Melt flow velocity in high pressure die casting: its effect on microstructure and mechanical properties in an Al Si alloy

Transcription

1 : its effect on microstructure and mechanical properties in an Al Si alloy D. R. Gunasegaram* 1, B. R. Finnin 1 and F. B. Polivka 2 Analysis of as cast tensile specimens of Australian alloy CA313 (an equivalent of A380) made using the high pressure die casting process at three different melt flow velocities has revealed that increasing melt velocity resulted in a finer microstructure and a reduced pore volume fraction and, consequently, better mechanical properties. Mechanical properties are shown to increase more than proportionately with reducing pore volume fraction and the reason for this is demonstrated to be the contribution from the more refined microstructure. The gate used in the investigation was unusual in that its geometry encouraged and prolonged the shearing of the melt. It is proposed that the microstructure was refined by flow shear, the rate of which increased with melt velocity and consequently enhanced the fragmentation of the externally solidified a-al grains and inclusions such as oxides, cold flakes and gas bubbles. Keywords: Al Si alloy, High pressure die casting, Mechanical properties, Melt flow velocity, Microstructure, Porosity List of symbols C d coefficient of discharge (ratio) P pressure on the melt, MPa DP pressure differential across restriction, MPa V velocity of plunger, m s 21 V gate velocity of melt at the gate, m s 21 r density (kg m 23 ) Introduction High pressure die casting (HPDC) is an established cost effective route to making high volume parts such as automobile chassis, power train components and light weight housings. A majority of light alloy castings are therefore made using this technology. Despite its many advantages, HPDC has traditionally suffered from an inherent defect in the form of porosity caused by entrapped gases and shrinkage. Rejects due to porosity add a substantial cost to the process in some cases. With increasing quality standards demanded of die castings, this cost continues to grow. Porosity has also limited the use of high pressure die castings for certain applications, for example: (i) where solution heat treatment and/or welding are/is required: due to surface blistering that is caused by the expansion of gases entrapped within pores just beneath the casting skin 1 CSIRO Manufacturing and Materials Technology, Private Bag 33, Clayton, South MDC, Vic. 3169, Australia 2 CSIRO Manufacturing and Materials Technology, Locked Bag 9, Preston, Vic. 3072, Australia *Corresponding author, dayalan.gunasegaram@csiro.au (ii) where castings experience cyclic loads in service: it is becoming increasingly clear that in the absence of gross defects such as cold shuts, fatigue properties of aluminium alloy castings are primarily determined by the size of pores and oxide films. 1,2 The subject of porosity reduction has been addressed in various ways in the past. High integrity die casting processes 3,4 that address the porosity issue have also emerged in the past few decades. These include vacuum die casting, squeeze casting, pore free die casting, semisolid metal working that includes thixocasting and the recently developed rheodiecasting (RDC). 5,6 However, little scientific effort has been expended in pushing the boundaries of process parameters currently in wide usage in the HPDC industry. In the present work, therefore, the authors aim to determine, within the bounds of existing cold chamber HPDC equipment, whether reduced porosity levels can be achieved through the employment of process parameters that are different from those used conventionally. More specifically, the authors study the effect of the melt flow velocity on the microstructure and mechanical properties of tensile specimens made using the workhorse secondary alloy CA313 (an A380 equivalent). Literature review Aluminium alloys Karban 7 has shown in his literature review of 11 previous works that six of these conclude that higher melt velocities increased porosity, two conclude exactly the opposite, another two point to an optimum melt velocity and the last found no relationship between melt ß 2007 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 30 August 2006; accepted 23 November 2006 DOI / X Materials Science and Technology 2007 VOL 23 NO 7 847

