Effects of NaBF 4 + NaF on the Tensile and Impact Properties of Al-Si-Mg-Fe Alloys

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1 Effects of NaBF 4 + NaF on the Tensile and Impact Properties of Al-Si-Mg-Fe Alloys ZONGNING CHEN, TONGMIN WANG, YUFEI ZHAO, YUANPING ZHENG, and HUIJUN KANG NaBF 4 + NaF were found to play three roles, i.e., Fe-eliminator, grain refiner, and eutectic modifier, in treating A356 alloy with a high Fe content. The joint effects led to significant improvement in both tensile and impact properties of thus treated alloy. The multiple reactions between the NaBF 4 + NaF and Al-Si-Mg-Fe system are suggested to form Fe 2 B, AlB 2, and Na in the melt, as per thermodynamic analysis. The three are responsible for Fe removal, grain refinement, and eutectic modification, respectively. When NaBF 4 and NaF are mixed in weight ratio of 1:1, an optimum addition rate is in the range between 1.0 and 2.0 wt pct for treating AlSi7Mg0.3Fe0.65 alloy, based on the results of tensile and impact tests. Excessive addition of the salt may deteriorate the mechanical properties of the alloy, basically owing to overmodification of Si and contamination of salt inclusions. DOI: /s x Ó The Minerals, Metals & Materials Society and ASM International 2015 I. INTRODUCTION AL-SI-MG casting alloys of the type A356 (AlSi7Mg0.3) are most versatile in the automotive and aircraft industries, owing to their attractive combination of favorable properties. [1 3] Increasing demands on the properties of these alloys have pointed to the need of microstructural control via tighter specification of final compositions, solidification processing, and subsequent heat treatment. [4] Of the main impurity elements in aluminum alloys, iron is the most pervasive and detrimental one. [4 6] Given that its solubility in liquid Al is very high, Fe is readily dissolved during casting with steel tools as well as during recycling process with Fe-containing scraps. [7,8] The detrimental effects of Fe are ascribed to the low solubility in solid Al, which causes the formation of brittle acicular Al-Si-Fe intermetallic compounds (IMCs) upon solidification, therefore deteriorating the strength and ductility of Al-Si alloys. [5,9,10] Consequently, special approaches must be adopted practically to reduce the negative effects of Fe in aluminum foundries. Unlike wrought alloys that can be shaped in solid state, the mechanical properties of aluminum cast components produced to net or near-net shape depend strongly on their as-cast microstructures. [11] One approach to improve the mechanical properties of Al-Si ZONGNING CHEN, YUFEI ZHAO, and YUANPING ZHENG, Students, and TONGMIN WANG, Professor, are with Key Laboratory of Materials Modification by Laser, Ion and Electron Beams (Ministry of Education), School of Materials Science and Engineering, Dalian University of Technology, No.2 Linggong Road, Ganjingzi District, Dalian, , Liaoning, P.R. China. Contact tmwang@dlut.edu.cn HUIJUN KANG, Lecturer, is with Laboratory of Special Processing of Raw Materials, Dalian University of Technology, Dalian, P.R. China. Manuscript submitted September 23, casting alloys is to add chemical modifiers, e.g., Na, Sr, Sb, Ba, and Eu, which influence the eutectic morphology. [12 14] It has been reported that addition of a few hundreds ppm of Na modifies the morphology of eutectic Si from coarse plate-like to fine fibrous, consequently favoring the improvement of the strength and ductility of the as-cast materials. [15 17] Another approach that can be employed is through refinement of the grain structure. [18,19] It is known that a fine and fully equiaxed grain structure is desirable for both cast and wrought aluminum alloys, as it facilitates downstream processing and imparts advanced mechanical properties to the as-cast components. [20 22] In practice, Fe removal, grain refinement, and eutectic modification are conducted in separated processes. These processes may be