Properties of Magnesium Die Castings for Structural Applications

Transcription

1 Materials Forum () 5, 8- Properties of Magnesium Die Castings for Structural Applications T. Abbott and M. Easton Cooperative Research Centre for Cast Metals Manufacturing School of Physics and Materials Engineering Monash University Victoria, 38 Australia ABSTRACT The use of magnesium for automotive components has grown rapidly in recent years. While considerable R&D effort has focussed on improving elevated temperature and corrosion properties, there is also a need for a better understanding of the factors influencing mechanical properties such as yield and tensile strength, ductility and impact properties. The majority of magnesium structural components are high pressure die cast. The literature on the mechanical properties of common die cast magnesium alloys gives a range of values, and while some of the variation shows correlation with factors such as porosity, non-metallic inclusions and section thickness, the remainder of the variation does not appear to show any systematic correlation with process parameters. While the properties of high pressure die cast magnesium components are well suited to many structural applications, there are opportunities to further the use of magnesium through process improvements or new processes. Institute of Materials Engineering Australasia, Ltd () 8

2 . INTRODUCTION Use of magnesium die castings has increased at a rate of % per year over the last ten years. The automotive industry has been the primary driver for the rapid increase in uptake of magnesium die castings (Cole 995). This is primarily because of the potential fuel efficiencies and subsequent reduction in green house gas emissions that can be obtained through the replacement of conventional ferrous materials with light weight materials such as aluminium and particularly magnesium. The density of magnesium is two thirds of aluminium and less than one quarter of iron. Other benefits of magnesium alloys, when cast using high pressure die casting, include the ability to produce complex parts with thin walls, low soldering of dies and a high production rate. Temperature ( C) Mg 649 C (Mg) Alloy Range (Mg)+ Liquid 437 C (Mg)+ β Liquid Weight % Aluminium Figure. The Mg-Al phase diagram showing the composition range of common commercial magnesium pressure die casting alloys From (Okamoto 998) Last year in Materials Forum, researchers from the Co-operative Research Centre for Cast Metals Manufacturing (CAST) discussed the developments and challenges in the utilisation of magnesium alloys β (Dahle et al. ). The purpose of this paper is to discuss the process - structure - property relationships in Mg-Al high pressure die castings with a particular emphasis on the microstructural features obtained during casting and the effect of these features on the properties relevant to structural applications. This paper is divided into three main parts. A general section which reviews the properties of magnesium and the effects of alloying elements is followed by a review of literature on factors influencing the properties of high pressure die cast magnesium. The final section discusses process developments.. MAGNESIUM ALLOYS FOR DIE CASTING Mg-Al based alloys comprise the majority of all magnesium die cast components. This is because aluminium is a low cost alloying element, improves the strength and hardness of magnesium alloys, and improves the fluidity. The Mg-Al phase diagram is shown in Figure. The commercial alloys range from -9wt% Al. Increasing the Al content increases the solidification range and the castability of the alloy (Westengen 993). It also increases the amount of β-mg 7 Al phase present in the alloy which increases the hardness and the strength. Unlike Al-Si casting alloys, the Mg-Al casting alloys have compositions within the solid solubility of Al. This is because higher Al content alloys are too brittle due to the presence of large amounts of brittle β- Mg 7 Al phase (Westengen 993). Because of the rapid solidification rate in high pressure die casting, solidification occurs under non-equilibrium conditions. 8

3 µm µm (a) AM6 (b) AS µm µm (c) AZ9 (d) AE4 Figure. Microstructures of the common HPDC magnesium alloys. Therefore, solidification will be best predicted by the Gulliver-Scheil equation. Han et al () have predicted using this equation, that AZ9 will contain up to 6% eutectic while even AS may contain % eutectic phase. The eutectic forms in the inter-dendritic regions (Figure ). The alloy is highly segregated within the α-mg and reaches the solid solubility limit of approximately % towards the grain boundaries, which corresponds well with the EDS measurements of Dargusch et al (998). According to Figure, the solid solubility of aluminium in magnesium decreases rapidly with temperature. Therefore on cooling, β-mg 7 Al precipitates from the matrix super-saturated in aluminium with either continuous or discontinuous morphologies (Celloto and Bastow ). Unlike precipitation sequences in most other alloy systems there are no intermediate phases formed during solid-state precipitation in the Mg-Al system (Polmear 995). Whilst discontinuous precipitation is a feature of sand cast and permanent mould 83