2 velocity and porosity. Karban s investigation itself showed that a lower plunger velocity (1. 7ms 21 v. 3. 4ms 21 ) produced less porosity. However, the melt velocities used are unknown since neither the gate velocity nor the gate area was mentioned. Three published works not reviewed by Karban 7 are examined below. Studying the effect of flow velocity (20 70 m s 21 at the gate) on rectangular plate castings of mm of alloy A380 made using a typical HPDC fan gate of mm cross-section, Ghomashchi 8,9 made the observation that while increasing flow velocities reduced pore volume fraction and dendrite cell size, there was an optimum velocity (y55 m s 21 )on either side of which the average pore size increased. Ghomashchi 8,9 did not measure mechanical properties. Niu et al. 10 also cast rectangular plates of mm of alloy DIN226 (AlSi9Cu3) with gate velocities of m s 21 using a typical HPDC rectangular gate of mm cross-section. They reported that increased gate velocities improved mechanical properties by keeping the melt within the gate molten for a longer duration, helping feed the casting better. These researchers determined through metallography that this was because the more rapid flow was able to wash out with it the partially solidified grains on the edges of the gates, thereby increasing the effective gate area. No attempt was made by the researchers to quantify porosity. The conclusions of Niu et al. 10 are in agreement with those of Davis and Robinson 11 who found similar metallographical evidence for gate blockage in aluminium alloy BS313 (9. 2%Si) at lower gate velocities, or lower effective coefficients of discharge C d. This coefficient is a measure of the hydraulic efficiency of the melt flowing under given process conditions and is defined as the ratio between the actual flowrate and the ideal flowrate. Davis and Robinson 11 also found that the porosity content in the casting decreased from y5 to 0. 5% as C d increased from 0. 1to0. 8. Their flat casting ( mm) was fed by a fan gate of mm. Meanwhile, in the mainstream literature that the industry largely follows, gate velocities of.40 m s 21 are seldom recommended for aluminium alloys (see Ref. 12) even though much higher velocities are achievable on many current production die casting machines. This is because a faster flowing melt may cause erosion of the die cavity resulting in shorter die life and occasional soldering at the gate resulting in lost productivity. In summary, there is currently no consensus in the literature regarding the effect of melt velocity on porosity or the mechanical properties of Al Si high pressure die castings. Non-aluminium alloys Recent research by Pitsaris et al., 13 Kittel-Sherri 14 and Klein 15 on high pressure die cast commercial magnesium alloy melts (gate velocities of m s 21 ) concluded that the mechanical properties of die castings improved with increasing gate velocities. In particular, Pitsaris et al., 13 who carried out their research on the same die that was used for the present work, found the increase in mechanical properties to be accompanied by a reduction in average pore size. However, the total pore volume fraction was noticed to increase with rising velocities. In relation to these results, it must be mentioned that Pitsaris et al. 13 calculated pore volume fractions in the entire bulk of the casting rather than in the gauge length alone as in the present work. The more homogeneous dispersion of pores was identified by them as the reason why the increased pore volume fraction did not adversely affect the mechanical properties. Pitsaris et al. 13 also observed a-mg grain sizes to decrease with increasing velocities and attributed this to the shearing of the externally solidified crystals at the gates and dendritic remelting. Indeed, reference to the shearing of crystals that externally solidified, e.g. in the shot sleeve and upstream stages of the runner, at the gates is not new. Such a possibility was mentioned by Robinson and Murray 16 following their work with zinc alloys which also found that increasing melt velocities refined and rounded the a grains. They also determined that pore volume fraction in zinc die castings reduced with an increasing effective discharge coefficient C d. In summary, unlike for aluminium die casting, investigations into magnesium and zinc die casting appear to have generally concluded that increasing melt velocities improve mechanical properties. Gap in knowledge In addition to the conflicting results reported above for aluminium die casting, a recent book 3 on high integrity die casting ignores flow velocity as an influencing variable in an equation presented for the calculation of percentage porosity. This further indicates that the influence of flow velocity on porosity and, therefore, on the resulting mechanical properties, remains largely unreported in the mainstream literature. The possibility that higher melt velocities could reduce the amount and size of porosity, and so present an option to T6 heat treat die castings, while potentially increasing their fatigue life also, warrants an objective investigation into the influence of melt velocity on microstructure and mechanical properties of high pressure die cast Al Si alloys. In addition, since shear at the gates has been identified by some researchers as a cause of finer grains, it is desirable to use a gate geometry that accentuates this advantageous effect. In justifying an investigation which included melt velocities that were outside