incompatible to each other. For example, Al-Si alloys are not efficiently modified by Sr if they have been grain refined by B beforehand. [23 25] This work introduces a simple but effective method for the melt treatment of Al-Si alloys. The addition of NaBF 4 + NaF combines Fe removal, grain refinement, and eutectic modification into a one-step process. The mechanisms of these effects are also investigated. II. EXPERIMENTAL PROCEDURE A. Sample Preparation The starting material was a commercial ZL101 (low- Ti version of A356) alloy, the Fe content of which was adjusted to 0.65 wt pct using high-purity iron (99.99 pct Fe). The chemical composition of the experimental material is given in Table I. Figure 1 presents a schematic illustration of the entire process. Melting campaigns were carried out in a 600 g batch scale. The alloy was heated in a resistance furnace up to 1073 K (800 C) followed by the addition of Fe. The melts were

2 Table I. Composition of the Fe-Rich A356 Alloy Used in This Work (Weight Percent) Element Si Mg Fe Zn Cu Ca Ti Al Content balance Table II. The Amounts of Flux Salts Used in Different Trials Addition Amount of Flux (g) Fig. 1 A schematic illustration of the processing procedure. held at that temperature for 30 minutes to ensure dissolving of the Fe. After degassing and refining, the melts were transferred into another furnace which was set at 1033 K (760 C) for salt addition. The NaBF 4 + NaF (1:1 in weight ratio) salts, mixed with 5 g NaCl and 1 g KCl, were incorporated into the melts once their temperature dropped down to 1033 K (760 C). The melts were stirred thoroughly and then held for 10 minutes. Measures were taken to keep the temperature of the melts within 1033 K ± 10 K (760 C ± 10 C) throughout the whole procedure before casting. Finally, the surface sludge was skimmed out, and samples were cast with a permanent mold at 993 K (720 C). In the meantime, mini samples with a diameter of 25 mm were also taken from the melt for macrostructural examination. A total of six samples (S1, S2,, S6), with different addition rates of NaBF 4 + NaF mixed salt, have been produced. The experimental details on the amounts of various salts can be checked in Table II. B. Structure Characterization The specimens were sectioned at 10 mm from the bottom of the samples. The chemical compositions of the final experimental alloys were studied using X-ray fluorescence spectrometer (XRF). For microstructural examination, the specimens were mechanically ground, polished with 1 lm diamond paste and then etched with Keller s reagent. The etched surfaces were observed under optical microscope (OM) and field emission scanning electron microscope (FESEM). Image analysis was carried out on the optical micrographs using the Image Pro-Plus software. A backscattered electron image (BEI) mode equipped with energy-dispersive spectrometer (EDS) was used to characterize the Fecontaining IMCs in the alloys. To reveal the change in grain structure, the mini samples were etched with Poulton s reagent, following a standard polishing procedure. Quantitative analysis of the cast grain size was done using the linear intercept method. C. Mechanical Characterization Tensile and impact tests were carried out on the alloys in both the as-cast and T6 states. The tensile specimens Sample No. NaBF 4 NaF NaCl KCl S S S S S S were machined to a dog-bone type according to ASTM B557 standard, having a gage length of 25 mm and a diameter of 5 mm. The crosshead speed was 1 mm/min. The 0.2 pct proof stress (r 0.2 ) was taken as the yield stress. For each alloy, three specimens were tested for each condition, with average values being reported. Fracture surfaces of the tensile specimens were examined with FESEM. Impact test bars were cut from the castings and then machined to the required ASTM B108 specifications. Unnotched specimens were employed to perform impact tests, and a computer-aided instrumented Universal Impact Testing Machine was