EFFECTS OF ALUMINIUM ADDITION AND AUSTEMPERING TEMPERATURES ON AL-ALLOYED DUCTILE IRON MICROSTRUCTURE AND MECHANICAL PROPERTIES

The research investigates the effect of varying amounts of aluminium (1.05, 1.575, 2.29, 3.02 and 3.74 wt.%) addition and heat treatment (austempering at 300, 350, 400 o C) on the microstructure and mechanical properties of ductile cast iron alloys. The graphitizing effects of the Al alloy and varied austempering temperatures on the hardness, tensile strength, and impact toughness of the ductile cast iron (DCI) were evaluated. The results of the influences of Al addition and heat treatment on the properties of the DCI determined were graphically presented while the characterization was done using a scanning electron microscope (SEM). The microstructures revealed that the addition of Al into the matrix brings about the precipitation of ferrite around the graphite nodules. The combined effects of the increase in Al content and austempering temperatures produced greater hardness values on the Al-alloyed DCI samples than the as-cast sample. The hardness values for the entire samples ranged from 27.25 to 57.03 BHN. Tensile strength increased with an increase in Al content and lower austempering temperatures, whereas, the impact toughness increased with increase in Al content and higher austempering temperature. The results of the research revealed that the addition of Al to ductile iron and austempering treatment led to an improvement in the hardness, tensile strength, and impact toughness of the alloys.


INTRODUCTION
Ductile cast iron (DCI) is a special class of cast iron that possesses appreciable level of ductility due to the presence of graphite nodules embedded in the ferrite/pearlite matrices. The material's properties can be improved by altering some processing parameters via heat treatment procedures [1][2][3]. In casting ductile iron, some of the parameters that influence solidification include melt-composition, Mg-treatment, and inoculation [4][5][6][7]. The cooling rate affects graphite structure formation and properties of the casting product [7]. The rare earth metals and elements such as Ce, Ca, Ni, Si, and Al promote graphite nucleation and growth, whereas, As, Bi, B, Sb, Sn, and Pb reduce the formation [4]. Pedro et al. [6] used FeSi and FeSiMg for inoculation and nodulation in a ladle with a nodulation pocket. Upadhyaya et al. [5] used Ce-Ca-Al-S-O-FeSi, Zr-Mn-Ca-Al-Ba-FeSi, and Sr-Al-Ca-FeSi as inoculants which were found to be very effective. Ductile cast irons are very versatile materials for the automobile and automotive industries. The higher demand for DCIs than ordinary cast iron is principled on their suitability for different engineering applications including automobiles, automotive engines, cutting tools, rolls, dies, and gears. They are employed as structural materials in civil construction, agricultural and mining equipment [8][9][10] for their higher degree of ductility, high strength, toughness, as well as shock, corrosion and wear resistance; excellent castability, moderate damping capacity and good combination of strength, ductility and toughness [7]. The need for the reduction in the weight of automobile engines as requisite for fuel-saving has ignited research into the development of various types of new light materials. Whereas, the common types of light metals and alloys widely used for such purposes include Al, Mg, Ti [11], the addition of Al to cast iron would not only reduces the density but also plays the lead role in enhancing the mechanical properties such as the ductility, toughness, and formability [2]. Diverse works on various properties of austempered ductile cast iron (ADCI) and Aluminium-alloyed austempered ductile cast iron (AADCI) have been widely circulated. These include contact and bending fatigue behaviour; microstructure and abrasion wear behavior, wear and friction behaviors [12]. Pereira et al. [13]studied the mechanical properties such as ultimate tensile strength (UTS), yield strength (YS), elongation); Brinell hardness (HB) and microstructures of austempered ductile cast iron (ADCI) alloyed with Cu-Ni-Mn-Mo in hot air austempering cycles in a two-step mechanism. It was stated that alloying with small additions of Cu, Ni, Mn, and Mo is more effective than significant addition of only one of these elements due to Cu and Ni having negative segregation, and Mn and Mo have positive segregation [14). The corrosion behaviors of steel, CI, DCI and ADCI of different alloying formations have been studied, and reported by authors [10,15,16]. Different heat treatment processes have contributed to the enhancement of diverse properties of DCI. The grains refinement and phase transformations have been achieved in many ways. ADCIs demonstrate more fascinating combinations of characteristics like low cost, elevated strength, fatigue and wear resistance compared with steels of the same hardness [17]. The hardness, strength and toughness are tripartite mechanical properties of repute that are much required in automobile engines, although, there are correlations and differences among these three properties [3,18]. Austempering modifies the structure to getting more ductility with improved softness. Pereira et al. [13] and Putatunda [19] among others adopted the two-step austempering process on ADCI. Despite the spectrum of literature reports that are available on diverse research undertaken so far on the DCI; it is unpleasant that research reports [20,21] are very scarce on Al-DCI alloys and, austempered Al-DCI alloys. The combination of Al alloying and austempering on DCI is the major interest in the present work, due to various advantages acclaim to Al in the formation of AADCI. Al addition produces Al2O3 in the melt and forms a film layer on the outside surface of the casting, thus, provides the resistance against oxidation at elevated temperatures. The graphitizing potential of Al is next to silicon with a higher tendency to produce thin-wall castings of cast iron without eutectic carbide formation; thus, reducing the overall weight of the castings (cylinder block or cylinder head); owing to the density of aluminium and thinning the wall of the castings. Al addition provides aluminium complexes (more eutectic cells, higher strength) during solidification [21]. The work evaluates the production of aluminium base DCI alloy. It investigates the casting procedure, heat treatment processing parameter variations, the consequences of the processing parameter on the mechanical properties (toughness, strength, and hardness) of the derived Al-alloyed DCI, assessment of microstructures, phase transformations and interactions between the cast iron matrix and aluminium alloy.