the envelope of current industrial usage, the authors have taken into consideration the existence of techniques 17,18 that circumvent the potential erosion and soldering issues associated with higher melt velocities in traditional high pressure die casting. Experimental A basic knowledge of the high pressure die casting process, described in detail elsewhere, 3,12,19 is assumed in describing the experimental conditions below. The castings were made on a Toshiba horizontal cold chamber die casting machine with a locking force of 2450 kn (250 tonne). The plunger of 50 mm diameter had a stroke of y280 mm when the sprue post length and biscuit (or wad) thickness was taken into consideration. The commercial CA313 alloy (US equivalent A380) with liquidus and solidus temperatures of 580 and 520uC respectively was injected into a hardened H13 steel die with three cavities to make two round tensile test specimens and a flat tensile test specimen during each shot 848 Materials Science and Technology 2007 VOL 23 NO 7

3 1 Spray from tensile specimen die: COP stands for change over position, grip ends G are locations from where specimens were obtained for comparison of porosity (see Fig. 9) (Fig. 1). Flat specimens were used to determine the composition of the alloy while the round specimens were used for tensile testing. The composition of the CA313 alloy determined using optical emission spectroscopy is Al 8. 77Si 0. 77Fe 2. 89Cu 0. 20Mn 0. 26Mg 0. 09Pb 0. 03Ni 0. 65Zn 0. 03Ti 0. 02Sn 0. 02Cr 0. 05Bi (wt-%). The three gates (see Fig. 1) were rectangles of 56 3 mm (width6height). The land (length) of these gates was 12 mm. The relatively narrow and longer-thanusual gate geometry was designed to encourage greater shear of the flow and to protract the shear length. The three nominal gate velocities investigated were 26, 48 and 82 m s 21, and were obtained by varying the second stage plunger velocity. The first stage plunger velocity was 0. 2ms 21 for all scenarios investigated. When the melt front was passing through the zone marked COP (change over position) in Fig. 1, the second stage plunger velocity was activated. The three different average second stage plunger velocities were 0. 61, and m s 21 respectively. Since the COP zone where the plunger acceleration commenced was located sufficiently in advance of the gate location, the plunger had reached the desired second stage velocities by the time melt entered the gates. The plunger stroke during the second stage was y33 mm, corresponding to the displacement of y165 g of melt past the COP. The total weight of castings and overflows was 90 g, which meant that y75 g of the runner was also filled during the second stage. The shot weight, which is equal to the weight of the dosed melt, was 440 g. The accumulator pressure used was MPa and the intensification (multiplication) pressure available to the melt was of the order of 120 MPa. Intensification was activated within 15 ms of the end of cavity fill. Two coordinates of the PV 2 linear graph 12 describing the capacity of the die casting machine (where P is the melt pressure in MPa and V 2 is the plunger velocity squared in m 2 s 22 ) were (0 m 2 s 22, 52 MPa) and (27 m 2 s 22, 0 MPa). Part of this graph is shown in Fig. 2. Evacuation of the gases from the die cavity during the filling stages was facilitated through vents that connected the overflows and both the grip ends of each of the castings to atmosphere. The vents were y0. 1mm thick so as to allow only gases to pass through. The furnace was maintained at 650uC. The die temperatures, measured using K type thermocouples, varied between the following limits: uC (moving 2 Measured pressure at plunger tip v. square of measured plunger velocity, along with part of PV 2 graph of machine half) and uC (fixed half). The measurement point on either die half was y5 mm away from the runner, at the end of the region marked COP (Fig. 1), and at a depth of 15 mm measured from the die parting plane. The die was sprayed with lubricant for 1 s after ejection of each shot, following the opening of the die 12 s after end of cavity fill. Several variables, including pressure at the plunger tip (which is equal to the pressure on the melt in contact with the tip), plunger velocity and plunger position, were measured in real time. 20 Obviously, the tip pressure also provided an indication of the intensification pressure transferred to the melt. The simulation of the solidification behaviour carried out using the Magmasoft software package indicated that in the 3 s elapsing between ladling of the melt into the shot sleeve and the start of plunger movement, the solid fraction had reached 12. 