BRPI0618517A2 - combination of casting process and alloy compositions resulting in castings with superior combination of high temperature creep properties, ductility and corrosion performance - Google Patents

Abstract

COMBINATION OF THE FOUNDRY PROCESS AND ALLOY COMPOSITIONS RESULTING IN FUSED PARTS WITH HIGHER COMBINATION OF HIGH TEMPERATURE DEFORMATION PROPERTIES, DUCTILITY AND CORROSION PERFORMANCE. The present invention relates to a process for casting a magnesium alloy consisting of 2.0-6.00 wt% aluminum, 3.00-8.00 wt% rare earth metals (RE metals). ), the ratio of the quantity of RE metals to the amount of aluminum expressed as a% by weight being greater than 0,8, at least 40% by weight of RE metals consisting of cerium, less than 0,5 wt.% manganese, less than 1.00 wt.% zinc, less than 0.01 wt.% calcium, less than 0.01 wt.% strontium, and the balance being magnesium and unavoidable impurities, If the total impurity level is less than 0.1% by weight, where the alloy is cast in a matrix, the temperature of the alloy being controlled in the range 180-340 <198> C, the matrix is filled in a time that , expressed in milliseconds, is equal to the product of a number between 5 and 500 multiplied by the thickness of the average part expressed in millimeters, the static metal pressures being maintained during casting between 2 0-70 MPa and subsequently intensified to 180 MPa.

Description

Patent Descriptive Report for "COMBINATION OF FOUNDRY PROCESS AND ALLOY COMPOSITIONS RESULTING IN FUSED PARTS WITH HIGHER TEMPERATURE DEFORMATION PROPERTIES, DUCTILITY AND CORROSION PERFORMANCE".

The present invention relates to a process for casting a magnesium alloy consisting of:

2.0-6.00% by weight aluminum,

3.00-8.00% by weight of rare earth metals (RE metals),

the ratio of the amount of RE metals to the amount of aluminum expressed as% by weight being greater than 0,8,

at least 40% by weight of RE metals consisting of cerium,

less than 0.5 wt% manganese, less than 1.00 wt% zinc, less than 0.01 wt% calcium, less than 0.01 wt% strontium, and the remainder being magnesium and unavoidable impurities, the total impurity level being less than 0.1% by weight.

Magnesium-based alloys are widely used as castings in the aerospace and automotive industries. Magnesium alloy alloy castings can be produced by conventional casting methods which include die casting, sand casting, permanent and semi-permanent mold casting, plaster mold casting and casting wrap. Mg-based alloys demonstrate several specifically advantageous properties that have stimulated an increasing demand for magnesium-alloy castings in the automotive industry. These properties include low density, high strength to weight ratio, good meltability, good machinability and good damping characteristics.

Most common magnesium die casting alloys, such as Mg-Al alloys or Mg-Al-Zn alloys, are known to lose their creep resistance at temperatures above 120 ° C. Mg-Al-Si alloys are designed for higher temperature applications and offer only limited improvement in deformation resistance. The Mg-Al-Ca and Mg-Al-Sr system alloys offer a further improvement in creep resistance, but a major disadvantage with these alloys is the problems with meltability. This is specifically a problem with high metal velocities impinging directly on the matrix surface, the so-called water hammer effect.

The alloy AE48 (4% AP, 2-3% RE) is known to offer significant improvement in high temperature and corrosion properties.

Mg-Al alloys containing Sr and Ca-like elements offer further improvement in deformation properties, however, at the cost of reduced casting capacity. The Mg-Al-Ca and Mg-Al-Sr system alloys offer further improvement in deformation strength, but a major disadvantage with these alloys is the casting capacity problems. This is specifically a problem with high metal velocities impinging directly on the matrix surface, the so-called water hammer effect.

The accompanying figures 1A and 1B schematically show cold chamber die casting machines and heated chambers respectively, each machine having a die 10,20 provided with a hydraulic damping system 11,21 respectively. The molten metal is introduced into the matrix by means of an alloy loading cylinder 12, 22 provided with a piston 13,23 respectively. In the cold room system, an auxiliary metal measuring system is required for the horizontal alloy loading cylinder. The heated chamber machine (figure 1B) utilizes a vertical piston system (12,23) directly on the molten alloy.

