Prevent Banded Defects in High-Pressure Diecast Magnesium Alloys.
Magnesium alloys offer attractive properties to diecasters, particularly a high strength-to-weight ratio, which makes them particularly ideal for components requiring low weight. For automotive applications, there is a resurgence in interest and research on magnesium, and high-pressure diecasting is the most attractive manufacturing route for these parts. Diecast magnesium also is receiving significant interest from the electronics industry where it can be used in housings for appliances such as mobile phones and notebook computers. With a heat content significantly less than molten aluminum and offering better fluidity, magnesium is suitable for production of intricate parts in high production. Also, since molten magnesium is not aggressive toward ferrous components, the containment materials of the casting operations are simpler and cheaper to construct. However, the surface of molten magnesium requires protection from burning in a reaction with atmospheric oxygen.
The fluidity of molten magnesium makes it highly suitable for production of components with long and thin sections. Examples of such applications include steering wheels, instrument panels and seat frames, and even smaller section thicknesses are made in notebook computer and mobile phone housings. Upon inspection of such castings, it is clear that the material often contains a significant amount of porosity. More significant is the presence of a continuous layered defect, or band, following the contour of the castings in thin sections or at large changes in cross-sectional areas, such as the transition from a thin to a thicker section. The appearance of the band may vary from being heavily segregated, which does not pose a major problem, to containing a high level of porosity to the extent of appearing torn.
Understanding the solidification processes is vital for the development of improved castings and alloys. At present, the production and use of high-pressure diecast components is growing rapidly, and they are increasingly being used in new applications. This is particularly the case with magnesium, and thus there is a substantial need for further understanding to allow process improvements and optimization. Examples include the importance of the casting's skin on mechanical properties in high-pressure diecast AZ91 and the importance of the [beta]-[Mg.sub.17] [Al.sub.12] component in this layer in improving corrosion resistance. The development of means to avoid the formation of undesirable bands of defects is another example of the type of required research.
Banded Defect Examples
Banded defects have been observed under a variety of casting conditions, alloys and metals (Fig. 1), including one commercial casting in a recent study. The casting consisted of different test bars for measurement of mechanical properties, and a cylindrical test bar in the center of the casting was studied. Inspection of the cross-section showed that it was possible to manipulate the location of the resulting ring--it could be moved closer or further from the die walls by either changing the casting conditions or alloy. It also was shown that the amount of porosity could be changed, for example, from being filled completely with segregate without porosity to showing full delamination of the outer skin and the central core. Several rings were observed in the same casting section.
Figure 2 shows an example of bands of porosity observed in a commercial component cast with a 319-type aluminum alloy by cold chamber high-pressure diecasting. The high level of porosity indicates that the banded defects were formed in the later stages of solidification when interdendritic feeding is difficult.
Research has been undertaken to develop an understanding of this phenomenon, and this article outlines a model that can be used to understand the formation of the bands. Based on this understanding, it is possible to suggest preventive measures to avoid this defect. The model explains both the location and appearance of the bands (the relative amounts of segregation, porosity and tearing), the alloy dependence and the effect of casting conditions. The rationale behind the formation of the banded defects lies in consideration of the solidification behavior in high-pressure diecasting and the resulting properties of the partially solidified material. The interplay between the solidifying material and the flow patterns during filling and feeding of the casting determine the material's response. The theory is not limited to any particular material or casting process, although bands of defects are more likely to appear in pressurized castings, such as high-pressure die castings, squeeze castings and semisolid castings.
Solidification in high-pressure diecasting has not received significant research attention. This is partly a result of the relative complexity of this process, which involves high filling rates and pressures and rapid cooling. With recommended fill times of 0.02 sec for small die castings of magnesium, a rapid second stage of injection and a final pressure intensification step, it is difficult to describe and study the actual solidification processes that occur. Solidification, therefore, is inherently difficult to study and cannot be reproduced easily in laboratory-scale experiments.
Most alloys solidify over a temperature range that results in the co-existence of solid and liquid for a shorter or longer period during casting depending on the cooling rate and the solidification range. One result of this is that solidification in cold chamber diecasting often occurs in two stages. First, there is relatively slow partial solidification during filling and holding in the shot sleeve before the shot is applied. Heating of the shot sleeve offers a way to reduce, or even avoid, this stage. Second, the remaining liquid solidifies in the die. Premature solidification in the shot sleeve, which may reach a solid fraction of about 20%, means that the cold chamber diecasting process has some similarity to the emerging semisolid forming processes. The resulting microstructure therefore often displays a bimodal grain size.
Upon application of the shot where the metal is injected into the cavity at speeds up to 20-50 m/sec, the solid particles in the slurry move away from the areas of highest shear. This is a common phenomenon observed in the flow of slurries. Figure 3 shows how coarse equiaxed dendrites formed in the shot sleeve have concentrated in the center of the cross-section of a magnesium high-pressure die casting. Plug flow of a central region containing a high solid fraction can be seen from this picture.