5% which is typical of industrial die casting. For purposes of the simulation, the shot sleeve preheat temperature was taken to be 150uC. Given that a temperature drop of y10uc has been observed 21 during ladling of melt into this shot sleeve, the melt initial temperature was assumed to be 640uC. The heat transfer coefficient between the shot sleeve and the melt was taken to be 2500 W m 22 K 21. The dimensions of the tensile specimens (Fig. 3) and the tensile testing procedure were in accordance with Australian Standard 1391 of 2005, with the exception of the gauge length section diameter which was 5. 6 mm, slightly higher than the mm recommended. The gauge length was 25 mm as prescribed. The as cast tensile specimens were tested at room temperature on a screw driven Instron machine with a crosshead speed of 2 mm min 21 (an initial strain rate of s 21 ). A total of 15 random samples were tested for each of the three gate velocities. These numbers were in addition to the four that failed prematurely during the tensile test. 3 Dimensions of round tensile specimens tested, mm Materials Science and Technology 2007 VOL 23 NO 7 849

4 5 Photograph of fracture surface, typical for all flow velocities investigated 4 Typical traces for nominal stress v. nominal strain with samples taken from each of three flow velocities studied: points of fracture were indicated by arrowhead Metallographical examination was carried out using an optical microscope on samples polished and etched with a 0. 5%HF solution. In quantifying grain size, grain count and pore volume fraction, the averages from five images (individual frame size mm taken at 1006 magnification) at equal intervals from the centreline of the longitudinal section of the gauge length for each specimen were calculated. A total of 15 specimens for each of the three gate velocities were analysed. The counting was carried out with the use of ImagePro Plus image analysis software. a Aluminium grains and pores of 2. 5 mm equivalent diameter and smaller were ignored in the analysis due to a lack of resolution at that level. Grain sizes were reported in terms of equivalent diameters calculated from the area of each grain. Results Hydraulic efficiency v. melt velocity Hydraulic efficiency C d is an indication of the resistance experienced by the melt during flow. The C d values shown in Fig. 2 were calculated from the relationship: C d 5V(r/(2DP)) 1/2. This is a modified form of the Bernoulli equation where, in practical terms, C d is the ratio between the actual flowrate and the theoretically expected flowrate. C d, which is reduced by vena contracta and frictional losses due to surface roughness and viscosity, has been used in the past 11,16 to quantify the filling efficiency of HPDC runner systems. This is in spite of the fact that the highly turbulent nature of the melt hardly satisfies the streamline flow conditions (zero radial velocity) assumed in the derivation of the Bernoulli equation. It is apparent from Fig. 2 that the pressure required to push the melt through the gate increased with greater melt velocities as expected. A similar trend detected in C d values is however noteworthy and will be discussed below. Mechanical properties v. melt velocity Typical traces of nominal stress v. nominal strain from tensile testing are shown in Fig. 4. Since the area under each curve is representative of the amount of work per unit volume that can be absorbed without causing the material to rupture, these data indicate that the specimens made at 82 m s 21 absorbed roughly twice as much energy before fracture compared to those made at 26 m s 21. Fracture surfaces (Fig. 5) show a brittle, intergranular fracture for all velocities. Typically, micropores were noticed on the fracture surface. These were not necessarily clustered. The variation of mechanical properties with melt velocity is presented in Fig. 6. Note that all error bars used in this work show 95% confidence intervals on either side of the plotted population mean for both x and y variables. It is clear from Fig. 6 that in all cases, the mean values of the properties increased with increasing gate velocities. The increases in the ultimate tensile strength (UTS), 0. 2% offset proof strength (yield strength) and elongation with increase in gate velocity from 26 to 82 m s 21 were 50 MPa (12%), 13 MPa (8%) and 1. 3% (72%) respectively. In general, scatter in the data is seen to decrease with increasing velocities, suggesting that the properties became more repeatable at higher melt velocities. a ultimate tensile strength; b 0.2% proof strength; c elongation. 6 Arithmetic means of mechanical properties v. measured gate velocities based on 15 tensile tests for each velocity 850 Materials Science and Technology 2007 VOL 23 NO 7

