Mechanical properties and microstructures of a modified Al–Si–Cu alloy prepared by thixoforming process for automotive connecting rods

SazianaSamat, Mohd Zaidi, OmarAmir Hossein Baghdadi, Intan Fadhlina Mohamed, Ahmad Muhammad Aziz
Department of Mechanical and Manufacturing Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 UKM, Bangi, Malaysia

Abstract

The thixoforming process with feedstock preparation yields a fine microstructure and enhanced mechanical properties relative to other traditional casting processes. However, the thixoforming process via cooling slope is limited to commercial cast alloys and requires specific thixoformability criteria for each type of alloy. Therefore, a modified Al–Si–Cu alloy with 2.0 wt.% Cu, 0.30 wt.% Mg, 0.5 wt.% Mn, and 0.1 wt.% Fe was developed from the thermodynamic analysis result in this work for the fabrication of automotive connecting rods. In this study, the thixoforming process was conducted at different liquid fractions of 0.40, 0.45, and 0.50. A good filling capability of the semisolid slurry was found at a liquid fraction of 0.50 without any defects. The microstructures of the α-Al dendrite and acicular silicon particles were transformed into globular and fibrous shapes in the thixoformed alloy, respectively. The intermetallic compounds of Al₂Cu, Al₅Cu₂Mg₈Si₅, and β-Al₅FeSi had a homogenous and fragmented shape. Due to the microstructural enhancement and the reduction in casting porosity owing to the high pressure during the thixoforming process, the mechanical properties of thixoformed were remarkability improved and reached to the values of 103 HV, 250 MPa and 340 MPa for hardness, yield stress, and ultimate tensile strength, respectively. Also, the T6 heat treatment process was applied to the thixoformed sample. Results indicated 95% and 22% improvements in the ultimate tensile strength and hardness of the thixoformed sample, with the resulting values being 340 MPa and 122 HV, respectively, relative to those of heat-treated gravity-cast alloys.

Korea Abstract

공급 원료를 준비하는 틱소 포밍 공정은 다른 전통적인 주조 공정에 비해 미세한 미세 구조와 향상된 기계적 특성을 제공합니다. 그러나 냉각 슬로프를 통한 틱소 포밍 공정은 상업용 주조 합금으로 제한되며 각 합금 유형에 대한 특정 틱소 포밍성 기준이 필요합니다.

따라서 자동차 커넥팅로드 제조를 위한 이 작업의 열역학적 분석 결과로부터 2.0 wt. % Cu, 0.30 wt. % Mg, 0.5 wt. % Mn 및 0.1 wt. % Fe를 함유하는 개질 된 Al-Si-Cu 합금이 개발되었습니다. 이 연구에서 틱소 포밍 공정은 0.40, 0.45 및 0.50의 다른 액체 분획에서 수행되었습니다.

반고체 슬러리의 우수한 충진 능력은 결함없이 0.50의 액체 분율에서 발견되었습니다. α-Al 수상 돌기와 침상 실리콘 입자의 미세 구조는 각각 thixoformed 합금에서 구형 및 섬유 모양으로 변형되었습니다. Al2Cu, Al5Cu2Mg8Si5 및 β-Al5FeSi의 금속 간 화합물은 균일하고 조각난 모양을 가졌습니다.

틱소 포밍 공정 중 고압으로 인한 미세 구조 향상 및 주조 다공성 감소로 인해 틱소 포밍의 기계적 특성이 현저하게 개선되었으며 경도, 항복 응력 및 340MPa의 값에 도달했습니다. 최대 인장 강도. 또한 T6 열처리 공정을 thixoformed 샘플에 적용했습니다.

결과는 thixoformed 샘플의 극한 인장 강도 및 경도가 95 % 및 22 % 향상되었으며, 결과 값은 열처리 된 중력 주조 합금에 비해 각각 340 MPa 및 122 HV입니다.