Materials Fabrication and Preparation
The Al-DCI samples were produced in the foundry laboratory at the EMDI, Akure, Nigeria with high purity raw materials using the rotary furnace. The liquid cast iron was tapped into a preheated ladle containing Al chips and Fe-Si-Mg alloy at tapping temperature of 1500 °C for the Al alloying and nodulizing process; before the inoculation with FeSi addition in the same ladle as the mixture is agitated mechanically at 1400 °C before being finally poured into the sand moulds. Rectangular shape ingots were cast, solidified, cooled to room temperature in the mould as described by [22,23]. X-Ray Fluorescence (XRF) analyzer (Rigaku ZSX Primus II model) was used to determine the chemical composition of cast DCI samples as shown in Tab. 1.

Heat Treatment Procedures
In this study, the samples were charged into the salt bath (50% KNO3and 50% NaNO3) and slowly heated to austenitizing temperature (ɣTemp) of 850°C from 25°C room temperature, held at the ɣTemp for homogenization and austempered. The procedure for the austempering process is as shown in Fig. 1 which includes: i. steady heating of the samples in a molten salt bath (at the rate of 11°C/min to ɣtemps of 850 °C for 75 mins); ii. held at 850 °C austenitizing temperature for homogenization for 15 mins; iii. removal from the furnace and quickly quenched in another molten salt bath to avoid pearlite transformation within 1 -3 secs; iv. holding at the austempering temperature in the molten salt bath for complete isothermal transformation to ausferrite at 300, 350, and 400 °C for 90 mins; v. finally removed and cooled in still air.

Microstructure and Hardness Test
The heat treated samples for hardness test were sliced to10mm (breadth) x 10mm (thickness) x 12mm (length) using the automatic cutting machine (Brillant 220, ATM, Germany). The sample surfaces were prepared for hardness and microstructure analysis. Grinding was done using Aka-Allegan 9 plate after smoothing with SiC grit paper (P320) and fine poly suspension (Dia-max 9 m). To obtain a very smooth and mirror-like finishing surface, the polishing entails using an Aka-Moran plate with 3 m poly suspension and later, Aka-Chemal plate with 0.2 m fumed silica respectively on the automatic polishing machine (Saphir 550, ATM, Germany). The microstructural and phase analysis of the produced Al-DCI was characterized using TESCAN VEGA3 scanning electron microscopy equipped with energy dispersive spectroscopy (EDS) and PANalytical Empyrean diffractometer (PW1710), with monochromatic Cu target Kα radiation at 40 kV and 40 mA. The constituent phases of the Al-DCI were revealed using Highscore X'Pert Software. The hardness test was done for the produced Al-DCI samples by using the Atico India Brinell hardness testing machine. Each of the specimens was first cleaned from dust, dirt and grease, then, placed and securely mounted on the machine. The ball indenter was impressed with an applied load of 10N for a dwell period of 15 secs. The Brinell hardness scale was read after the pointer had come to rest. The indentation was viewed through a microscope and the diameter was measured with the attached micrometer. The procedure was repeated three times and average results were taken.