5 7 Pore fraction (%) in gauge length v. measured gate velocity At this juncture, a comparison of properties obtained at 26 m s 21 gate velocity with typical properties published for the as cast CA313 alloy die castings is appropriate. According to the Australian Die Casting Association Handbook, 22 the UTS, 0. 2% offset proof strength and elongation in an as cast condition are 270 MPa, 160 MPa and 2% respectively. It is obvious that the values reported in the present work at 26 m s 21 (274 MPa, 168 MPa and 1. 8% respectively) are consistent with those in the handbook. Such agreement is not necessarily expected because mechanical properties determined from different sources are known 13 to vary with process conditions such as melt temperature, intensification pressure and die temperature, while elongation may also change with specimen dimensions. 23 Pore volume fraction and pore size v. melt velocity The variation of pore volume fraction determined in the gauge length of the cylindrical tensile specimens with gate velocity is shown in Fig. 7. The pore volume fraction from the gauge length permits a more accurate correlation with mechanical properties than a bulk pore volume fraction obtained from the entire casting. 24 Significantly, the castings with a pore volume fraction of 0. 40% (at 82 m s 21 ) belong in the realm of high integrity die castings (below 0. 5% porosity) as described by Ji et al. 5 Figure 7 demonstrates that the pore volume fraction reduced significantly as the gate velocity increased from 26 to 48 m s 21, while a further increase to 88 m s 21 had less effect. The changing porosity levels are also visible from a comparison of photographs of specimens made at 26 and 82 m s 21 gate velocities (Fig. 8). These confirm that increasing melt velocities led to reduction of overall porosity level, rather than simply displacing porosity from the gauge length to other locations such as grip ends. Metallography of the centreline regions of the grip ends connected to the overflows (Fig. 9) showed that both porosity levels and the size of pores decreased with increasing melt velocity. Further evidence for the reduction in pore size is found in Table 1 where the percentage of smaller pores (,20 mm) is found to increase with velocity at the expense of larger pores (.100 mm) while the proportion of intermediate size pores remains practically the same. The authors have not attempted to 8 Photographs of typical longitudinal sections of round specimens: 26 m s 21 (left) and 82 m s 21 (right), with circle on latter showing evidence of semisolid mush pushed in at high pressures during intensification stage metallurgically discriminate the shrinkage pores in Fig. 9 from gas pores. Mechanical properties v. pore volume fraction The effect of pore volume fraction on mechanical properties is graphically shown in Fig. 10. The sensitivity of mechanical properties to pore volume fraction increased as the pore volume fraction decreased. This may indicate that factors other than the reduced pore volume fraction have also contributed towards improved mechanical properties. Microstructure v. melt velocity Typical microstructures obtained from the centreline of the gauge length are shown in Fig. 11, which should be read in conjunction with Fig. 12. The cumulative percentage areas occupied by a-al grains counted on five photographic frames ( mm each and at 1006) taken at equal distances on the centreline of the gauge length are plotted in Fig. 12 as a function of grain diameter. (It may be noted that the total area occupied by the a-al grains in the matrix remained unchanged at y60% for all velocities.) The data plotted in Fig. 12 were obtained from investigating 15 specimens for each of the three flow velocities. The lighter a-al grains set in a darker background of the eutectic (Fig. 11) are bimodal in that some of these are large and dendritic while the rest are very small and non-dendritic. The dendritic grains are most prevalent at the lowest velocity. The smaller a grains occupied a greater percentage of the total a grain area as melt velocity increased (Fig. 12). Because the parameters determining the large grain formation (pouring Table 1 Area occupied by pores of given size as fraction of total pore area Pore size (mean diameter), mm Gate velocity, m s Total Materials Science and Technology 2007 VOL 23 NO 7 851

6 9 Close-ups of grips at far side of gates (marked G in Fig. 1) for three gate velocities investigated: actual width from left to right in pictures equals 20 mm a 26 m s 21 ; b 48 m s 21 ; c 82 m s Typical microstructures from near centre of gauge length for three nominal gate velocities investigated (CA313 etched with 0.5% HF solution) a ultimate tensile strength; b 0.2% proof strength; c elongation 10 Mechanical properties v. pore fraction temperature and dwell time in the shot sleeve) were independent of filling velocity, this result indicates that as melt velocity increased, the larger a-al grains were reduced to smaller grains by some mechanism operating during the casting process. The reduction in pore size with increasing velocity (Fig. 9) is found to be in line with the refinement in a-al grains (Figs. 11 and 12). It may be pointed out that the size of the majority of primary particles (Fig. 12) at 82 m s 21 melt velocity is comparable to those obtained from rheodiecasting, 5,6 where the melt is prepared in an inline twin screw slurry maker which imparts both shear and turbulence, but is 852 Materials Science and Technology 2007 VOL 23 NO 7