For optimum performance of Mg-AI-RE alloys, it is imperative that the alloys be melted under extremely rapid cooling conditions. This is the case for the high pressure die casting process. The 10.20 steel die is equipped with an oil (or water) cooling system controlling the die temperature in the range of 200-300 ° C. A prerequisite for good quality is a short die filling time to avoid solidification of the metal during filling.

A matrix fill time of the order of 10 2Sx average part thickness (mm) is recommended. This is achieved by forcing the alloy through the door at high speeds, typically in the 30-300 m / s range. Piston speeds of up to 10 m / s with sufficiently large diameters are being used to achieve desired volume flows in the alloy loading cylinder for the required short fill times. It is common to use static metal pressures of 20-70 MPa and sufficient pressure intensification of up to 150 MPa. With this casting method, the resulting cooling ratio of the component is typically in the range of 10-1000 ° C / s, depending on the thickness of the component being cast. For AE alloys, this is a key factor in determining properties, both due to the generally high part cooling ratio and specifically the extremely high surface layer cooling ratio. In the accompanying figure 2, the relationship between the solidification range and the microstructure is illustrated. The horizontal geometric geometrical axis shows the solidification ratio expressed as ° C / S, and the vertical scale on the left shows the spacing of the secondary dendrite arm, whereas the vertical scale on the right is shown as the grain diameter expressed in μm. Line 30 indicates the grain size obtained, considering that line 31 is the value obtained for the spacing of the secondary dendrite arm.

The die provides the refining of the molten grain for cooling reasons. As mentioned above, cooling ratios in the range of 100-1000 ° C / s are usually obtained. This typically results in grain sizes in the range of 5-100 μm.

It is well known that fine grain size is beneficial for the ductility of an alloy. This relationship is illustrated in the attached figure 3, where the relationship between grain size and relative elongation has been shown. On the horizontal geometric geometry axis, the grain arrangement by size was represented as being expressed in μηι, whereas the vertical geometric geometry axis gives the relative elongation expressed as a percentage. In the graph two different compositions are shown, the first pure Mg, line 35 and the second Mg alloy named AZ91, line 36.

It is also well known that fine grain size is beneficial for the tensile strength of an alloy. This relationship (Hall-Petch) is shown in the attached figure 4. The horizontal geometric geometry axis shows the grain diameter, expressed as d (-0.5), where it was expressed in μm, and the vertical geometric geometry axis shows the stress yield strength expressed in MPa.

Therefore, it is evident that the fine grain size provided by the very high cooling rates facilitated by the die casting process is a necessity for obtaining tensile strength and ductility.

The term castability describes the ability of an alloy to be cast into an end product with the required functionality and properties. It usually contains 3 categories: (1) the ability to form a part with all desired geometry aspects and dimensions, (2) the ability to produce a dense part with the desired properties, and (3) the effects in die casting tools, die casting equipment and die casting process efficiency.

German Patent Application No. 2122148 describes Mg-AI-RE-type alloys primarily Mg-AI-RE alloys with RE content <3 wt%, although alloys with higher RE content are well discussed. AE42 alloy (4% Al, 2-3% RE) is known to offer significant improvement in high temperature and corrosion properties. Additions of small RE to Mg-Al alloys have been shown to lead to a significant improvement in corrosion properties, but a deterioration in meltability, as adhesion problems occur more frequently. The attached figure 5 shows the regions of excellent, weak and very weak melting capacity in the Mg-AI-RE system. In the horizontal geometric axis, the amount of Al expressed in% by weight is shown, whereas in the vertical geometric axis, the amount of RE expressed in% by weight is mustard. Line 40 is the line indicating RE solubility at 680 ° C, whereas line 41 indicates the solubility of RE at 640 ° C. Region (dark) 42 represents the composition with very poor melt capacity. Region (intermediate) 43 represents composition with poor meltability and region 44 (light) represents compositions with excellent meltability. As shown in Figure 5, the meltability becomes worse as the RE content of the alloy increases. However, as shown in Figure 5, there is a region with RE> 3.5% by weight (the upper bound restricted by RE solubility). Al in the range of 2.5% to 5.0% and further described with a% Re /% AI ratio greater than 0.8, where the high pressure die casting capability is excellent.