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Keywords

Al–Si–Cu alloyThixoformingCooling slopeMicrostructureMechanical propertiesAutomotive connecting rod

1. Introduction

Automotive industries are currently focusing on fuel efficiency improvement. Consequently, advanced lightweight materials have attracted extensive attention in the aluminum alloy-manufacturing industries toward the substitution of automobile components, which are made from steel and cast iron. They provide components with enhanced properties, such as high strength-to-weight ratio, high elongation values, and excellent corrosion resistance. Many process technologies, typically involving forging, casting, rolling, and extrusion, are associated with aluminum alloy parts [[1], [2], [3]]. These processes encompass numerous pre- and post-activities to obtain the final shape, requisite surface finish, and dimensional accuracy. However, they cause inconsistencies in mechanical strength, dendritic microstructure, low dimensional accuracy, and defects in the form of gas or shrinkage porosity within the cast parts. The advent of advanced casting techniques, such as high-pressure die, gravity, and squeeze casting, has compensated those shortcomings and produced refined microstructure and improved productivity [[4], [5], [6]]. Nevertheless, these processes should be commensurate with various casting process parameters. For the squeeze casting process, although it can compensate for the porosity problem, the entire process requires long cycle time, thus limiting the process yield capability [7].

In this regard, semisolid metal (SSM) processing becomes known as an alternative method to obtain products with industrial capability in terms of the absence of porosity, short cycle times, high strength requirement, and cost effectiveness [8,9]. The discovery of thixotropic behavior, as reported by Flemings [10], has led to a new development in processing. Thixotropic materials behave like a solid if allowed to stand and flow like a liquid when sheared. One type of SSM processing is thixoforming. This method consists of the preparation of a feedstock material with a nondendritic structure, followed by the reheating of the billet to the appropriate semisolid temperature, and, finally, the flowing of the semisolid slurry into a die [11]. Compared with conventionally cast parts, thixoformed components offer near-net shaping of complex shapes and better mechanical properties. The microstructure is spheroidal solid particles surrounded by liquid phase and is free of oxides and pores [12].

Therefore, thixoforming is presently a potential forming process in industrial manufacturing. The first step in this process is feedstock preparation for nondendritic microstructure in thixotropic behavior before the forming process. Nevertheless, most early works on the fabrication of thixoformed automobile components focused on the mechanical stirring method, magnetohydrodynamics, and strain-induced melt-activated processes [[13], [14], [15], [16], [17]]. Although they offer a fine spheroidal size, uniformity is lacking with rosette-shaped structures. The cooling slope (CS) route by pouring molten metal over an inclined CS holds benefits for obtaining near-globular grain sizes (GSs) for thixoforming. CS is a simple process with a few types of equipment and low running costs [[18], [19], [20]]. The thixoformed microstructure prepared by CS casting is metallurgically sound with homogenous dispersion of α-Al and Si particles in an aluminum matrix [21].

Aluminum alloys, which are in demand, are adaptable to thixoforming and can be used to exploit the advantages of this process further. Aluminum products may be heat-treated to achieve high mechanical properties and are superior to cast products. However, the currently used Al alloys, such as A357, A356, A388, and A319, have initially been designed for conventional casting techniques. These alloys have a complex semisolid transition and are unsatisfactory in processes with inconsistent temperatures due to the variations in liquid fractions [22]. A suitable alloy must exhibit good thixoformability, such as a wide working window, low liquid fraction sensitivity, and age hardening potential [23]. Hence, to increase the number of alloys in thixoformed automotive parts, a range of aluminum alloys with development and modification in the alloying elements is considered to exploit the potential for the thixoforming process.

Each compositional change in aluminum alloy affects the thixoformability criteria. Salleh et al. [24] investigated the effect of the compositional variation of Mg and Cu elements on thermodynamic modeling to fulfill the criteria of thixoformability. They found that the result obtained from the liquid fraction percentage versus temperature curve differed for each compositional variation with a decrease in solidification temperature and an increase in eutectic temperature. Hu et al. [25] found that increasing the alloying element Mg and reducing Mn and Fe improved the thixoformability criteria, i.e., the window temperature processing.

The development of low-cost alloys with acceptable mechanical integrity remains a crucial and significant target for thixoforming process. The effects of alloying elements on the thixoformability criteria of the Al–Si–Cu alloy have been frequently reported in recent research. Nonetheless, alloying element optimization in the thixoforming process of Al–Si–Cu requires further study for automotive component fabrication. Therefore, this work aims to develop a modified Al–Si–Cu alloy for the fabrication of automotive connecting rod components. The fabrication process was performed through a thixoforming process on the modified alloy prepared using the CS method. The modified alloy was obtained through the optimization of alloying elements via thermodynamic analysis. The microstructural and mechanical properties of the thixoformed sample were investigated. In addition, the effect of T6 heat treatment was determined to investigate the improvement in the mechanical properties of the modified Al–Si–Cu alloy.