In most diecasting processes, the metal starts solidifying on contact with the relatively cold die walls. Therefore, a solid layer enclosing the partly solidified metal often is formed. Depending on the injection rate, the runner and gating system, and the geometry of the casting, it also is possible that the molten metal stream will not fill the cross-section, and the stream will ricochet back from the end of the casting, often the overflow. This backward flow may occur next to the walls if the cross-section is too wide for the in flowing liquid to fill the entire cross-section. Proper design of runner and gating systems prevents this effect. Furthermore, there may be a centripetal, rotational component of the velocity profile of the molten metal during filling of approximately round or square sections. However, microstructures similar to that in Figures 1-3 still would be observed.
Compared to aluminum, magnesium is ore suitable for hot chamber diecasting, which eliminates premature solidification in the shot sleeve and, therefore, only occurs by the second stage (inside the die). However, the driving forces for the formation of defect bands also are present in this case.
Behavior of the Solidifying Material
There is a strong relationship between the mechanical behavior of alloys during solidification and the evolution of microstructure. The part of the casting where the material is between the liquidus and the solidus temperatures (the mushy zone) can be divided into three different regions with different properties. Various techniques have been developed to measure the position of these regions, and mechanical methods seem to be the best.
Molten metals and slurries with a low solid fraction have little resistance to deformation and are characterized by a viscosity that defines their ability to flow. However, further cooling and an increase in the solid fraction results in the evolution of dendrites within the material, and this causes a continuous increase in viscosity. The equiaxed dendrites geometrically impinge on each other at a characteristic solid fraction. This is called the dendrite coherency point and is a function of the alloy and the cooling rate through their effects on microstructural evolution. Generally, it can be expected that a material with small globular crystals displays a late coherency point at about 40% solid, whereas a material with large and branched dendrites exhibits early coherency at 10-15% solid. At the coherency point, the mush develops resistance to deformation where a certain threshold stress must be exceeded for the material to start behaving as a liquid. Further solidification results in growth and coarsenin g of the dendrites, and a point is reached where the crystals start to mechanically interlock. The increasing strength of the dendritic network with small increases in solid fraction is called the maximum packing solid fraction.
Figure 4 schematically represents the general trend in the evolution of mechanical strength of different microstructures during solidification based on the measurements of the mechanical behavior of solidifying aluminum alloys during shear. The significance of the dendrite coherency point and the maximum packing point can be observed easily as can the effect of microstructure. It is important to emphasize that the relationships shown would be strongly dependent on the alloy constitution, the governing cooling conditions and shear rate. At higher shear rates and smaller cooling rates, the curves are shifted to the right. The results show the ordinate is shear strength in kPa, which characterizes the instantaneous resistance to deformation of the material for different solid fractions. Considering that the strength during solidification would reach about 1 MPa at the solidus temperature, it is clear that the maximum packing point represents the most significant change in properties, whereas only small changes in strength occur between coherency and maximum packing.
These recently discovered properties of solidifying alloys are of significant importance for the description and understanding of the processes that occur during solidification. For example, they have shed new light on the identification of mechanisms of feeding to avoid porosity in solidifying metals. The observations reported in Fig. 4 also provide the foundation for understanding the formation of the bands of defects in castings produced in pressurized casting processes and where the liquid flow rate and shear is high. This indicates the importance of further characterizing and understanding the solidification processes in high-pressure diecasting.
Understanding the Formation of Banded Defects
A cross-section of a cold chamber magnesium die casting illustrates the principle explanation for the formation of defect bands in pressurized casting processes. This is a simplification, and the principle is applicable to any geometry, alloy and casting process.
Since many magnesium alloys solidify over a wide solidification range, often exceeding 2l2F(100C), the mushy zone consisting of a range of solid fractions occupies a larger area in the casting than an aluminum alloy, in which the solidification range of the most common diecasting alloys is 68-86F (20-30C). This increases the likelihood of the formation of banded defects.
The principle of the model is best explained by considering two extreme cases that can lead to band formation. The first case is when the material contains solid upon application of the shot, a case that would commonly occur in cold chamber diecasting and in semisolid casting. The second case is the hot chamber case of injection of a liquid with a temperature above the liquidus temperature, without any solidified material.