4 Strength (MPa) cast Mg-Al alloys, it is not as prevalent in die castings due to the rapid solidification rate and since die castings generally do not undergo ageing treatments.. Properties of Mg-Al Alloys Figure 3 shows that the yield strength and tensile strength of high pressure diecast alloys increase with aluminium content. The elongation decreases with aluminium content. This is due to the increased concentration of Mg 7 Al at the higher aluminium contents AS AE Weight % Aluminium AM/AZ Alloys Figure 3. Variation of room temperature tensile properties with aluminium content for die cast AM series alloys and AZ9. Also plotted is data for AS and AE4. Solid symbols are tensile and yield strength, open symbols are % elongation. After Aune and Westengen (995). Increasing the aluminium content decreases the impact strength (Figure 4), probably due to increased volume fraction of interconnected Mg 7 Al. In unnotched specimens the impact strength remains relatively constant up to 6-7% Al, after which there is a dramatic decrease in the impact strength. For this reason, alloys containing 5-6% Al are used, i.e. AM5, AM6, for applications requiring good impact properties. 5 5 % Elongation Impact Strength (J) 5 5 Machined-In Notch Cast-In Notch Unnotched Weight % Aluminium Figure 4. The variation of impact strength of high pressure diecast AM series alloys with aluminium content. After (Aune, Westengen et al. 993). Increasing the aluminium content decreases the creep resistance (Figure 5). This has been attributed to the low melting point of Mg 7 Al, the presence of discontinuous precipitation and the poor effectiveness of Mg 7 Al in retarding dislocation motion (Humble 997). Increasing the aluminium content improves the corrosion properties of magnesium alloys to some extent; the main problem with corrosion is the presence of cathodic impurities such as Ni, Fe and Cu. More recent alloy designations such as AZ9D and AM6B have much lower tolerances for these elements and consequently much improved surface corrosion resistance, comparable to aluminium casting alloys (Makar and Kruger 993).. Minor Alloying Elements Zinc The most commonly used magnesium alloy is AZ9. It contains.5-% Zn as well as 9% Al. The zinc improves fluidity by increasing the freezing range. However, zinc additions between -5% may cause 84

5 hot-cracking (Foerster 975). Zinc additions also lead to a small increase in hardness and strength. The increase in strength may be due to zinc decreasing the solid solubility of aluminium, thereby increasing the content of Mg 7 Al (Celloto ) or by solid solution strengthening. Zinc may also combine with magnesium to form MgZn (Han et al. ). Creep Strain (%) AM/AZ Alloys AS AE4 AZ Aluminium Content (wt%) Figure 5. Variation of creep strain at 5 C, 5MPa and hours for die cast AM series alloys and AZ9. Also plotted is AS and AE4. After (Dargusch 999). Manganese Between.-.4% manganese is added to virtually all magnesium die casting alloys to improve corrosion resistance. Manganese has a high affinity with iron and is able to neutralise its detrimental effects to a large extent(makar and Kruger 993). The intermetallics formed settle to the bottom of the furnace causing an effective decrease in the content of iron in the casting. The presence of manganese has little effect on mechanical properties. Silicon AS and AS4 were developed by Volkswagen for use in engine components in which high temperature creep is problematic for AZ9 and AM series alloys. Figure 3 shows that AS has an order of magnitude lower creep rate than AZ9. Silicon also increases the yield strength and decreases the elongation (Figure 3). These effects are attributed to the presence of Mg Si. These alloys cannot be sand-cast because coarse Mg Si particles are formed at low cooling rates, which make the alloy brittle. Rare Earth Elements AE4 is an alloy containing 4%Al and % rare earth mischmetal for further improved creep resistance (Figure 5). The improved creep resistance is attributed to the presence of Al RE 3 at grain boundaries which decreases the rate of grain boundary sliding (Humble 997). The cost of rare earths makes these alloys relatively expensive. Also, rare earth containing alloys are more reactive in the molten state..3 Common Alloys, Properties and Applications The composition, properties and uses of the four most common die cast alloys are listed below (Table ). Most of the automotive structural components are made from AM6 and AZ9. A low cost, creep resistant magnesium alloy for powertrain and underthe-bonnet applications has yet to be developed, although there are some potential candidates and considerable research is continuing in this area. Until such a creep resistant alloy is produced, AM6 and AZ9 will remain the most commonly used Mg alloys. 85