2. Materials and methods

2.1. Material selection and thermodynamic analysis

Al–Si–Cu aluminum alloys with different ranges of alloying elements that are commonly used in automotive component fabrication were used in this study. The experimental design was established to determine the effects of chemical composition factors on the results of SSM processing [24,25]. Table 1 shows the alloying elements with four factors at three varying levels.

Table 1. Alloying elements with their specific values at three different levels (wt. %).

The Taguchi method was applied to exhibit the influences of different factors at multiple levels, which included a series of Al–Si–Cu alloys, and obtain the ideal chemical composition for thixoformability, as shown in Table 2. This method was used to reduce the number of experiments by applying an orthogonal array design [26,27] A thermodynamic prediction software package, JMatPro® (Sente Software Ltd.), was used as a guide to evaluate the thixoformability criteria for developing a thixoformed alloy.

Table 2. Al–Si–Cu alloys used in this study (wt. %).

2.2. Differential scanning calorimetry (DSC) analysis

DSC analysis was applied to determine the solidus, liquidus, and liquid fraction profiles. DSC testing was performed on a 25 mg-sized piece of Al–Si–Cu alloy by using a Netzsch-STA(TG-DSC)-449 simultaneous thermogravimeter in a neutral gas atmosphere of nitrogen (N₂) with a scanning rate of 10 °C/min. The heat flow rate (HFR) signal curves versus temperature were then used to generate the transformation curve between the liquid fraction (L_f) and temperature (L_f in % vs. T in °C).

2.3. Thixoforming process

A schematic of the thixoforming process is presented in Fig. 1. The experimental device was self-designed and began with dendritic feedstock preparation that was subjected to a CS casting processing route. The initial material of Al–Si–Cu alloys was melted up to superheated 750 °C in crucible carbide silicon under an argon gas temperature before being poured via CS at 640 °C.

A cooling cast feedstock billet of 25 mm diameter and 90 mm length was then heated at semisolid temperature by using an inductive heating system (60 kHz, 35 kW) and induction coil geometry, with the induction coil having a diameter of 50 mm and a length of 130 mm [28]. The slurry was compressed into a preheated stainless steel die at 150 °C after degassing by argon via a hydraulic press for thixoforming for 60 s holding times at respective liquid fraction temperature (Fig. 1(b)). The punch velocity and pressure were 85 mm/s and 20 kN, respectively. The die assembly design cavity of the thixoformed connecting rod product is shown in Fig. 1(c).

Table 3 shows the processing condition for the studied alloy for connecting rod fabrication via a gravity casting method (melting at 750 °C, cooling up to 630 °C, and pouring into a permanent mold) and thixoforming. The thixoforming process was performed at different temperatures and liquid fractions (referring to the DSC result). The connecting rod sample was heat-treated to a standard T6 heat treatment. The solution heat treatment was performed at 505 °C for 8 h, followed by water quenching at 60 °C and artificial aging at 160 °C for up to 3 h [29].

Table 3. Processing condition for connecting rod fabrication.

2.4. Analyses of microstructural and mechanical properties

As shown in Fig. 2, the sample was extracted from the position of the connecting rod at the piston pin end, shank, fork part, and crank pin end for microstructure observation, phase analysis and tensile and hardness testing. X-ray analysis was also conducted to recognize the type and discontinuities in the gravity-cast and thixoformed connecting rod, as outlined in the ASTM E2422−11 Standard for the Inspection of Aluminum Castings.

The metallography specimens were prepared as per the guide in the ASTM-E407-2007 standard. Each sample was polished to a mirror surface with 1 μm diamond paste by using 400–1200 grinding paper and then etched in Keller's agent (1 ml of hydrofluoric acid, 1.5 ml of hydrochloric acid, and 2.5 ml of nitric acid in 95 ml of distilled water) for 15–20 s. The etched specimens were investigated using an optical microscope (Olympus) and further observed using a scanning electron microscope (Zeiss Merlin Compact) equipped with an energy-dispersive X-ray spectroscope. Phase identification was also performed by utilizing a Bruker D8 Advance X-ray diffraction (XRD) analyzer.

Hardness was measured using a Vickers hardness tester by applying 10 kgf for a dwell time of 10 s. All of the samples were polished before the test was conducted. Each hardness value was compiled from the average of 30 measurements. Tensile test specimens were machined in accordance with the ASTM-E8M standard with a rectangular cross-section sample of 32 mm gauge length. The tensile test was performed using a 100 kN Zwick-Roell universal testing machine at a tensile rate of 1 × 10 s⁻¹ at room temperature. The test was conducted on three samples. As shown in Fig. 2, the tensile specimen was collected from the shank and fork part area of each thixoformed connecting rod.