used for testing. For each alloy condition, five specimens were tested and an average value of the total absorbed energy was taken to represent the impact toughness. III. RESULTS A. Fe Removal The BEI micrographs of the NaBF 4 + NaF-treated Fe-rich A356 alloys are shown in Figure 2. It can be seen that addition of NaBF 4 + NaF was effective in eliminating Fe-rich IMCs. The microstructure of the as-cast AlSi7Mg0.3Fe0.65 alloy consists of A356 matrix and acicular AlSiFe phase (Figure 2(a)). The population of AlSiFe phase was gradually decreased with increase in the addition rate of NaBF 4 + NaF and its morphology changed from acicular to rod-like structures. No acicular AlSiFe was present when the addition rate exceeded 2.0 wt pct (Figure 2(d)). Figure 3 shows the EDS analyses, acquired in spot mode on the Fe-rich IMCs in the alloys. The EDS results confirm that the Fe-rich IMCs are basically b-alsife phase, corresponding to Al 5 SiFe or Al 9 Si 2 Fe 2, with a composition range of to at. pct Fe and to at. pct Si (Table III). It is also noted from Table III that the Fe content in the b-alsife phase shows a decreasing trend with the addition level of NaBF 4 + NaF being raised. Figure 4 depicts the Fe content of the alloy, measured by XRF, as a function of the addition level of NaBF 4 + NaF. It shows a noticeable decline in Fe content of the alloy, thanks to the

3 Fig. 2 SEM micrographs of (a) S1, (b) S2, (c) S3, (d) S4, (e) S5, (f) S6, showing the morphology and distribution of Fe-rich IMCs. addition of the mixed salt. The Fe content was decreased to ~0.25 wt pct, after adding 4.0 wt pct NaBF 4 + NaF of the melt. B. Grain Refinement The grain structure change with the addition of NaBF 4 + NaF is shown in Figure 5. Increasing the addition rate also produced an overall inverse trend to the grain size of the alloy. The average grain size of S6 was ± 26.7 lm, while that of the original alloy was ± 215 lm (Figure 6). This inverse relationship is ascribed to the reaction between NaBF 4 and Al, yielding dissociative B in the melt, which is deemed to be the most efficient grain refining element for Al-Sibearing alloys. [26] Of the several alloys produced with different addition levels of NaBF 4 + NaF, S2 appears to have the coarsest grain size, which is even an inferior grain structure with respect to the untreated alloy. This may be linked with the reduction in the Fe content of S2 and then the impaired growth restriction of a-al during solidification. While not directly obvious, grain refinement seems to have saturated after 3.0 wt pct NaBF 4 + NaF was added. Only a slight decrease in grain size from 223 ± 30.9 to ± 26.7 lm was achieved when the addition level was further increased to 4.0 wt pct a generally similar grain refining efficiency. This implies that grain refinement of the experimental alloy saturated at about 180 to 200 lm under the present solidification circumstances. C. Eutectic Modification Another important change that was brought to the microstructure of the alloy upon addition of NaBF 4 + NaF, was eutectic modification. This effect is asexpected because NaF is known to be a strong modifier for A356 alloys. Figure 7 shows the OM micrographs of the samples with SEM inserts highlighting the morphology change of Si particles with different addition levels. It is found that modification of the eutectic structure is very sensitive to the NaBF 4 + NaF addition rate. Figure 7(a) shows the as-cast microstructure of

4 Fig. 3 EDS analyses of the Fe-rich IMCs in the alloys (a) S1, (b) S2, (c) S3, (d) S4, (e) S5, (f) S6. Table III. Point Identification of the Phase Composition in the EDS Spectra of Fig. 2 Elements Content (At. Pct) Sample No. Al Si Fe S S S S S S non-modified alloy. Lamellar form of eutectic Si throughout the entire matrix is the predominant feature. At addition levels lower than 1.0 wt pct, the coarse, lamellar eutectic Si in the alloy gradually transformed into fine, fibrous network (the inset in Figure 7(c)) with increasing NaBF 4 + NaF addition rate. But an opposite trend with coarsening eutectic Si (the inset in Figure 7(e)) was observed at higher addition levels. In order to study the effects of the mixed salt on the modification level in each case, Si particle length and aspect ratio were chosen as the representative