6 Table. The compositions, properties and applications of the four most commonly used magnesium die casting alloys. Alloy Composition (wt%) Properties Applications AZ Al,.5-.Zn Good room temperature strength, excellent castability AM Al,.5Mn Improved impact properties/ductility, good castability AS.Al,.Si Improved creep resistance, poor castability AE4 4Al,. misch metal Good creep resistance, high cost, reactive melts General die casting alloy for most applications Applications that require higher crashworthiness, eg. steering wheels, IP beams Higher creep resistance, eg. some engine parts Under the bonnet automotive parts, transmission housings 3. PROCESS VARIABLES, STRUCTURE AND PROPERTIES OF HIGH PRESSURE DIE CAST MAGNESIUM While the intrinsic characteristics of alloys are important determinants of properties, the high pressure die casting process is also a major factor influencing properties. In some applications, especially where parts consolidation is a driving force for the use of magnesium, die castings are replacing components assembled from rolled steel. It is important that these castings are able to provide similar or better properties, such as energy absorption in crash situations, compared to the original components. The rapid injection of molten metal and the high cooling rates in high pressure die casting results in a fine microstructure and other distinctive characteristics. The microstructures, and hence properties, are amenable to some control through casting parameters. The high pressure die casting process has many parameters capable of influencing properties. These include: Melt temperature Shot velocity (low and high speed stages) Intensification pressure Die temperature Section thickness Distance from ingate Process parameters influence the microstructure which in turn influences the mechanical properties. In this section the microstructural features of high pressure die cast magnesium will be discussed followed 86

7 by a review of process parameter - microstructure - property relationships. 3. Microstructural Features The microstructure of high pressure die cast magnesium can be characterised in terms of a number of different features including the following: Porosity Externally solidified dendrites (ESDs) Skin Defect bands Non-metallic inclusions Porosity can arise either from dissolved gasses, solidification shrinkage and entrapped air and is typically in the range of -5%. The amount of gas porosity is influenced by metal temperature and the turbulence of filling, while the shrinkage porosity can be affected by casting designs that alter feeding behaviour and intensification pressure. Porosity can occur in a dispersed manner or concentrated in bands as discussed below. In cold chamber machines it is common for some solidification to take place in the shot sleeve prior to the commencement of the shot. The solid phase formed in this way is able to grow considerably larger than the material solidified during die filling, giving rise to a bimodal distribution of primary magnesium grains. The larger grains formed in the shot sleeve are referred to as externally solidified dendrites (ESDs). The amount of ESDs varies depending upon the metal temperature and other factors, and can range from none up to approximately % (Dahle et al. 999). Another common feature of high pressure die cast magnesium is the presence of a surface layer or skin. The skin is characterised by a higher volume fraction of second phases, Mg 7 Al in the case of Mg- Al alloys and, often, a finer microstructure (Mao et al. 999). The formation of the skin suggests the segregation of solute elements to the surface. This may occur due to the semi-solid state of the metal as it flows in the cavity. The depletion of solid phase adjacent to surfaces is a common phenomenon for flowing suspensions (McCormack and Crane 973). Alternatively the higher Mg 7 Al content may be a consequence of a faster cooling rate near the surface without enrichment in solute. Possibly the most significant features of high pressure die cast magnesium are bands of defects which, in most cases, form parallel to the casting surface. The make up of these bands seems to vary considerably and may consist of cracks, porosity (shrinkage and/or gas porosity) and /or segregation. Examples of defect bands are shown in Figure 6. The bands may be single or multiple and can sometimes intersect with the surface. A theory for the formation of these bands was proposed by Dahle and StJohn (999). They suggested that metal is in a semi-solid state during filling and that the solid phase migrates away from the surface and towards the centre. While this effect raises the solid fraction towards the centre, heat extraction through the die results in increased solid fraction near the surface. As a consequence there is an intermediate zone with relatively high liquid fraction. As solidification and filling proceeds, the centre and surface regions begin to develop some mechanical strength resulting in a localisation of shearing in the high liquid fraction region. This mechanism should result in the central region having a lower solute content than the outer region, which has been reported to be the case in at least one instance (Dahle et al. 999). 87