3. Results and discussion

3.1. Thermodynamic analysis and thixoformability of the Al–Si–Cu alloy

The effects of the variations in alloying elements, mainly Cu, Mg, Mn, and Fe, on the thixoformability of the Al–Si–Cu alloy were examined using the JMatPro® simulation software. The results of JMatPro® simulation were based on the solidification process that occurred under nonequilibrium conditions. Fig. 3 shows the liquid fraction and identification of the solidus, liquidus temperature, and highest eutectic “knee”-point temperature (amount of eutectic in the structure) for all the studied alloys. The liquid fractions and temperatures fluctuated at different alloy compositions. Table 4 lists the solidification parameters for all the studied alloys given by JMatPro® software, as presented in Fig. 3.

Table 4. Solidification parameters for all studied alloys given by JMatPro® software.

Fig. 4 summarizes the thixoformability criterion parameters given by JMatPro® software for the studied Al–Si–Cu alloys. As suggested by Liu et al. [30], a relatively wide working window temperature with a high knee point should occur between 0.3 and 0.5 L_f, and an adequate solidification range (between 55 °C and 150 °C) and small liquid fraction sensitivity (less than 0.025 °C⁻¹) are required for thixoforming process.

The identification of working window allows materials to be operated in the semisolid temperature range. Therefore, a wide process window is favorable to improving the process stability, leading to an optimum slurry viscosity and die filling behavior. As shown in Fig. 4, a wide working window temperature (between 0.3 and 0.5 L_f) at 11 °C with a “knee” point in between the range was achieved in Alloys G and H via Cu reduction to 2 wt.% and Mg addition between 0.3 and 0.5 wt.%. Cu and Mg provided a substantial increase in the material strength. Nevertheless, the high Cu content in the Al–Si–Cu alloy was able to increase the amount of eutectic in the structure at the highest “knee” point, which tended to slow down the rate of liquid formation.

Solidification temperature is defined as the range between liquidus and solidus temperatures for an alloy. As shown in Fig. 4, the solidification temperature range was between 100 °C and 115 °C for all alloys in this study. These solidification temperatures were in good agreement with the solidification temperature range suggested by Patel et al. [23] and Liu et al. [30], who reported between 10 °C and 150 °C. Alloys G and H also obtained enlarged solidification temperatures of 109 °C and 115 °C, respectively. The reduction of Cu in the Al–Si–Cu alloy system decreased the solidification range, which could improve the ability of the alloy to have good resistance to hot tearing products as it became susceptible to the formation of material porosity.

The sensitivity of liquid fraction is also another important criterion for thixoforming, considering that the process deals with reheating via an induction heating system, which is related to the skin depth effect phenomena [28]. The sensitivity of liquid fraction can be defined as the slope of the liquid fraction versus temperature. To achieve a well-controlled condition of temperature during the billet-heating process, liquid fraction sensitivity should be less than 0.03 °C⁻¹ [22]. As presented in Fig. 4, low sensitivity of liquid fraction of 0.4 L_f was achieved for all alloys with different chemical compositions. However, lower sensitivity is advisable for thixoforming to have better processability. The sensitivity of liquid fraction for the Al–Si–Cu–Mg alloy decreased with rising Cu content from 2 wt.% to 4 wt.% and increasing Fe from 1.0 wt.% to 0.1 wt.%.

Overall, all Al–Si–Cu alloys with variations in chemical compositions can be considered thixoformable in accordance with the thixoformability criterion mentioned above. Nevertheless, Alloy H showed better criteria in terms of a wider working window with the existence of a knee point at approximately 0.47 L_f, lower liquid fraction sensitivity, and a higher solidification temperature range, as shown in Fig. 4. Thus, Alloy H was selected as a modified Al–Si–Cu alloy for further analysis to fabricate automotive connecting rods.

3.2. DSC characterization

To evaluate the liquid fraction versus temperature curve calculated using JMatPro® software with the experimental result, thermal analysis was conducted on the modified Al–Si–Cu alloy (Alloy H) through DSC experiment. Fig. 5 shows the result of the DSC transformation curve for the modified Al alloy. The DSC results were obtained by applying the method of differential calculus to a semisolid transformation curve [31] to identify the critical temperature in thixoformability analysis.