When the injected material contains prematurely solidified material, the pre-solidified dendrites concentrate toward the center of the flow (Fig. 3). Therefore, the solid fraction is high in the center and less toward the edges of the material. Upon contact with the cold die walls during filling, the material starts to solidify so that the walls are quickly covered by a layer of solid. The resulting fraction solid distribution therefore quickly approaches a "W" shape (Fig. 5). As there is a strong relationship between fraction solid and shear strength (Fig. 4), the fraction solid distribution corresponds to a distribution of strength (Fig. 5b) for the fraction solid distribution (Fig. 5a). The strength distribution indicates that the strength is at a minimum in a region between the wall and the center of the casting. This region will have zero strength as long as the fraction solid there is below the dendrite coherency point. If the die still is not filled completely, the strength curve will indicate that th e central region will flow in a plug-like fashion, "lubricated" by the liquid in the low-strength region against the relatively immobile solid next to the wall. The deformation is concentrated to the band because it represents the least resistance to deformation. Continued solidification of the casting combined with further flow would result in continued development of the segregated band in the low-strength region. As solidification occurs, the liquid is enriched in solute, and the band is the last region to solidify in the casting, which may act as a feeding path during the later stages of solidification. As long as the solid fraction in the shear band stays below the maximum packing solid fraction, the band will be segregated and contain little porosity. The source of porosity in that case is limited to shrinkage of the liquid in the band and feeding of adjoining sections. The location of the band is related to the thickness of the layer solidified on the die walls and is a function of the heat content of the molten metal, the cooling capacity of the die and the amount of flow.
If significant solidification occurs during filling and deformation occurs while the entire casting cross-section has reached a high fraction solid, the band may develop a high level of porosity in addition to segregation. The level of porosity increases with increasing amounts of deformation and solid fractions in the band when deformation occurs (when lower die temperatures or more extensive cooling are used).
The rationale behind the formation of these bands is similar to that outlined above, but how several bands may form requires some explanation. In most cases, the outermost band is formed first, and the one closest to the center is formed last. The reason for this is a direct result of the heat flow conditions in the casting. While plug-flow of the central region occurs (Fig. 5), the casting is cooled by heat flow through the walls. The temperature in the region adjacent to the wall, therefore, is continuously decreasing until it reaches a temperature sufficient to solidify the molten, segregated liquid in the band. This freezes the band and, most likely, some primary solid further toward the center. However, if there still is a driving force for flow present, a new band will form by a similar mechanism (deformation of a region of lowest strength). In this case, the solid fraction is higher than when the first band formed, and the amount of porosity in the band is higher than that in the first band closest to the walls. Subsequent bands can form by repeating this cycle, and the number of bands is a direct consequence of the heat extraction effectiveness of the die, the amount of flow required and the properties of the solidifying alloy. Alloys with wide solidification ranges are more prone to the formation of bands because of a broader mushy zone within the solidifying casting.
In hot chamber diecasting the input material generally contains no solid, and the solid fraction profile now is a simple parabolic shape. For small cross-sections and high flow rates, segregated bands still may form, although they are expected to be less apparent, being a simple result of a velocity boundary layer around the maximum packing solid fraction where material against the die wall is essentially immobile.
The properties of the solidifying material also are a function of the amount of deformation and shear rate that the material experiences. A high shear rate has a direct impact on the microstructural evolution, normally causing fragmentation and small, more globular crystals. Therefore, it is not straightforward to obtain direct measurements of the strength development characteristics under the governing casting conditions, although Fig. 4 indicates a transition to the right as the size and shape of the solid phase changes. At high shear rates it is likely that the strength development characteristics are quite similar for different alloys and limited by the size of the mushy zone and volume fraction of eutectic rather than microstructural evolution.
Bands of defects appear to be limited to pressurized casting processes. The reason for this result is that high fluid flow rates, high shear stresses and/or shear rates are experienced in these processes. The cooling rates also are normally high so that there is no time for rearrangement of the microstructure. In thin sections, the shear stresses are higher, and the bands are clearly observed in the final microstructure.
The above description outlines the mechanisms responsible for formation of bands in thin-section castings. The other factors affecting the location of banded defects are changes in cross-sectional area and changes in flow direction, both of which create regions of high local stress. Shearing and plug flow again are the reasons for the formation of the bands of defects, and the position of the stress raisers and the resulting velocity profile explain the location of the band.
Optimizing Casting Conditions
Based on this model, some precautions can reduce the problem of defect bands. Because the origin of the bands is the shear that occurs in the two-phase material, the remedies lie in reducing or controlling shear and controlling the mushy zone. In cold chamber diecasting, the input material must be controlled, specifically reducing premature solidification, by a higher pouring temperature, heating the shot sleeve and reducing the dwell time before the shot. Although grain size is not a parameter that can be controlled for a specific alloy in high-pressure diecasting, a smaller grain size may be beneficial. Increasing the die temperature and using a higher pouring temperature can be beneficial if the filling can be completed before significant shear of the partially solidified material occurs. Many casting processes and design parameters can be manipulated to reduce the amount of stress generated. The tuning of these parameters also strongly depends on other casting considerations such as porosity, cold shuts and solidification mode. To control shear defects in high-pressure diecasting, the following approaches can be considered: reduce the number and/or size of overflows, reduce shot sleeve solidification, increase die temperature, increase melt temperature and ensure that pressure intensification occurs after the die is completely filled.
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|Comment:||Prevent Banded Defects in High-Pressure Diecast Magnesium Alloys.|
|Author:||StJohn, David H.|
|Date:||Feb 1, 2000|
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