8 (a) (b) Figure 6. Example of defect bands (a) in a mm thick cast tensile bar, (b) in a commercial casting (3mm thick). Pore appear as white. Mao (999) observed bands of pores that appeared spherical suggesting they were due to gas porosity. The porosity was thought to have formed due to a sudden drop in pressure, possibly as a result of a small back movement of the plunger tip during intensification. This mechanism would appear only to explain instances of gas porosity and not shrinkage porosity or segregation. Rodrigo and Ahuja () investigated the effects of casting parameters on the formation of pore/segregation bands. Although they did not propose a formation mechanism they did assess the validity of the above mechanisms in light of their experimental findings. The model put forward by Dahle and StJohn (999) was disputed on the basis that the presence or absence of ESDs did not influence the appearance of the pore bands and that when ESD were present they appeared either side of the pore bands. However, Dahle and StJohn s theory (999) also applies to the situation where ESDs are not present and therefore it is not clear whether this is in fact contradictory evidence. The theory put forward by Mao (999) was also disputed as the observed porosity was due to shrinkage rather than gas. A patent application by Murray and Cope (999) suggests an alternative mechanism for the formation of these bands. Although they do not explicitly refer to bands, they claim, from microstructural examination, that in the runner and in sections of the casting where flow is uni-directional, flow occurs through a cylindrical section much smaller than the physical cross-section. The size of the cylindrical section appeared to be determined by the flow velocity, which typically was in the range of 4-65m/s. It was claimed that initially solidification occurred in the runner, reducing the effective cross section and increasing the velocity. Once a velocity of about 5m/s was attained, viscous heating prevented further solidification. It was claimed that because viscosity rises rapidly for solid percentages above 5%, the viscous heating process would stabilise the solid fraction of material entering the cavity. As this phenomenon was also observed in sections of the cavity where flow was unidirectional, this suggests that the bands of 88

9 defects occur where solidification was temporarily arrested by viscous heating. Rodrigo and Ahuja () also claim that pore / segregation bands are more defined in regions where flow is largely unidirectional. Stress (MPa) Stress (MPa) AZ9 % Elongation Yield Strength (MPa) Tensile Strength (MPa) 3 % Porosity AM5 % Elongation Yield Strength (MPa) Tensile Strength (MPa) 3 4 % Porosity Figure 7. Relationship between porosity level and mechanical properties for mm thick pressure die cast tensile samples. Alloys are AZ9D (top) and AM5 (bottom). From (Liu, Chen et al. ) Murray and Cope (999) also claim that once semi-solid flow is established in the runner, filling of the cavity occurs by the smooth progression of a semi-solid front. Apparently with the percentage solids remaining relatively constant during filling, allowing flow over large distances. This is despite the fact that the cross-section of the cavity is larger than the runner and therefore % Elongation % Elongation the velocity would be inadequate to maintain viscous heating. Other factors must then come into play to stabilise the solid fraction. One possibility is that the less turbulent nature of the flow reduces heat loss. These conditions may also allow for particle migration to the centre as proposed by Dahle and StJohn (999) giving rise to a solute enriched liquid at the edges. This liquid would resist solidification due to its lower freezing temperature (Dahle et al. ) and would produce a solute enriched region upon eventual solidification. In this case the defect bands would mark the location where the semi-solid flow regime was established, but not necessarily indicate that a flow rate of 5m/s was attained. Defect bands occur in a range of forms, such as cracks, bands of porosity (gas and/or shrinkage) and segregation. These all have superficial similarity, however it is not clear whether they form by the same process or whether multiple mechanisms are involved. Further investigations are necessary before their formation mechanism is fully understood. The effects of porosity on mechanical properties of pressure die castings are shown in Figure 7 for AZ9 and AM5 alloys. The level of porosity does not have a significant effect on the shape of the stress-strain curve but instead affects the point along the curve where failure occurs. Consequently, increasing levels of porosity reduce the tensile strength and elongation while yield strength is relatively unaffected (Elmahallawy et al. 998; Liu et al. ). Variations in porosity levels account for some of the property variations with section thickness as porosity levels sometimes increase with section thickness (Sequeira et al. 996; Rodrigo et al. 999). 89