Therefore, it is an object of the present invention to provide magnesium base alloys at a relatively low cost with improved high temperature performance and improved casting capacity.

Due to the formation of AIxREy dispersoid phases, the compositions of the present invention minimize the volumetric fraction of the brittle Mg17Ah2 phase (the RE / razão ratio in the dispersoid phases increases with the increasing% Re /% AI content in the alloy). Because the eutectic phase of Mgi7AI12 melts around 420 ° C, conventional Mg-Al alloys such as AM50, AM60 and AZ91 will have a solidification range close to 200 ° C, as shown in the attached figure 6. Figure 6 shows the solid in fraction (expressed as weight percent) on the horizontal geometrical axis versus the temperature (0C) on the vertical geometrical axis for various alloys. Mg-AI-RE alloys with% / RE /% AI ratios as specified in the present invention will solidify completely around 570 ° C, since the solidification range is only approximately 50 ° C.

In general, the increasing aluminum content in Mg-Al die casting alloys improves the die casting capability. This is because Mg-Al alloys have a wide range of solidification, which makes them inherently difficult to melt unless a sufficiently large range of eutectic is present at the end of solidification. This may explain the good meltability of AZ91D consistent with the cooling curves shown in figure 6. Since the Al content is reduced to 6, 5 and 2% at AM60, AM50 and AM20, respectively, the eutectic The remainder decreases to a level where feeding becomes difficult during the final stages of solidification, which means that for thick-walled parts, microporosity and even larger spaces may be present. For thin-walled parts, the feed capacity during the final stages is less important (although alloy fluidity becomes the significant factor) since volume shrinkage is partially absorbed by the reduction in thickness due to shrinkage of the alloys. matrix walls. Alloys AE44 and AE35 show very different cooling characteristics than Mg-AI alloys. The solidification range is significantly shorter, indicating concentrated shrinkage porosity that may be decreased during solidification. These alloys have good flowability during mold filling and thus can be easily cast into end products with fewer casting defects. The melting capacity of AE44 and AE35 is relatively equal to that of AZ91D.

An additional issue related to the narrow solidification interval is the fact that reverse segregation generally observed to occur in AZ91D as well as AM alloys does not occur. This is illustrated by the fact that AE alloys with high RE contents have a bright surface without Mg-AI eutectic phase segregation. The surface layer solidifies during and immediately after die filling and the temperature drops rapidly below the solids temperature, thereby preventing the molten metal from being forced toward the die surface when shrinkage begins. This will be beneficial in preventing reactions between the matrix wall and the molten metal, which could lead to matrix adhesion.

An example with a matrix thickness of about 3 mm showing three layers with different microstructure in AE44 is given in the attached figure 7. The surface layer, having an approximate thickness of 50 μm, consists of equal axial grains with a size of about 10 μm. This is a very small grain size, which can be explained by the rapid cooling conditions on the matrix wall. The middle layer is between 100 μm thick and extremely fine grained. The morphology is different from the previous one and DAS in the 2-4 μm range is observed. The change in melting point balance due to pressure may explain this observation. When the metal becomes pressurized, the melting point equilibrium increases, ie the metal cools abruptly. This is theoretically the case for all Mg alloys, but there remains a significant difference in the solidification characteristics between the alloys. The nucleus consists of equal axial grains of -20 μm. Core solidification is restricted by heat flow out of the core toward the matrix. Both heat transport through the already solidified layer and heat transfer to the die / die interface will provide a slower cooling ratio than the skin and thus a thicker microstructure will be formed.

When the RE content is low or the% RE /% AI ratio is low as in AE42 or AE63 there is a possibility that Mg-Al is present which would segregate to the surface and lead to reduction. This may explain why AE42 is superior with a poorer casting capacity.

In figure-8, a box matrix is shown at the (upper) part of the drawing. Micrographs of node 3 (near door) examples for AM60, AM40, AE63, AE44, and AE35 alloys are as shown below. Hot cracked are observed in AM40 and AE63.