In Fig. 5, points A and B represent the solidus and liquidus temperatures at approximately 510 °C and 628 °C, respectively. Point C represents the eutectic temperature at which α-solid solutions started melting, and the amount of liquid fraction at the highest “knee” point represents the amount of eutectic in the structure. The highest knee point occurred at the temperature of 574 °C with 0.36 L_f. Points D and E denote the initial and final temperatures for the working window of thixoforming process, which were 568 °C with 0.25 L_f and 600 °C with 0.54 L_f, respectively. However, the thixoforming process is suggested to be conducted at above the “knee” point temperature [32,33]. Therefore, the thixoforming process was performed in the temperature range between 583 °C and 597 °C with 0.40 and 0.50 L_f, respectively, as presented in Table 2.

The DSC results were different from those points in JMatPro® software (Scheil calculation) analysis due to few factors. Scheil calculated a result for a bulk material, but the DSC results were obtained from a very small-scale sample [34]. The DSC results were gained with uncertainty in the specification of the compositions and an inhomogeneous distribution of the alloying elements. Nonetheless, the calculated result of the JMatPro® software considered the ideal result.

3.3. Connecting rod fabrication process

The fabrication of three trials of connecting rod samples was conducted through a thixoforming process (each with different liquid fractions) and a gravity casting process. Table 5 shows the representative samples in the gravity-cast and thixoformed samples in 0.4, 0.45, and 0.50 L_f and the die-filling behavior of the modified alloy in semisolid state. In this case, the casting dies were designed for gravity casting, and the thixoforming process was similar to a horizontal-type gate to ensure that the slurry fed from the shank area would flow to fill the entire piston pin and crank pin ends. Kang et al. [35] found that a thick gate for the thixoforming process affects the filling slurry capability and flow behavior of semisolid slurries.

Table 5. Configuration of connecting rod fabrication.

Macro views	Analysis of X-ray Image Scan	Remarks

As shown in Table 3, unfilled cavities were evident in the thixoformed samples at 0.40 and 0.45 L_f, whereas the thixoformed sample at 0.50 L_f showed complete die cavity filling and excellent surface finish. The die filling behavior capability of semisolid slurries was enhanced at a high liquid fraction. Here, the high liquid fraction corresponded to a high temperature that could provide adequate liquid for primary grain or nuclei growing into large grains in the die at short solidification time [36].

The low viscosity characteristic at low shear stress is highly dependent on processing temperature, such as liquid fraction [37]. A high-viscosity slurry encounters difficulty when flowing into a die and could not fill the part. Moreover, the high liquid fraction associated with deformation behavior affects the outflow phenomena, microstructure GS, and yield strength (YS) of the thixoformed sample [38].

From Table 4, the macroscopic observation indicated incomplete casting products, casting defects, cracks, and cold shuts; the X-ray radiograph shows black spots, which were found to be casting defects (interdendritic shrinkage or pore type) due to improper filling. The microstructure of the thixoformed connecting rod at 0.50 L_f was free from any microporosity. On the contrary, the thixoformed connecting rods at 0.40 and 0.45 L_f showed small rounded pore microporosity at approximately 60 μm maximum length in the fork part, as indicated by the red circles on the image. In the case of the connecting rod via gravity casting, interdendritic shrinkage microporosity at approximately 200 μm maximum length was observed at the piston pin end and crank pin end areas. The main formation of porosity in gravity casting was caused by the turbulent flow and exposure to a high temperature of the molten metal during cavity filling, which would lead to solidification shrinkage and hydrogen porosity [39,40].

For the thixoformed connecting rods at 0.40 and 0.45 L_f, the presence of pore porosity might be due to the gas entrapment and a contaminant from graphite lid that mixed with the semisolid slurries during the heating of the CS cast billet. The thixoforming process is responsible for reducing the possibility of microporosity formation because the laminar flow behavior within the die cavity facilitates the formation of homogenous microstructural morphology [41]. The formation of microporosity can remarkably decrease the mechanical properties and promote crack propagation. Therefore, the thixoforming process via CS casting had good flowability and could produce extremely high-quality components to reduce and even remove casting defects. The experimental result suggested that thixoforming at 0.50 L_f should be set at 597 °C.

3.4. Microstructural analysis

Microstructural investigation was performed to evaluate the potential implementation of a thixoformed connecting rod produced by CS casting. A comparison of the microstructures of modified Al–Si–Cu alloy parts through gravity casting and thixoforming processes from different cross-sectional regions, i.e., piston pin end, shank, fork part, and crank pin end, is shown in Fig. 6. The microstructure of the gravity-cast alloy was composed of coarse α-Al dendrites with acicular-shaped eutectic Si and intermetallic located between the secondary dendrite arms with shrinkage pores, which were easily found in the sample.