5 parameters to quantify the modification of eutectic Si. Table IV summaries the two characteristics for each alloy. The average Si lengths of S1 and S3 were measured to be 44.8 ± 15.5 and 3.2 ± 1.2 lm, respectively, suggesting a 93 pct reduction in the average Si length by addition of 1.0 wt pct NaBF 4 + NaF. It may also be observed from the table that the aspect ratio was simultaneously reduced, a fact that can be taken to demonstrate the spheroidization of Si. With excess addition of NaBF 4 + NaF, the increases in Si particle length and aspect ratio are in agreement with the observations in Figure 7. This phenomenon is termed as overmodification. [26] D. Tensile and Impact Properties Benefitting from these changes in microstructural features, one may expect marked improvement in mechanical properties of the alloy. Figures 8(a) and (b) present the tensile properties (r 0.2, ultimate tensile strength (UTS), and elongation) of the samples in their as-cast and T6 states, respectively. In both states, the elongation maximized when 1.0 wt pct NaBF 4 + NaF was added, while the UTS was not observed to decrease until the addition rate exceeded 2.0 wt pct. A quality index, Q ¼ UTS þ 150 lgðelongationþ, was introduced to evaluate the tensile test data. [1] Q combines both strength and ductility, describing the comprehensive Fig. 4 The Fe contents of the alloys treated with different NaBF 4 + NaF addition rates. Fig. 6 The average grain sizes of the alloys plotted as a function of the NaBF 4 + NaF addition rate. Fig. 5 Macrostructures of the mini samples representing the grain structure of (a) S1, (b) S2, (c) S3, (d) S4, (e) S5, (f) S6.

6 tensile properties of a casting than either the UTS or the elongation alone. Table V lists the calculated Q values of the experimental samples in the as-cast and T6 states. The Q value of S3 after T6 treatment (389.5 MPa) is almost identical to that of S4 (394.5 MPa), despite their distinct UTS and elongation. S4 recorded a 45.8 pct improvement in Q compared with the untreated alloy. While tensile properties are the most popular method to assess the mechanical properties of aluminum alloys, the impact toughness is also of importance. Moreover, it can provide a useful estimation of the performance under conditions of rapid loading. Figure 9 shows the variation in impact toughness of the experimental alloys as a function of the salt addition levels. The toughness values of the alloys in the as-cast state tend to distribute between 7 and 15 J. Again the addition levels in the range of 1.0 to 2.0 wt pct led to the best impact results. A discernible drop occured when the addition rate of NaBF 4 + NaF exceeded 2.0 wt pct. These results may be attributed to the conjunct effects of Fe removal, grain refinement, and eutectic modification of the mixed salt. Table IV. Summary of the Si Particle Characteristics in the Samples of This Work Particle Length (lm) Aspect Ratio Sample No. Average SD Average SD S S S S S S Fig. 7 OM micrographs of (a) S1, (b) S2, (c) S3, (d) S4, (e) S5, (f) S6, in which the insets are the SEM micrographs showing the morphology of eutectic Si in each sample.