Tensile Strength Test Procedures
Three tensile test samples of gauge length of 28 mm with gauge diameter of 7 mm were machined from the same class each of DCI and Al-DCI alloys (M1, M2, M3, M4, M5, M6), and the tests were carried out following ISO 6892-1 (2016). The average values of results were obtained as the tensile strength of the alloys. A table-top universal tensiometer of Model KPL 2000-1 with a self-aligned Instron 8800 digital controlled panel was used to perform the tensile test. Test specimens were clamped on opposite ends on the Instron machine cramp and stretched at a slow constant rate until fracture.

Impact Toughness Test
The impact strength was determined with the aid of Tinius Olsen model JBN-300 capacity impact testing machine with a direct reading gauge. Three specimens each of size (10x10x50) mm with v-notch at the centre at an angle of 45° and a striking load of 75 kg weight was also used for each sample, and the average values determined in accordance with ISO 148-1 (2016) and JISG0202-1987specifications. To calculate energy E absorbed on the Charpy impact test, equation 1 was used.

Characterization of Samples
The effects of chemical composition on the ductile cast iron are similar to that of grey iron with quantitative and qualitative differences in the influence on graphite morphology. The presence of minor elements like Cr, Cu, Mg, Mn and Ca appreciably modified the microstructure of graphite morpholo-gy and matrix structure in that; they can encouraged the spheroidization of graphite [4,7,14,19] or negatively influence flake graphite shape [22] which is the single most vital feature responsible for the enhancement of the mechanical properties of cast iron as obtained in the study and illustrated in the microstructures. Fig. 1 shows the procedures for the single step austempering processes adopted in the study. M1iswas ascast DCI without Al addition, though, contains Al as a trace element (0.024%). M2 to M6 have an increasing amount of Al  (1.05, 1.575, 2.29, 3.02 and 3.74 wt.%) in the alloyed ductile cast iron produced [23] and were compared with the as-cast sample.