10 Strength (MPa) TS (MPa) YS (MPa) %Elongation Cleanness (Brightness) % Elongation Figure 8. Effect of cleanliness (quantity of non-metallic inclusions) on mechanical properties (Haerle et al. 997). Non-metallic inclusions may also be regarded as microstructural features. They differ from the features described above in that they do not form within the die but rather are exogeneousoxide particles that were present in the molten metal. Control of molten metal handling practices is key to controlling their levels. The presence of non-metallic inclusions, such as oxides produced during melt handling, act in a similar manner to porosity, reducing strength and ductility, but having little effect on yield strength (Haerle et al. 997) (Figure 8). 3. Effect of Casting Design on Tensile Properties A number of factors relating to the design, or geometry of a casting, may influence mechanical properties. The factors considered here are section thickness and the effect of distance, and section changes with distance, from the ingate. The effect of section thickness has been studied for the most common alloys with data from a range of sources shown in Figure 9. There have been a number of different studies (Sequeira et al. 996; Sequeira et al. 997; Rodrigo et al. 999; Stich and Haldenwanger ) on the effects of section thickness on the properties of AZ9D. Some studies report an increase in yield and tensile strength with decreasing section thickness (Sequeira et al. 996; Stich and Haldenwanger ). However, this is not always the case (Rodrigo et al. 999) and when results from different studies are combined, a wide range of values is observed as shown in Figure 9. In the case of tensile strength and elongation, the combined results do not show any clear trend. In the case of yield strength, the values are confined to a more narrow range and a slight increase in thinner sections is still evident in the combined data (for AZ9). A number of studies attempt to relate the formation of a surface layer to mechanical 9

11 properties (Sequeira et al. 997; Mao et al. 999; Rodrigo et al. 999). Metallographic examinations of castings often reveal a surface layer with finer microstructural features and, in the case of AZ9 at least, a higher volume fraction of second phase particles (Sequeira et al. 997). Other characteristics reported for the surface layers include a higher hardness and a lower volume fraction of voids. The higher hardness contributes to increased yield strengths in thinner sections if, as reported in some papers, the surface layer thickness is partially independent of section thickness (VanFleteren 996). Other reports, however, suggest that the surface layer thickness can vary in proportion to the section thickness (Mao et al. 999). The wide spread of tensile strength and elongation values suggests that casting quality issues have a more significant effect on these properties than section thickness. Variations in casting quality (amount of porosity and casting defects) are suspected when the shape of stress-strain curves (and hence yield stress) remain largely unchanged, but the point along the curve where failure occurs changes (and hence tensile strength and elongation vary together). Round bars tend to have better properties, with yield and tensile strength and elongation values at the high end of the range for flat samples (see Figure 9). In summary, section thickness may be of some value in predicting yield strength of AZ9 but not for tensile strength or elongation. One investigation (Rodrigo et al. 999) has taken this assessment further and found that even after the effects of porosity are corrected for, section thickness is still not a good predictor of these properties. Yield Strength (.%Proof) (MPa) % Elongation Tensile Strength (MPa) Section Thickness (mm) Section Thickness (mm) 8 4 Sequeira et.al. (996) Rodrigo et.al. (999) Stich () Round Bars (Various) Section Thickness (mm) Figure 9. Effects of section thickness on tensile properties of pressure die cast AZ9 alloy from various sources. Results from Sequeira et al. (996) (squares) and Rodrigo et al. (999) (circles) are from cast test bars. The dashed lines are from Schindelbacher and Rosch (998) and represent results from tensile specimens cut from a stepped casting. The results presented by (Stich and Haldenwanger ) appear to be from the same source as (Schindelbacher and Rosch 998). The results from round cast tensile bars are from Aune and Westengen (99), Aune and Westengen (995) and Sequeira et al. (996). 9

12 Density (g/cm 3 ) Cross-Section (mm ).6 8m/s 7MPa 8m/s 3MPa 6m/s 7MPa 4m/s 7MPa Distance from Ingate (mm) Figure. Effect of distance from ingate and section size on porosity in cast test bars of alloy AZ9D for various gate velocities and intensification pressures, from (Elmahallawy et al. 998). Although the results indicate an increase in porosity with distance from ingate, they are influenced by the increase in section size (shown by the dashed line) associated with the grip sections. As the distance from the ingate increases, the ability to feed solidification shrinkage decreases, so porosity levels will tend to increase as shown in Figure (Elmahallawy et al. 998). This effect is exacerbated if the cross sectional area increases with distance from the ingate. In cast-to-size test bars the section increases at the grip sections relative to the central test region. Consequently, the grip section closest to the overflow tends to have relatively high levels of porosity. A higher applied pressure during solidification tends to reduce the severity of this effect (Elmahallawy et al. 998). 3.3 Effect of Casting Parameters on Microstructure, Porosity and Properties Studies into the effects of a number of casting parameters have been reported (Gutman et al. 997; Renaud et al. 997; Sequeira et al. 997; Elmahallawy et al. 998; Gutman et al. 998; Rodrigo and Ahuja ). The parameters include: metal injection rate or velocity, applied pressure during solidification, die temperature, metal temperature, venting gate area relative to ingate, cavity fill time and ram position at the start of the high speed shot. The metal injection rate or velocity has several variants. All the papers examined quoted this in units of velocity (m/s or equivalent). In some instances the velocity of the ram is quoted, while in others the flow velocity through the ingate is used. The casting cycle usually has a low velocity stage and a high velocity stage and the effects of both have been reported. In one paper an intermediate velocity is reported (Renaud et al. 997). Other papers refer to an injection rate which suggests a volumetric measure, however, the units quoted are velocity (m/s) (Gutman et al. 997; Gutman et al. 998). The values 9