The thixoformed sample was produced by CS casting, the α-Al dendrite-shaped microstructure was transformed into a globular grain, and a uniform distribution of eutectic Si particles with no micropores was observed. Dissimilar microstructures and GSs were found at different positions in the thixoformed sample. The microstructure of the globular grain suspended in the eutectic phase allowed it to move smoothly. However, eutectic phase segregation was found in the thixoformed sample at 0.45 L_f along the connecting region, except the shank area. Liquid segregation dominated in unfilled regions at a low liquid fraction. The liquid segregation occurred when the reheated metal was fully solidified the moment it flowed into the cavity. The adjacent irregular α-Al grain microstructures were stack together and agglomerated, resulting in the liquid to drain out through the grain network due to insufficient sliding.

The fraction liquid parameters and the size of primary α-Al and shape factor (SF) were correlated. A high liquid fraction exhibited small GS and enabled extensive grain movement. Thus, this feature led to high viscosity and consequently permitted the simultaneous flow of solid and liquid that determined the filling behavior capability.

In conventional casting, most criteria for the characterization of α-Al grain structure are generally based on secondary dendrite arm spacing. The semisolid processing in thixoforming characterized on the basis of the size and sphericity of globules and α-Al grains corresponded to the calculation of SF and average GS obtained from image analysis [17]. The equations are given as follows:(1)GS=[∑i=1N2(Aiπ)1/2]/N(2)SF=4πAP2where A and P is the area and perimeter of a particle, respectively. Also, N is the total number of particles in each image.

Table 6 shows the quantitative value of the average GS and SF of the studied alloy at different liquid fractions for each main position in the connecting rod. The α-Al GS was low at 0.50 L_f with an average of 57 ± 6.0 μm. Nevertheless, the SF sphericity of α-Al was considerably improved from the irregular shape of 0.81 ± 1.0 at 0.45 L_f to a spherical one of 0.85 ± 0.2 at 0.50 L_f. The results in GS and SF met the expectations in thixoforming via CS casting.

Table 6. GS and SF measurement at different liquid fractions.

Fig. 7 and Fig. 8 show the high magnification of the optical images of the gravity-cast and thixoformed Al–Si–Cu alloys at 0.50 L_f before and after T6 heat treatment. In both figures, zones 1 and 2 mark the detailed morphology, shape phase, and compound in the eutectic region of the Si particles and other intermetallics.

The morphology of the eutectic silicon particles in zone 1 in the gravity-cast sample was transformed from a coarse acicular-shaped structure to a fine-shaped structure in the thixoformed sample, as shown in Fig. 7(a) and 7(b), respectively. However, the structure of the Si particles in heat-treated condition showed a fine spheroidal grain microstructure, as presented in Fig. 8(a) and 8(b). The Si morphology transformation is extremely important because it is responsible for the tensile strength improvement and the stress concentration reduction at the grain boundary [42].

The intermetallic compounds (zone 2) of the gravity-cast sample (before and after heat treatment) were observed in the interdendritic region. By contrast, the thixoformed sample at 0.50 L_f (before and after heat treatment) was fragmented and separately distributed in the eutectic region with small intermetallic precipitate inside the solid phases. The gravity-cast and thixoformed samples at 0.50 L_f intermetallic compounds were homogenous after T6 heat treatment.

Fig. 9 and Fig. 10 show the energy-dispersive X-ray spectroscopy (EDS) mapping analysis of the modified Al–Si–Cu alloys acquired from the gravity-cast and thixoformed samples at 0.50 L_f of dark structures at grain boundaries in Fig. 7. The analysis revealed the distribution of the main alloying elements in the samples. The Si eutectic morphology of the gravity-cast and thixoformed samples was intensified at grain boundaries. Nonetheless, Mg, Mn, and Fe elements were distributed homogenously in both microstructures. The Cu elements had a homogenous distribution at grain boundaries in the thixoformed sample compared with that in the gravity-cast sample.