7 12 Cumulative percentage area occupied by a aluminium grains as function of grain size finer than those obtained from many other semisolid processes. 3 The average percentage of eutectic in the matrix (Table 2) was determined using photographic frames of the microstructure in the centreline of the gauge length (e.g. Fig. 11). A total of 20 frames were used for each of the three melt velocities investigated. The eutectic fraction remained unaffected by melt velocity at y40%, although eutectic colonies were expected to become finer as velocity increased (Fig. 12). The fact that the eutectic fraction did not vary could be expected since alloy composition remained independent of melt velocity. Unsurprisingly therefore, the result is in agreement with the observation of Ghomashchi, 8,9 although he found only a 30% eutectic fraction. The eutectic fractions obtained in both the present work and that of Ghomashchi 8,9 were less than that expected from the phase diagram. The reason for this, at least in the present case, was that the measurements were made in the centreline region of the gauge length where a-al grains were dominant. The eutectic fraction, however, was visually observed to increase monotonically towards the casting skin region where it was higher than the a-al fraction. Discussion Hydraulic efficiency v. melt velocity Potential reasons for the increase in C d (Fig. 2) are that the melt flow through the runner was non-newtonian (shear thinning) in nature due to fragmented grains flowing with increased ease compared with dendrites and/or that the resistance presented by the runner reduced as melt velocity increased due to diminished amounts of materials solidifying along the runner walls. Significantly, porosity levels in the casting decreased with increasing C d (see Fig. 5), in agreement with earlier Table 2 Comparison of eutectic fraction (area-%) for different melt velocities investigated shown with twice the estimated standard deviation Melt velocity at gate, m s 21 Eutectic fraction, % works on aluminium 11 and zinc 16 alloys. It is probable that the reduced coating of the runner walls that resulted in a higher C d also facilitated a more effective transfer of the subsequently applied intensification pressure to the casting cavity, thereby increasing feeding efficiency. Consequently, die casters seeking reduced porosity would probably benefit from operating a given die at the highest possible melt velocity the machine could manage, provided other considerations allow this. Mechanical properties v. melt velocity The improvements in mechanical properties with increasing melt velocity (Fig. 6) may be attributed to a reducing pore volume fraction (Figs. 7 9) and a refinement of the a-al grains (Figs. 11 and 12). The observed greater dispersion of finer pores and other defect-forming inclusions such as oxides and cold flakes at higher velocities is likely to have resulted from increased turbulence and a more finely atomised spray of melt entering the casting cavity. (Atomisation is the result of liquid layers getting peeled off from a liquid jet due to the aerodynamic forces caused by its relative velocity to the ambient gas.) This would have further assisted in improving UTS and elongation. The improvement in 0. 2% offset proof strength, which is a bulk property, may also be related to a greater refinement of a-al grains at higher velocities. The area weighted average grain diameter (calculated from raw data used to plot Fig. 12) reduced from 33 mm at26ms 21 to 29 mm at48ms 21 and 21 mm at 82 m s 21. The R 2 statistic between these grain diameters and melt nominal velocity was a statistically significant This meant that the variation in the grain diameter was described by a linear relationship with the melt velocity and, consequently, the rate of melt shear at the gate. The R 2 statistic between the grain diameters and 0. 2% offset proof strength, elongation and UTS were , and respectively, showing varying degrees of dependence of mechanical properties on the area weighted average grain size. The graphs in Fig. 6 indicate that the principle of diminishing returns operated. There is therefore an optimum melt velocity for any given runner system at which the advantage of improving properties and the disadvantages of using higher melt velocities balance out. The smaller error bars associated with ordinate values of Fig. 6 indicate that the properties became more repeatable at higher melt velocities, in agreement with previous work 14 on Mg alloys. Since improved repeatability in commercial production is becoming an important requirement 25 towards reducing the cost of quality (including inspection, warranty and timely delivery), the present results suggest that the use of higher melt velocities is one of the factors that the die casters could consider as a potential solution. It may be concluded from this work that higher melt velocities not only improved mechanical properties in high pressure die castings, but also made the properties more repeatable. It is therefore suggested that the conventional understanding that higher melt velocities are always detrimental to part quality because of the risk of turbulence (see Refs. 3 and 4) should be questioned. Materials Science and Technology 2007 VOL 23 NO 7 853