Figure 8 demonstrated that AE44 and AE35 are less susceptible to hot breakage than AM alloys. This is explained by the complete and rapid solidification of the surface layer which results in the relatively fine grain structure as described above.

Partly due to the fine grain structure and partly due to the absence of the brittle phase of Mg17AI12 this layer becomes very ductile and therefore able to deform when thermal resistances are developed during solidification. A coarser grain surface layer, as typically shown in higher solidification range alloys, and / or a Mg17AI12 rich layer, would have a much lower ductility and would tend to crack, forming heated ruptures rather than deforming.

Testing of large (~ 1.5 m) thin-walled (~ 3 mm thick) pieces showed that the die filling characteristics of AE44 and AE35 are excellent and since long feed is not necessary for Thin-walled parts, as discussed above, are expected to be a viable alternative for these types of components, where matrix filling is of vital importance.

The properties of the various alloys of AE are explained from the observations that Al alone provides the bonding of the solid solution, while RE combines with Al forming dispersoidal phases in the grain boundary regions. In alloys AE44 and AE35, the dispersoid phase (mainly AI2RE) constitutes a continuous 3D network, effectively preventing the emergence of deformation by thermal activation and sliding of the grain boundary. This is shown in Figure 9, which shows Backscatter Electronic Composition (SEM) images showing the die casting microstructure (left to right) of AE44, AE35 and AE63. Although Al alone provides binding of the solid solution, RE combines with AL to form dispersoidal phases in the grain boundary regions.

A further enlargement of SEM-BEC images for AE44 is shown in Figure 10, which also shows the lamellar structure of AIxREy phases in AE44. As seen from Figure 10, the AIxREy dispersoidal phases in the AE alloys consist of an extremely thin lamellar structure. This submicron-sized lamellae structure stiffens the grain contours and thus prevents deformation. On the other hand, these lamellae are not brittle (or not as brittle as eutectic Mg-AI) as die casting alloy AE44 experiences a ductility that is similar to that of AE42. In AE63, the network (mainly AI11RE3) becomes fragmented and the grain boundary regions are probably influenced by a substantial amount of eutectic Mg-Al, reducing ductility and deformation properties. In AE42 there is also probably a significant amount of eutectic Mg-Al that limits deformation properties. Alloy AE35 has slightly lower ductility than AE44, but still higher than AE63.

Several examples of mechanical properties including properties of ductility, tensile strength, creep and corrosion resistance of AE alloys are shown below. The unique combination of creep strength and ductility compared to existing alloys is illustrated in figure 11. In figure 11, the ductility (horizontal geometric axis) is shown as versus creep resistance for various known Mg alloys. Zone 50 comprises AM alloys, AE alloys 51, AZ91 alloys zone 52 and other high temperature alloys zone 53. The AE alloys of the present invention are the only die casting alloys that combine ductility and high temperature properties in this way and consequently offer several new and untapped opportunities for builders and designers specifically in the automotive industry.

It is a more specific objective to provide relatively low cost die-cast magnesium aluminum-rare earth alloys with good creep resistance and tensile strength as well as good clamping-load retention, specifically at elevated temperatures of at least 150 ° C.

SUMMARY OF THE INVENTION

The present invention therefore provides:

- the alloy which is cast in a matrix, the temperature of which is controlled in the range 180-340 ° C,

- the matrix which is filled at a time which, expressed in milliseconds, is equal to the product of a number between 5 and 500 multiplied by the thickness of the average part expressed in millimeters,

- static metal pressures being maintained during casting at 20-70 MPa and subsequently intensified to 180 MPa. By combining a specified Mg-Al-RE alloy with a special casting process, products with excellent creep resistance at high temperature, high ductility and generally good mechanical properties would be obtained. as well as corrosion properties.

In general, various RE metals may be used as an alloying element, such as, for example, Ce, La, Nd and / or Pr and mixtures thereof. However, it is preferred to use cerium in substantial amounts, as this metal provides the best mechanical properties. Mn is added to improve corrosion resistance, but its addition is restricted due to limited solubility.

Preferably, the aluminum content is between 2.0 and 600 wt%, more preferably between 2.60 and 4.50 wt%.