Microstructural and Phase Evaluation
The combined effects of Al addition and the austempering on the trends of microstructural transformations at different temperatures are presented in Figs. 2-5. The microstructures displayed even distribution of graphite nodules within the DCI matrices. This may be the right quality required in DCI material to have high impact strength, giving room for the applied stresses to be distributed evenly throughout the bulk component in addition to high material yield strength. According to Hernandez-Rivera et al. [25], as the treatment proceeds for 15 mins, the carbon content in the austenite increases to allow the formation of a more significant portion of stabilized austenite in the final microstructure. Graphite nodules appeared as dark round areas on the un-etched micrographs [6]. Microstructures in Fig. 2 affirms that post inoculation is a result-oriented method for DCI production; which its efficiency depends on the early nucleating potential of the melt. The ascast sample was not austempered, thus, Fig. 2a  The result revealed that besides the graphitic nodules within the DCI matrix; graphites are noticed to be precipitated along the grain boundaries in many considerable quantities in the Al-alloyed DCI alloys in Fig. 3b-f. The number of nodules is relatively small when 1.05 wt.% Al is added to as-cast DCI. This established the fact that Al presence substantially alters the graphite morphology and matrix structure through spheroidization as shown in Fig. 3d-e. At 350 °C austempering, the microstructure of the cast sample in Fig. 4a does not show a significant difference as with 300 °C. In Fig. 4a, [5,23].These properties are due to the characteristic of ADI microstructure, which consists of graphite nodules dispersed in a matrix of acicular ferrite and retained austenite ("ausferrite") [17,23]. The micrographs generally revealed the presence of nodularity of graphite and the morphology of pearlite. The ferrite regions around graphite nodules are due to the diffusion of carbon towards the graphitizing area [5]. Since a material's strength depends on its microstructure, the engineering processes can alter it. A variety of strengthening mechanisms such as workhardening, solid solution strengthening, precipitation hardening, and grain boundary strengthening alter the strength of a material and can be quantitatively and qualitatively explained. In all instances, the microstructure after the heat treatment consisted of graphite nodules and carbides within an ausferritic matrix.  The earlier works of Adebayo et al. [12,23] had reported the presence of theformation of some intermetallics compounds which showed the presence of Al-carbide phases in all the samples that contain Al as indicated in the form of intermetallic compounds of Al such as Al4SiC4,Al4C3 and MgAl2C2; in addition to Fe3C and Fe2C. This correlates and validates the carbide formation reported by Pimentel [26] on Niobium carbide and Chromium carbides (as eutectic of M3C) that were formed from the addition of Nb and Cr to cast iron. Other intermetallic compounds earlier identified includeMg2Si, Al2Fe3Si4, Al5FeSi, AlFe, AlFe3, Al2FeSi and Al8FeSi. Fig. 6 shows the hardness of M1-M6 austenitized at 850 °C and austempered at 300, 350 and 400 °C for 90 mins; the hardness increases with an increase in Al content and with a decrease in austempering temperature. In each instance, the combined effects of increased Al content and austempering temperatures produced greater hardness values on the Alalloyed DCI samples than the as-cast sample. The hardness   [27]. The austempered samples may also experience a high increase in hardness because of the formation of strain-induced [17]. The tensile test conducted was to establish the influence of the Al additions on the enhancement of the tensile strength of DCI. The test is a universal method to evaluate the mechanical properties of a material and give some clear information on the microstructure and chemistry (composition, phases) [28]. In this study, a tensile test was selected to analyze the qualities of cast iron products required in making automobile engine parts, relative to their tensile strength and greatly dependent on the carbon content. Fig. 7  ). However, the tensile strength increased with an increase in Al content. This is due to the precipitates of fine ferrite and pearlite with embedded graphite in the as-cast structure observed in different proportions. The more the precipitates, the more the strength increases. For the austempered samples, aside the influence of Al, the higher degree of under-cooling favour nucleation and growth of finer ferrite and austenite (finer ausferrite) resulting in higher yield and tensile strengths but at the expense of ductility. The impact toughness of the M1-M6 specimen was austenitized at 850 °C and austempered at 300, 350 and 400 °C for 90 mins are shown in Fig. 8 In the present result, the microstructure obviously influences mechanical properties and fracture behaviour. The degree of under-cooling has a pronounced effect on the volume of carbon diffusion. The austempering at a lower degree of under-cooling would enhance carbon diffusion. Thus, an increase in austempering temperature assists the carbon diffusion process to stabilize the non-transforming austenite; hence, the higher carbon content in the austenite (γcCc parameter). An increase in this parameter further improves the fracture toughness of the materials [9]. The presence of pearlite increases strength, hardness but reduces impact energy. Engineering design stresses obtained from the ultimate strength or yield point values of the materials give safe and reliable results only for the case of static loading. As illustrated in Figs. 6 to 8, error bars for each plot provide the upper and lower limit (maximum and minimum amount) of hardness (Fig. 6), tensile strength ( Fig. 7) and impact toughness (Fig. 8) obtained from each of wt.% Al values.

CONCLUSIONS
The conclusions that can be established in accordance with the obtained results are as follows: i. The microstructures revealed that the addition of Al into the matrix brings about the precipitation of ferrite around the graphite nodules. ii. The addition of Al to ductile cast iron increases the hardness and tensile strength of the alloys. iii. Austempering further improves the tensile and hardness properties of the alloy which increase with a higher degree of under-cooling. iv. Austempering also improves the impact toughness of the alloy but decrease with increase in the degree of undercooling.

Acknowledgement:
The authors express their sincere gratitude to the Centre of Nanomechanics and Tribocorrosion, University of Johannesburg, South Africa for providing the laboratory facilities for this research.