8 Pore volume fraction and pore size v. melt velocity Pore sizes undoubtedly decreased with increasing melt velocities in the present work. Pore volume fractions also followed the same trend (Fig. 7). It is unlikely that the amount of entrapped gases decreased with increasing melt velocities. This is because the same amount of gases was present in the runner and casting cavity at the start of the second stage and less time was available for venting with higher velocities (11 ms at 82 m s 21 v. 33 ms at 26 m s 21 ). Consequently, the reduced levels of absolute porosity are mainly attributable to an increased level of feeding enjoyed during the intensification stage. Figure 8b shows evidence of mush being pushed into a casting under high pressures for the 82 m s 21 case and this proof is substantiated by increased plunger movements recorded during the intensification stage. The averages of the actual total stroke of the plunger recorded for the 26, 48 and 82 m s 21 melt velocities were , and m respectively, with every m in the displacement of the plunger amounting to 1. 1% of the volume of the melt dosed. The better feeding in the higher velocity case is perhaps attributable to, inter alia, one or both of the following: (i) the shorter duration of the fill in the higher velocity case and the greater turbulence resulted in the melt solidifying less on the walls of the runner and gate regions during filling. Since the deposit on the walls was narrower, a wider path was available for the intensification pressure to be transferred to the solidifying casting as (ii) velocity increased the higher impact pressure of the plunger at the end of cavity fill in the higher velocity case perhaps fragmented networks of larger dendrites along the runner and gate more effectively, in addition to helping clear up the narrow gate region of dendritic debris and other solid inclusions. This allowed a wider path for the intensification pressure. The impact pressures recorded for the 26, 48 and 82 m s 21 melt velocities were 46, 55 and 71 MPa respectively. The fact that castings with markedly improved mechanical properties were obtained with traditional HPDC equipment is a significant result. This indicates that die casters may not always need to invest in additional capital equipment in search of castings with reduced porosity. In addition, although.5000 Al alloy and Mg alloy shots had been made with the same die, the authors did not come across any soldering at the gate, where the velocity was highest. Even though the number of shots made in the authors die to date is relatively small, any serious soldering problems with this die are not envisaged in its lifetime. This is because it has been shown 26 that as long as the ratio between the surface area of the die/melt interface and the volume of the melt in the zones in question (in the present case gates) is sufficiently large, the die material surrounding the melt channel would not experience temperatures required for soldering. Furthermore, any erosion of the gate or casting cavity regions were not noticed. Therefore, commercial die casters may do well to assess the use of higher melt velocities on a case by case basis where quality demands are greater. Accordingly, they could also appraise the use of runner design techniques such as the ATM philosophy 17,18 that circumvent the issues of soldering and erosion traditionally associated with higher melt velocities. It should be pointed out that while the effect of melt velocity on pore volume fraction is in agreement with many past works in both the aluminium and nonaluminium categories as highlighted in the literature review, it is at odds with some investigations focusing on aluminium alloy HPDC. A tentative reason for this might be that the venting efficiency in the present case and those in agreement was higher than that of the other works, resulting in less gas entrapment in comparison. This speculation may form the hypothesis of a future investigation. Mechanical properties v. pore volume fraction As mentioned before, mechanical properties increased more than proportionately as the pore volume fraction reduced (Fig. 10). This trend is strikingly similar to that obtained by Caceres and Selling 24 when plotting true tensile strength and true elongation to fracture in their Al 7Si 0. 4Mg (A356. 