Fig. 10 depicts the scanning electron microscopy (SEM) microstructure of the phases and intermetallic compounds of the modified Al–Si–Cu alloys with conjunction findings from the literature for a similar A319 alloy. As shown in each sample, α-Al phases, eutectic Si, and intermetallic compounds, i.e., Al₂Cu, Q-phase (Al₅Mg₈Cu₂Si₆), and Fe-containing phases of β-Al₅FeSi, were formed in the microstructure. The EDS point analysis results obtained from each sample in Fig. 11 are provided in Table 7. The intermetallic compounds were determined using the chemical compositions.

Table 7. EDS point analysis of the intermetallic compounds of the gravity-cast, thixoformed, and thixoformed+T6 samples at 0.50 L_f

The Al/Cu intermetallic present in the blocky morphology was elongated and distributed along the interdendritic region in the gravity-cast sample and the eutectic region in the thixoformed sample at 0.50 of liquid fraction. However, in the thixoformed+T6 sample, the Al/Cu intermetallic had dissolved inside the α-Al particle, which showed that the solution treatment reached the dissolution temperature of Cu-rich. Fe is the most harmful impurity affecting the tensile strength and ductility; consequently, the morphology of the needle-like shape of β-Al₅FeSi in gravity casting was transformed to a small plate needle-like one in the thixoformed sample, as clearly illustrated in Fig. 11. The intermetallic morphology changed the size and shape after the thixoforming process owing to isothermal heating and the pressure applied during the forming process aside from the difference in solidification rates. Addition of the alloying element Mn (Fe/Mn ratio lower than 1) to the Al–Si–Cu alloy could suppress the development of needle-shaped particles to a less harmful and more compact morphology [43], as illustrated in the SEM images (Fig. 11(b), point 5). Hence, the mechanical properties of the thixoformed Al–Si–Cu alloy should be improved.

Furthermore, XRD analysis was performed on all of the samples to identify the phases, as shown in Fig. 12. The peak diffraction of major elements, i.e., α-Al and Si phases, was obtained in all of the samples. The XRD data confirmed the presence of Al₂Cu, Al₅Cu₂Mg₈Si₆, and β-Al₅FeSi phases, as discussed previously. Given that the interface energy propagated the growth of α-Al during the thixoforming process, the diffraction peak intensity related to α-Al at 2θ = 38.5° (111), 44.7°(002), 65.1° (022), 78.2°(113) in the thixoformed samples increased compared with that in the cast alloy.

3.5. Mechanical properties

The Vickers hardness value was measured on the surface of the connecting rod at different regions, which depended on the indenter contact, primary α-Al grain, and quenched liquid phase. The average microhardness values of the modified Al–Si–Cu alloys before and after T6 heat treatment are reported in Table 8. The hardness of the gravity-cast sample was 99 ± 2 HV, and that of the thixoformed sample was 103 ± 2 HV. The increase in hardness measurement was attributed to the low porosity, fine α-Al, and uniform distribution of the silicon and intermetallic compounds [44]. The hardness increased substantially in the gravity-cast and thixoformed alloys after T6 heat treatment. However, the hardness value of the gravity-cast sample was similar to that of the thixoformed sample at 100 ± 3 HV. During solution treatment, an alloying element was dissolved and dispersed into α-Al to generate the solid solution-strengthening mechanism [45]. Precipitated intermetallic compounds in α-Al in the alloy that occurred during aging at 160 °C increased the hardness value to 122 ± 2 HV.

Table 8. Hardness values of the gravity-cast and thixoformed samples at 0.50 L_f.

Table 9 shows the ultimate tensile strength (UTS), YS, and elongation of the modified alloys through different casting processes and heat treatment conditions. The processing parameters used for the tensile test samples were defined on the basis of complete die filling at 0.50 L_f at 597 °C, and the tensile specimen sample was tested to determine tensile strength and elongation. The YS, UTS, and elongation to failure of the modified Al–Si–Cu by thixoforming reached 205 ± 10.5 MPa, 248 ± 9.4 MPa, and 2.3 ± 0.7%, respectively. The YS, UTS, and elongation of the gravity-cast alloy were 138 ± 12.7 MPa, 148 ± 11.3 MPa, and 0.5 ± 0.1%, respectively. Accordingly, the YS, UTS, and elongation of the thixoformed sample improved by 49%, 68%, and 3.6%, respectively, compared with those of the gravity-cast sample. This result was related to the refinement microstructure attributed to the sphericity of α-Al globules and the porosity reduction [46].

Table 9. Tensile properties of the modified Al–Si–Cu alloys in various processing conditions.