13 reported for injection rate do not match typical gate or ram velocities so it is unclear what they refer to. The different casting parameters appear to interact strongly and it is therefore difficult to separate out the effects of individual parameters. The metal injection rate or velocity sometimes increases (Sequeira et al. 997; Gutman et al. 998) and sometimes reduces (Gutman et al. 997; Renaud et al. 997; Elmahallawy et al. 998; Gutman et al. 998) porosity (see Figure, Figure ). In one report where porosity increased the effect was attributed to increased gas entrapment (Sequeira et al. 997) (Figure ). It is likely that, in cases where the effect is opposite, solidification shrinkage induced porosity is more significant. % Porosity or % Elongation mm Flat test bars Gate Velocity (m/s) % Elongation Tensile Stress Stress (MPa) % Porosity Yield Stress % Porosity or % Elongation mm Flat test bars Stress (MPa) Gate Velocity (m/s) Figure. Effect of gate velocity on porosity and tensile properties of AZ9 (Sequeira et al. 997). 93

14 % Porosity % Porosity AM 5m/s AM 8m/s Solidification Pressure (MPa) AS4 4m/s AS4 8m/s Solidification Pressure (MPa) % Porosity AM5 4m/s AM5 8m/s Solidification Pressure (MPa) AZ9 6m/s AZ9 8m/s Solidification Pressure (MPa) Figure. Effects of gate velocity, solidification pressure and alloy on porosity (Elmahallawy et al. 998). % Porosity Increases in applied pressure during solidification usually reduce porosity (Elmahallawy et al. 998). However, one report shows the reverse for AZ9(Elmahallawy et al. 998) (Figure ). The effects of die and metal temperature are also variable, with one report suggesting that porosity levels reach a maximum level at certain temperatures (Gutman et al. 997) (Figure 3). Another report shows that porosity decreases with increasing metal temperatures (Renaud et al. 997). An investigation into the relative effects of different parameters (Renaud et al. 997) found injection velocity to be the most significant factor for porosity but not significant for strain at fracture. The only parameter which showed a significant effect on both porosity and strain at fracture was metal temperature. Other discrepancies between the effects on porosity and properties have been reported. In one study (Sequeira et al. 997) where porosity continued to increase with injection velocity the mechanical properties initially increased then decreased (Figure ). The poor properties at low injection velocities (and low porosity levels) were attributed to poor surface finish. 94

15 % Porosity Injection Rate m/s % Porosity Casting Temperature (C) % Porosity Mold Temperature (C) Figure 3. Effects of Injection rate, casting temperature and mould temperature on porosity (Gutman et al. 997). Reports on the effects of casting parameters on other microstructural features are scarce. One report (Rodrigo and Ahuja ) investigates the effects on defect bands. The intensity of this band seems to be affected in an opposite manner to total porosity. The intensity of the defect band increased with applied pressure and die temperature and decreased with slow shot speed (see Figure 4). The effects of changes to defect bands on mechanical properties have been investigated to a limited extent. An investigation (Sannes and Westengen 998) using as-cast test bars found little effect of casting parameters (shot profile and melt temperature) on defect bands or mechanical properties. However, the aluminium content was shown to strongly influence defect bands, with the bands becoming less well defined as the aluminium content was increased from AM levels through AM5 and AM6 to AZ9 levels. For AM5 alloy, a slight effect of the distance of the defect band from the surface on fracture elongation was noted. The spread of elongation values was large with an extreme range of 6-7% observed. In order to attain the highest elongation values it was necessary for the defect band to be close to the centre of the section (Figure 5). It was noted that material between the defect band and the surface was sound while the material contained within the band had distributed porosity. The increased proportion of sound material would account for the improved ductility results for samples with defect bands closer to the centre. 3.4 Comments on Casting Parameter - Structure - Mechanical Property Relationships The literature data demonstrates that considerable variation in properties occurs for a given high pressure die cast magnesium alloy. For example, the tensile strength of AZ9D can vary over a range of nearly MPa. While the literature demonstrates that a range of property values can occur, it does little to quantify the factors contributing to this range. This is partially due to there being a limited number of investigations, and partly due to the various factors interacting in a complex manner making them difficult to separate. The few currently discernible parameter - property relationships include: 95

