP on wrought and cast aluminum alloy. The properties are also enhanced by the FSP removal of casting defects [35, 36].
3.5 Fracture analysis
The fracture surface was examined using a scanning electron microscope (SEM), as shown in Figure 10. The A356 and A356/Al3Ni tensile specimens' fracture surfaces are depicted in the SEM micrograph both in their cast and post-friction stir processing (FSP) states.
Figure 10. Fracture surface (a) A356 as cast, (b) A356 as cast after FSP, (c) A 356/Al3Ni in situ composite as cast, (d) A 356/Al3Ni in-situ composite after FSP
The in-situ composites (Figures 10 (a) and (c)) do not exhibit distinct dimple development. The crack's upper surface is rather level and flat. Holes have been noticed on the split surface. Based on these numbers, it looks like the group failed weakly. There is not much solid evidence that the metal matrix changed shape significantly before it collapsed. The crack surfaces of alloys that have been processed with friction stir (Figure 10 (b) and (d)), on the other hand, show that the aluminum matrix has been deformed plastically. There is a clear pattern of small depressions on the broken surface. It looks like the breaking mode is flexible [37, 38].
The work employed stir casting to produce in-situ composites including an Al3Ni reinforcing phase, Al alloy A356, and 15% weight percent pure Ni powder and determine how FSP impacted the stir-cast A356/Al3Ni in-situ composite's mechanical properties and microstructure. From the present investigation, the following important conclusions are derived:
|
FSP |
Friction stir processing |
|
FSW |
Friction stir welding |
|
XRD |
X-ray diffraction |
|
SEM |
Scanning electron microscope |
|
SZ |
Stir zone |
|
TMAZ |
Thermo-mechanically affected zone |
|
HAZ |
Heat affected zone |
|
BM |
Base metal |
|
MMC |
Metal matrix-composites |
|
AMCs |
Aluminum matrix composites |
|
HSS |
High-speed steel |
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