The T6 heat treatment considerably improved the tensile properties of the gravity-cast and thixoformed samples. As shown, the YS, UTS, and elongation to fracture of the gravity-cast sample were 143 ± 9.9 MPa, 174 ± 7.6 MPa, and 1.0 ± 0.2%, respectively; after thixoforming, the corresponding values were 250 ± 10.5 MPa, 340 ± 12.1 MPa, and 4 ± 0.5%. The tensile ductility improved after heat treatment. The high strength value was due to the T6 heat treatment as the main strengthening element through solution heat treatment and spheroidizing the Si particles. During the solution treatment, the Cu and Mg elements dissolved in the α-Al matrix at the grain boundaries at high solubility, and the ageing process promoted the precipitation of Al₂Cu in the alloy [47].

Fig. 13 shows a comparison of tensile strength and elongation results in connecting rod fabrication with different semisolid processing and alloying composition by Dao et al. [48] and Wang et al. [49] in semisolid squeeze casting via electromagnetic stirring. The UTS value of the modified Al–Si–Cu alloy (2% Cu and 0.32% Mg) by thixoforming via CS casting in the current study could reach 340 MPa after heat treatment, and the elongation to fracture decreased to 4.0%. The UTS of this modified alloy was higher than that of the commercial alloys A319 [50], ZL104 [49], and AlSi9Mg [48] for automotive component fabrication, as presented in Fig. 13. The addition of Cu content of this modified Al–Si–Cu alloy lowered the ductility because the Cu solute atoms enhanced the Al₂Cu precipitation hardening in the Al matrix. Moreover, with reference to Bergsma [51], the UTS value of the modified connecting rod met the reference data of a connecting rod made using a wrought aluminum alloy and indicated its success as an alternative material for fabrication. The elongation should be maintained in the range of 1%–5%.

The SEM fractography of the tensile fracture surface of the gravity-cast samples before and after T6 heat treatment is shown in Fig. 14. The gravity casting before heat treatment failed in a brittle manner, indicating the fracture of the long Si particle and some oxide pores that led to the deterioration of tensile properties. Nevertheless, after heat treatment, the presence of clustered shrinkage acted as fracture ignition points. The existence of porosity in the gravity-cast sample was responsible for the decrease in tensile strength [52].

The SEM fractography of the tensile fracture surface of the thixoformed samples before and after T6 heat treatment is shown in Fig. 15. Both samples showed fine dimple ruptures with void ignition at eutectic silicon particles detected on the fracture surfaces, and the fractography indicated a ductile fracture mode.

4. Conclusions

A connecting rod has been successfully fabricated using a modified Al–Si–Cu alloy via a thixoforming process with CS. In accordance with the obtained results, the conclusions are as follows:1.

A thermodynamic prediction analysis (JMatPro® software) was performed to investigate the effects of alloying element compositions on the alloy thixoformability. The modified Al–Si–Cu alloy with 2.0 wt.% Cu, 0.30 wt.% Mg, 0.5 wt.% Mn, and 0.1 wt.% Fe exhibited a wide working window at 11 °C with the existence of knee point at approximately 0.47 of liquid fraction, low liquid sensitivity at 0.018 °C⁻¹, and a high solidification temperature of 115 °C.2.

The thixoformed connecting rod successfully filled the die at 0.50 L_f without any defects. The microstructure consisted of spheroidal α-Al, uniformly distributed silicon particles, and fragmented intermetallic particles.3.

The hardness, YS, and UTS values of the thixoformed sample were 103 ± 2 HV, 250 ± 10.5 MPa and 340 ± 12.1 MPa which about 4%, 49%, and 68% higher than those of the gravity-cast connecting rod sample, respectively, which were attributed to microstructural enhancement and the reduction in casting porosity owing to the high pressure during the thixoforming process.4.

The tensile strength of the modified Al–Si–Cu alloy connecting rod increased remarkably after the T6 heat treatment. Improvements of 95% and 22% in UTS and hardness, with the resulting values being 340 MPa and 122 HV, respectively, in comparison with those of heat-treated gravity-cast alloys, were observed.5.

The gravity-cast sample showed a brittle-type fracture with cleavage rupture, whereas the thixoformed samples before and after T6 heat treatment showed a dimple rupture.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The authors would like to thank the Universiti Kebangsaan Malaysia and the Ministry of Education (MoE) Malaysia for the financial support under research grants MI-2019-025 and DIP-2016-007.

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© 2020 The Authors. Published by Elsevier B.V.

© 2020 The Authors. Published by Elsevier B.V.