16 8 8 Total Porosity Rating 6 4 Rating 6 4 Pore/Segregate Band Intensity Total Porosity Pore / Segregate Band Intensity Slow Shot Speed m/s Rating Die Temperature ( C) Defect Band Intensity (Intensification ON) Total Porosity (Intensification ON) Defect Band Intensity (Intensification OFF) Total Porosity (Intensification OFF) Figure 4. Effects of casting parameters on the distribution of porosity (concentrated in pore bands vs randomly distributed) (Rodrigo and Ahuja ). Tensile strength and elongation tend to decrease with increasing porosity and oxide inclusion levels. Tensile yield strength increases with decreasing section thickness for thicknesses less than 3mm. Porosity levels tend to increase with distance from the ingate, particularly if there is an increase in section size. Defect bands, which are highly prominent in metallographic examination, do not appear to have a strong effect on tensile properties. However, this assessment may give a misleading impression of their role in actual magnesium components. In tensile bars the bands are orientated parallel to the stress direction which would minimise their effects. In an automobile crash situation, magnesium components would be subject to a range of stress conditions including bending. In bending the defect bands would be subject to shear forces and consequently their influence is likely to be more pronounced. A second factor is the orientation of defect bands. In cast tensile bars they tend to form parallel to the surface. In actual components this is not always the case as 96

17 the example in Figure 6 shows. In situations where they intersect the surface their effects on mechanical properties may be more pronounced. % Elongation % of Section Outside of Defect Bands Figure 5. Effect of defect band location on elongation for as cast tensile bars (Sannes and Westengen 998). For applications where it is critical to achieve optimal mechanical properties it is important that porosity, oxides and presumably also defect bands, be reduced or eliminated. The presence of porosity can also have an influence beyond that of mechanical properties. For example, it limits the possibilities for welding and heat treatment of die castings. With a view to overcoming some of these limitations alternative casting processes have been developed and adopted. These processes may expand the potential end uses of magnesium and are summarised briefly in the next section. 4. CASTING PROCESS DEVELOPMENTS The magnesium high pressure die casting process has many advantages (Mordike and Ebert ) which has allowed it to become the most commonly used magnesium component manufacturing process. Process improvements to high pressure die casting such as vacuum pressure die casting has the potential to enhance properties and expand further the applications of magnesium. Alternatively a number of other processes are currently in use or under development. These include: Squeeze casting Thixocasting / Thixoforming Thixomolding Rheocasting The squeeze casting process introduces liquid metal into a die at high pressure but at a lower speed. The high pressure ensures feeding of solidification shrinkage and prevents porosity. There are two types of squeeze casting processes. The direct process introduces the liquid into an open die which is then closed and pressurised. In the indirect process the melt is introduced using a shot sleeve and piston arrangement (Hu 998). Considerable property improvements are possible with squeeze casting, especially in ductility as shown in Figure 6. The thixoforming or thixocasting process uses a pre prepared billet that is heated to a semi-solid state, transferred to a shot sleeve and then forced into the die cavity. In the semi-solid state the billet forms a globular solid phase that allows flow to occur when subjected to shear. This material flows smoothly into the cavity, reducing air entrapment, and, as it is already partially solid, solidification shrinkage is also reduced. In addition to reduced porosity levels, the thixocast microstructure is also significantly different to high pressure die cast magnesium due to the globular nature of the 97