7 Likewise, the T6-treated samples have produced similar results, but with higher toughness values in the range of 9to18J. IV. DISCUSSION The one-step approach involves addition of NaBF 4 + NaF-based mixed salts to obtain Fe removal, grain refinement, and eutectic modification of Al-Si alloys. It strongly depends on a well-designed series of chemical reactions. The following reactions are supposed to take place after adding NaBF 4 + NaF into the Fe-rich aluminum melt: 3NaF(s) þ Al(l) ¼ AlF 3 ðsþþ3na(l) ½1Š 3NaBF 4 ðlþþ3al(l) ¼ Na 3 AlF 6 ðsþþ2alf3ðsþþ3bðsþ ½2Š AlðlÞþ2B(s) ¼ AlB 2 ðsþ ½3Š 2Fe(l) þ B(s) ¼ Fe 2 B(s); where s, l, and g in the bracket represent solid, liquid, and gas state, respectively. Summing Reactions [1] through [4], the overall reaction is 3NaF(s) þ 3NaBF 4 ðl) þ 5Al(l) þ 2Fe(l) ¼ Na 3 AlF 6 ðs) þ 3AlF 3 ðs) þ 3Na(l) þ AlB 2 ðs) þ Fe 2 B(s): ½5Š ½4Š Fig. 8 The variation in the tensile properties of the alloys with different salt addition rates in the (a) as-cast and (b) T6 states. Reaction [5] implies that Fe is removed in the form of Fe 2 B by settling, [5] AlB 2 is a potent nucleating substrate for Al-Si alloys, [27] and Na is known as a superior chemical modifier. [26] We now discuss the feasibility of Reaction [5] in terms of thermodynamic calculations. The Gibbs free energy, DG T, of Reaction [5] should be calculated according to DG T ¼ DG 0 T þ RT ln a Na 3 AlF 6 a 3 AlF 3 a 3 Na a AlB 2 a Fe2 B a 3 NaF a3 NaBF 4 a 5 ; ½6Š Al a2 Fe where DG 0 T is the standard Gibbs free energy at a given temperature, R = J/mol K, T is the temperature in K, and a is the activity of the substance in the system. For substances in solid state, a can be considered as 1. a NaBF4 is also taken as 1 because of its emulsification with metallic melt. a Na, a Al,and a Fe are approximately replaced with their mole fraction [Na] mole, [Al] mole, and [Fe] mole. Equation [6] now becomes DG T ¼ DG 0 ½NaŠ 3 mole T þ RT ln ½AlŠ 5 mole ½Fe Š2 mole : ½7Š Fig. 9 The variation in the total absorbed energy of the alloys with different salt addition rates in the as-cast and T6 states. In the case of this experiment, [Al] mole, and [Fe] mole can be calculated from their weight concentrations, Table V. Calculated Q Values from the Tensile Data of the Samples S1 to S6 Sample No. S1 S2 S3 S4 S5 S6 Q (MPa) As-cast T6-treated

8 which are recognized as and wt pct, respectively; [Na] mole is estimated from the NaBF 4 + NaF addition rate, x salt, as follows: It should be noted that the ZL101 alloy in the present work contained as much as ~300 ppm Ti. This level is not neglectable as the boride of Ti (TiB 2 ) is much more ½AlŠ mole ¼ ½FeŠ mole ¼ ½NaŠ mole ¼ 92:0519=27 ¼ 0:9264 ½8Š 92:0519=27 þ 6:9373=28:1 þ 0:2966=24:3 þ 0:6513=56 0:6513=56 ¼ 0:00316 ½9Š 92:0519=27 þ 6:9373=28:1 þ 0:2966=24:3 þ 0:6513=56. 0:5x salt 23 23þ :0519=27 þ 6:9373=28:1 þ 0:2966=24:3 þ 0:6513=56 ¼ 0:003235x salt: ½10Š DG 0 T can be calculated by Eq. [11]: DG 0 T ¼ DH0 T TDS0 T ; ½11Š where DH 0 T and DS 0 T are the standard enthalpy change and entropy change, respectively. Using the standard enthalpy of formation, D f H Kð760 CÞ, and standard entropy, S Kð760 CÞ data at 1033 K (760 C), as given in Table VI, DG Kð760 CÞ of Reaction [5] is calculated to be DG K ¼ 221:6kJ=mol: ½12Š stable than that of B (AlB 2 ). In this regard, TiB 2 should form instead of AlB 2 once B is reduced from NaBF 4.In this work, the addition level of NaBF 4 was in the range of 0.25 to 2.0 wt pct. Considering complete reduction of NaBF 4, the B level released into the melt should be in the range of 246 to 1969 ppm. 300 ppm Ti level will consume 136 ppm B to form TiB 2, according to the stoichiometric ratio of TiB 2. Based on this analysis, Substituting Eqs. [8] through [10] and[12] into [7], the practical Gibbs free energy of Reaction [5] at 1033 K (760 C) becomes DG 1033 K ¼ 267:1 þ 25:8 ln x salt kj=mol: ½13Š Table VII shows the calculated values of DG 1033K(760 C) in the cases of S1 to S6. The negative values of DG 1033K(760 C) indicate that Reaction [5] can occur spontaneously in the preparation of all samples. The calculations theoretically support the presence of Fe 2 B, AlB 2, and Na in the melt. Fig. 10 SEM fractographs of the AlSi7Mg0.3Fe0.65 alloy and EDS spectrum generated from the point of the needle-like IMC. Table VI. Thermodynamical Data of the Substances in Reaction [5] Substance NaF NaBF 4 Al Fe Na 3 AlF 6 AlF 3 Na AlB 2 Fe 2 B D f H Kð760 CÞ S Kð760 CÞ The data were derived from Ref. [29]. D f H Kð760 CÞ is in kj/mol and S0 1033Kð760 CÞ in J/mol. Table VII. Calculated DG 1033K(760 C) Values of Reaction [5] in the Cases of S1 to S6 Sample No. S1 S2 S3 S4 S5 S6 DG 1033K(760 C) (kj/mol)