If larger amounts of aluminum are present, this can easily lead to the formation of Mg17AIi2 phases which is detrimental to creep properties. A very low Al content is negative for melting capacity. For RE metals, it is preferred that the RE content is present between 3.50 and 7.0 wt%, the upper limit being restricted by the RE solubility in the Mg-Al-RE system as shown in Figure 1. .

If more than 3.50% by weight of RE is present, this provides a significant improvement in creep properties. More than 7.00% by weight is not practicable due to the restricted solubility of RE metals in liquid magnesium aluminum alloys.

Additionally, it is preferred that the ratio of RE / ΑΙ be greater than 0.9.

For specific applications the alloy composition is selected such that the aluminum content is between 3.6 and 4.5 wt% and the RE content is between 3.6 and 4.5 wt%, provided that the ratio of RE / Αl is greater than 0,9.

This type of alloy can be used for applications up to 175 ° C while still showing excellent creep and tensile strength properties. Moreover, this alloy does not show any deformation of its properties due to aging and has a good casting ability.

For applications above 175 ° C, the alloy composition is such that the aluminum content is between 2.6 and 3.5 wt% and the RE content is greater than 4.6 wt%.

Apart from the excellent deformation properties and tensile strength, this alloy shows no degradation of aging properties.

Preferably, the RE metals are selected from the group cerium, lanthanum, neodymium and praseodymium.

RE metals contribute to ease of bonding, but also increase corrosion resistance, creep resistance and improve mechanical properties.

Preferably, the amount of lanthanum is at least 15 wt% and more preferably at least 20 wt% of the total RE metal content. Preferably, the amount of lanthanum is less than 35% by weight of the total RE metal content.

Preferably, the amount of neodymium is at least 7 wt% and more preferably at least 10 wt% of the total RE metal content. Preferably, the amount of neodymium is less than 20% by weight of the total RE metal content.

Preferably, the amount of praseodymium is at least 2 wt% and more preferably at least 4 wt% of the total RE metal content. Preferably, the amount of praseodymium is less than 10% by weight of the total RE metal content.

Preferably, the amount of cerium is greater than 50% by weight of the total RE metal content, preferably between 50 and 55% by weight.

Calcium and strontium are known to provide an increase in creep resistance and the addition of at least 0.5% by weight of calcium will improve tensile strength.

However, Ca and Sr could be avoided due to the fact that even at very small concentrations these elements lead to considerable adhesion problems and thus influence the alloy's melting ability.

The present invention is described in more detail with reference to the following examples, which are for illustration purposes only and are not to be construed as indicating or employing any limitation of the invention broadly described herein.

Example 1

In order to account for the influence of the binding elements, various Mg binders were prepared with the compositions as given in Table 1.

Several test bars were manufactured to perform the tests described in the following examples. The tests performed are as follows:

Tensile strength and ductility

6mm test bars according to ASTM were manufactured and the following:

Test conditions were used:

- 10 kN Instron machine

- room temperature at 2100C

- at least 5 parallels at each temperature

- resistance ratio

- 1,5 mm / min to 0,5% strength,

- 10 mm / min over 0.5% strength

- Test according to ISO 6892

Stress Strain Test

For this test, the following test material was employed:

- diameter: 6 mm

- gauge length: 32.8 mm

- radius of curvature: 9 mm

- claw head diameter: 12 mm - overall length: 125 mm

The test is performed in accordance with ASTM 139.

Stress relaxation test

- Test Material

-12 mm in diameter, 6 mm in length

- cutting the arbitrary end of the strain bars

- Test according to ASTM E328-86

Corrosion property

Corrosion has been tested according to ASM 117.

Example 2

Resistance was measured as a function of temperature for various compositions.

The results are shown in figures 12, 13 and 14. At these figures, the geometric axis y is representing the resistance to stress expressed in MPa, whereas the geometric axis χ is representing the temperature expressed in degrees Celsius. .

Example 13

Creep resistance was measured as a function of time for various compositions.

The results are shown in figures 15 and 16. In figure 15, the measurement is performed at 175 ° C with a force of 40 MPa and in figure 16 the measurement is performed at 150 ° C with a force of 90 MPa.

In these figures, the geometric axis y is representing resistance to deformation expressed as a percentage, whereas the geometric axis χ is representing time expressed in hours.