0) alloy as a function of area fraction of defects on the fracture surface. The trend observed shows that because the higher melt velocities reduced the pore volume fraction to very low levels, the influence of additional factors became more pronounced. These factors were most likely: a more homogeneous dispersion of pores and/or the refined a-al grains. Microstructure v. melt velocity It is clear that increasing melt velocity multiplied the number of smaller a-al grains at the expense of the larger grains (Fig. 12) and through this refinement contributed to the improvements in mechanical properties. These results appear to confirm the opinion of Robinson and Murray 16 that shear at the gates would break down the larger grains into smaller, rounded forms. It is also plausible that the increased melt velocity removed more dendritic fragments, which can act as nuclei, off the runner and die walls during the filling stages, thereby increasing the nucleating sites in the casting cavity and as a consequence aiding in the refinement of the a-al grains. Increasing shear rate also may have contributed to the refinement of gas bubbles trapped earlier in the shot sleeve and/or the runner. Refinement of gas porosity and its greater dispersion might have been further aided by a finer atomisation of the flow at the higher velocities. It is therefore proposed that increased shear of suspensions (such as externally solidified dendrites, oxides picked up from the furnace and elsewhere, cold flakes created in the shot sleeve and the runners, and gas bubbles entrained at the shot sleeve) at the narrow gate during injection caused the refinement of the general microstructure as flow velocities increased. An investigation into the size of oxides in a related but unpublished work revealed that oxide particles downstream of a narrow gate were smaller than those upstream of it, lending weight to the shear theory. It is therefore suggested that all potential defect forming suspensions, including the oxides, became less harmful by being reduced in size. Mechanical properties have a greater dependence on internal defects of random 854 Materials Science and Technology 2007 VOL 23 NO 7

9 Table 3 Comparison of flow shear rates at gates and their influence on mechanical properties Mechanical properties Work Alloy Gate aspect ratio Average gate velocity, m s 21 Average nominal flow shear rate at gate UTS s 21 % change* MPa 0. 2% offset proof strength Elongation % change* MPa % change* % % change* Current CA Niu et al 10 DIN226S Not measured *Between the lowest and highest velocity tested. orientation when these are larger and may occasionally lie in an unfavourable orientation. In this latter case, the mechanical properties would reduce markedly and otherwise not. The unpredictability of the orientation of the larger defects was the reason that mechanical properties such as UTS and elongation were less repeatable at lower melt flow velocities. The bulk property 0. 2% offset proof strength, however, would have benefited from a refined a-al grain structure (Fig. 12) according to the well known Hall Petch relationship as melt velocity increased. The shear rates experienced in the present work and those from a previous work 10 on an Al Si alloy are shown in Table 3 along with the resulting mechanical properties. For purposes of calculating average shear rate, which is the velocity gradient in the melt flow, the average velocity at the gate is used. In spite of the many differences observed between the works, including the vastly dissimilar aspect ratios of the gates used (1. 7 in the present work v ), the fact that properties improved with increasing shear rates emerges as a common result. Conclusions Cold chamber high pressure die castings with markedly improved mechanical properties have been made using traditional HPDC equipment and a commercial workhorse Al Si alloy. The improvements were made possible by the employment of melt injection velocities higher than those normally practised in the industry. Along with improved mechanical properties, the repeatability of the properties was enhanced by the use of the higher melt velocities. Increasing gate velocities were shown to result in a reduction in the size of pores as well as an increase in their dispersion, a reduction in the pore volume fraction, and a reduction in the number of larger grains and a concomitant increase in the number of smaller grains in the gauge length of the tensile piece castings. It was inferred that an increased feeding efficiency during the intensification stage was the reason for the improvement in the absolute pore volume fraction at higher flow velocities. Mechanical properties were shown to improve more than would be expected from the reduced pore fraction alone and the reason for this was postulated to be the contribution from the more refined a-al grains at higher melt velocities. A higher rate of flow shear was put forward as the cause of the refined a-al grains observed. 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