18 solid phase. In the case of AZ9, the globular magnesium rich phase is surrounded by a network of Mg 7 Al containing eutectic phase. It is claimed that this structure exhibits an elastic modulus more than % higher than high pressure die cast magnesium (Suk et al. ). Thixomolding of magnesium is a process similar to plastic injection moulding. Magnesium chips are used as feed material. The chips pass into a reciprocating screw, are heated to a semi-solid state, and moved into the shot accumulator via a non return valve (Walukas, LeBeau et al. ). The solid fraction is usually lower than for thixocasting and porosity levels are between those of high pressure die casting and thixocasting. Typical porosity levels for thixomolding are -.7% (Peng and Hsu ). A major advantage of the thixomolding process is the elimination of molten magnesium handling facilities and it is especially suited to operations already experienced in plastic injection moulding. The rheocasting process is similar to thixocasting, except that the semi-solid feed is generated by cooling a liquid rather than reheating a solid billet. This process has existed since the 97's. However, it is the least developed process for magnesium. It is now being actively developed by a number of organisations (Peng and Hsu ), (Hall et al. ). 5. CONCLUSIONS The use of magnesium die castings has grown considerably in recent years, particularly for automotive applications. Considerable research has been undertaken to improve the critical properties of corrosion and elevated temperature creep resistance. However, mechanical properties, such as yield and tensile strength, ductility and impact properties are also critical, especially in structural components. While a number of studies have investigated the factors influencing the properties of high pressure die castings it would appear that much of the property variability is unpredictable. Further investigation is necessary to understand the formation and role of microstructural features such as defect bands in determining properties. This is especially true for situations relevant to in-service conditions where the actual stress is often not purely tensile. Stress (MPa) % Elongation UTS (MPa) Yield Strength (MPa) Elongation (%) 4 Sand cast Gravity die cast Pressure die cast Squeeze cast Figure 6. Mechanical properties of as cast AZ9 for various casting processes (Hu 998). 98

19 While factors such as porosity, non-metallic inclusions, section thickness and distance from ingate show some influence on mechanical properties it is probably unlikely that simple correlations with properties will be found for other factors (for example metal temperature). This is due in part to the inherent complexity in the filling and solidification of industrial castings. While our understanding and optimisation of magnesium high pressure die casting continues to improve, there is also considerable opportunity to improve component properties through the use of newer processes, particularly those employing semi-solid processing. However, high pressure die casting will probably remain the process of choice for most components as it combines good properties with efficient, low cost production and can be used for complex thin walled components. ACKNOWLEDGEMENTS The authors wish to thank their colleagues in the CRC for Cast Metals Manufacturing (CAST) for valuable discussions and for input on this manuscript. CAST was established under the Australian Government's Cooperative Research Centres Scheme. REFERENCES Aune, T. K. and H. Westengen (99). Mechanical Properties of Pressure Die Cast Mg-Alloys. Magnesium Alloys and Their Applications. Aune, T. K. and H. Westengen (995). Magnesium Die Casting Properties. Automotive Engineering 3(8): Aune, T. K., H. Westengen, et al. (993). Mechanical Properties of Energy Absorbing Magnesium Alloys. Celloto, S. (). TEM Study of Continuous Precipitation in Mg- 9wt%Al-wt%Zn Alloy. Acta Materialia 48: Celloto, S. and T. J. Bastow (). Study of Precipitation in Aged Binary Mg-Al Alloys and Ternary Mg-Al-Zn Alloys using 7 Al NMR Spectroscopy. Acta Materialia 49: 4-5. Cole, G. S. (995). Magnesium in the automotive industry. Die Casting Management October: Dahle, A. K., Y. C. Lee, et al. (). Development of the as-cast microstructure in magnesium - aluminium alloys. Journal of Light Metals (): 6-7. Dahle, A. K., S. Sannes, et al. (999). Optimisation of the Quality of High Pressure Die Cast Magnesium Alloys. Automotive Alloys, TMS. Dahle, A. K. and D. H. StJohn (999). The Origin of Banded Defects in High Pressure Die Cast Magnesium Alloys. NADCA Conference Cleveland. Dahle, A. K. and D. H. StJohn (999). Rheological Behaviour of the Mushy Zone and Its Effect on the Formation of Casting Defects during Solidification. Acta Materialia 47(): 3-4. Dahle, A. K., D. H. StJohn, et al. (). Developments and Challenges in the Utilisation of Magnesium Alloys. Materials Forum 4: Dargusch, M. (999). Department of Mining, Minerals and Materials Engineering. Brisbane, University of Queensland. Dargusch, M., G. L. Dunlop, et al. (998). Elevated Temperature Creep and Microstructure of Die Cast Mg-Al Alloys. Magnesium Alloys and Their Applications, Verksoff- Informationsgesellschaft, Wolfsburg Germany. 99

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