9 Fig. 12 The total absorbed energy plotted as a function of the Q index for the alloys in different conditions. Fig. 11 The fracture surfaces of the tensile specimens (a) S3 showing well-defined dimples and (b) S6 with a number of inclusions. there will always be excess B in the melt to participate in the formation of AlB 2 and Fe 2 B. In the case of S1, where 0.25 wt pct NaBF 4 was added, only 110 ppm B was left and 136 ppm B had been converted into TiB 2, which is known to be a lessefficient nucleating particle when Si is present. This partly accounts for why the average grain size of the 0.5 wt pct NaBF 4 + NaF-treated alloy is a bit coarser than that of the original alloy, as shown in Figure 6. However, with the increase in the addition rate, more B will be supplied to form AlB 2. As a result, the grain size shows a decreasing trend when the addition rate of NaBF 4 + NaF exceeds 0.5 wt pct. The poor ductility is ascribed to the presence of acicular b-alsife IMCs. This is evidenced by the SEM fractograph of S1 as shown in Figure 10. The EDS result shows that the IMC contains at. pct Si and at. pct Fe, corresponding to the b-alsife with a composition range of 17.0 to 19.0 at. pct Si and 15.5 to 16.5 at. pct Fe. The acicular b-alsife IMCs lead to early fractures by creating notch effect during tensile test, and the ductility will thus be reduced. In the present work, the best mechanical properties were obtained when the addition of NaBF 4 + NaF fell in the range of 1.0 to 2.0 wt pct. The improvements in UTS and elongation happen to coincide with the change in Si particles of the alloys. This indicates that for Al-Si casting alloys, the substructural features, such as the Si size, are the prime controlling features in influencing the mechanical properties. [28] Figure 11(a) shows the fractograph of S3. Dimples across a transgranular fracture surface are the predominant features. The smaller Si particles act as barriers to dislocation movement. The enhanced strength is due to resistance to dislocation slip by Si particles, and the improved ductility is attributed to the enhanced dislocation accumulation. Deterioration of mechanical properties was observed when further increasing the addition rate over 3.0 wt pct. This can be attributed, only partly, to the overmodification of Si. Another factor is the presence of residual reaction product (Na-Al-F) in the final alloy, which is clearly observed from the fractograph of S6 as shown in Figure 11(b). To relate the tensile and impact properties, the impact toughnesses, represented by total absorbed energies of the as-cast and T6-treated samples, are plotted as a function of the Q index as shown in Figure 12. Most of the points, although some of which cluster in the range of Q 250 to 300 MPa, somehow exhibit a linear correlation between the impact toughness and Q values except for two points, as indicated by the circle. It should be noted that both of the two points are of S6, the alloy treated with 4.0 wt pct NaBF 4 + NaF. This is because impact property is more readily affected by variations in the microstructure, while not as sensitive as tensile properties to the inclusions. Thus, the impact toughness of S6 appeared an unexpected relatively higher value, in both the as-cast and T6 states. V. CONCLUSIONS In summary, we reported a new approach to treat Fe-rich Al-Si-Mg alloys by addition of NaBF 4 + NaF. The NaBF 4 + NaF play three roles, i.e., Fe-eliminator, grain refiner, and eutectic modifier. The combining effects improve the mechanical properties of the alloy with superior strength, ductility, and impact toughness, in both the as-cast and T6 states, but only when an

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