Example 4

For the various compositions according to table 1, the stress ratio was defined, expressed as the remaining load versus time. Results are shown in figures 17, 18 and 19.

In these figures, the geometric axis y is representing the remaining charge expressed as a percentage of initial charge, whereas the geometric axis χ is representing the time expressed in hours.

Example 5

For various compositions, corrosion properties have been defined according to ASTM B117. In this test, a large amount of data was incorporated in order to define the influence of RE content versus Al content. The results are shown in figure 20.

In this figure, the geometric axis y is representing the content of RE expressed as% by weight, whereas the geometric axis x is representing the content of Al also expressed as% by weight.

The boundary lines between the zones with different shadows are the same lines of corrosion resistance.

From these test results, it is clear that a process for casting a magnesium alloy has been provided, considering that the products are obtained with a superior combination of high temperature deformation properties, ductility and corrosion performance. Table 1

<table> table see original document page 16 </column> </row> <table>

Claims (18)

1. Process for the casting of a magnesium alloy consisting of: 2,0-6,00% by weight of aluminum, 3,00-8,00% by weight of rare earth metals (RE metals), the ratio of amount of RE metals to the amount of aluminum expressed as wt% being greater than 0.8, at least 40 wt% of RE metals consisting of cerium, less than 0.5 wt% of manganese, less 1.00 wt% zinc, less than 0.01 wt% calcium, less than 0.01 wt% strontium, and the remainder being magnesium and unavoidable impurities, the total impurity level being less than 0.1% by weight. - the alloy is cast into a die, the temperature of the die being controlled in the range 180-340 ° C, - the die is filled at a time which, expressed in milliseconds, is equal to the product of a number between 5 and 500 multiplied by the average part thickness expressed in millimeters, - the static metal pressures being maintained during casting between 20-70 MPa and subsequently intensified to 180 MPa. A process according to claim 1, wherein the matrix temperature is controlled at a temperature in the range from 170 to 390 ° C, preferably in the range from 200 to 270 ° C. A process according to claim 1 or 2, wherein the matrix fill time expressed in milliseconds is equal to the product of the average part thickness expressed in millimeters multiplied by a number between 8 and 200, preferably between 5 and 50, more preferably between 5 and 20. A process according to any one of claims 1-3, wherein the static metal pressure during casting is maintained between 30-70 Mpa. A process according to any one of claims 1-4, wherein the cooling ratio after casting is in the range 10-1,000 ° C / s. A process according to any one of claims 1-5, wherein the aluminum content is between 2.50 and 5.50 wt%, preferably between 2.60 and 6.50 wt%. A process according to any one of claims 1-6, wherein the RE content is between 3.50 and 7.00% by weight. Process according to any one of claims 1-7, wherein the aluminum content is between 3.6 and 4.5% by weight and the RE content is between 3.6 and 4.5% by weight and RE-AI ratio is greater than 0.9. A process according to any one of claims 1-8, wherein the aluminum content is between 2.6 and 3.5 wt% and the RE content is greater than 4.6 wt%. A process according to any one of claims 1-9, wherein the RE metals are selected from cerium, lanthanum, neodymium and praseodymium. A process according to claim 10, wherein the amount of lanthanum is at least 15 wt% of the total RE metal content, preferably at least 20 wt%. A process according to claim 10 or 11, wherein the amount of lanthanum is at most 35% by weight of the total RE metal content. A process according to any one of claims -10-12, wherein the amount of neodymium is at least 7 wt% of the total RE metal content, preferably at least 10 wt%. A process according to any one of claims 10-13, wherein the amount of neodymium is at most 20% by weight of the total RE metal content. A process according to any one of claims -10-14, wherein the amount of praseodymium is at least 2 wt% of the total RE metal content, preferably at least 4 wt%. A process according to any one of claims -10-15, wherein the amount of praseodymium is at most 10% by weight of the total RE metal content. A process according to any one of claims 10-16, wherein the amount of cerium is greater than 50 wt% of the total RE metal content, preferably between 50 and 55 wt%. A process according to any one of claims 10-17, wherein the amount of calcium and / or strontium is at most 0.01% by weight.