JP5290764B2 - Casting method and alloy composition for forming a cast part having a combination of excellent high temperature deformation characteristics, malleability and corrosion performance - Google Patents

Description

  The present invention relates to 2.0 to 6.00 mass% aluminum, 3.00 to 8.00 mass% rare earth metal (RE metal), wherein the amount of RE metal relative to the amount of aluminum expressed in mass%. The ratio is greater than 0.8, at least 40% by weight of the RE metal is cerium, and less than 0.5% manganese, less than 1.00% zinc, less than 0.01% calcium, 0% The present invention relates to a method of casting a magnesium alloy comprising less than 0.01% by weight of strontium and magnesium and the remainder being an inevitable impurity and having a total impurity level of less than 0.1% by weight.

  Magnesium-based alloys are widely used as casting parts in the aerospace and automotive industries. Magnesium-based alloy cast parts can be manufactured by conventional casting methods such as die casting, sand casting, permanent / semi-permanent casting, gypsum casting, and investment casting. Mg-based alloys have demonstrated many particularly beneficial properties that are driving the increasing demand for magnesium-based alloy cast parts in the automotive industry. These properties include low density, high strength / weight ratio, good castability, easy machinability, and good damping properties. Most common magnesium die-cast alloys, such as Mg-Al alloys and Mg-Al-Zn alloys, are known to lose creep resistance at temperatures above 120 ° C. Mg-Al-Si alloys have been developed for higher temperature applications and present only limited improvements in creep (deformation) resistance. Although Mg-Al-Ca and Mg-Al-Sr alloys present a further improvement in creep resistance, the major drawback of these alloys is a castability problem. This is the so-called water hammer effect and is particularly problematic at high metal velocities that directly impact the mold surface.

  Alloy AE48 (4% AP, 2-3% RE) is known to present significant improvements in high temperature properties and corrosion.

  Mg-Al alloys containing elements such as Sr and Ca present a further improvement in creep properties, but at the expense of reduced castability. Although Mg-Al-Ca and Mg-Al-Sr alloys present a further improvement in creep resistance, the major drawback of these alloys is the problem of castability. This is the so-called water hammer effect and is particularly problematic at high metal velocities that directly impact the mold surface.

  1A and 1B of the accompanying drawings schematically show a cold chamber and a hot chamber die casting machine, each of which has a mold 10, 20 with a hydraulic damping system 11, 21, respectively. Molten metal is introduced into the mold by shot cylinders 12, 22 with pistons 13, 23, respectively. In a cold chamber system, an auxiliary system for metering metal into a horizontal shot cylinder is required. The hot chamber machine (FIG. 1B) uses the vertical piston system (12, 23) directly in the molten alloy.

In order to obtain the excellent performance of the Mg—Al—Re alloy, it is essential to cast the alloy under extremely rapid cooling conditions. This is the case for the high pressure die casting process. The steel molds 10, 20 are provided with an oil (or water) cooling system that controls the temperature of the molds in the range of 200-300 ° C. A prerequisite for good quality is a short mold filling time to avoid solidification of the metal during filling. A mold filling time of the order of 10 ^-2 seconds x average part thickness (mm) is recommended. This is typically obtained by forcing the alloy through the gate at a high speed in the range of 30-300 m / sec. In order to obtain the desired volume flow in the shot cylinder for the short filling time required, a plunger speed of 10 m / sec or less with a sufficiently large diameter is used. It is common to use a static metal pressure of 20 to 70 MPa and a subsequent pressure increase to 150 MPa or less. Using this casting method, the resulting cooling rate of the component is typically in the range of 10 to 1000 ° C./second, depending on the thickness of the component being cast. For AE alloys, this is an important factor in determining properties, both because of the general high cooling rate of the parts and in particular the extremely high cooling rate of the surface layer. FIG. 2 of the accompanying drawings shows the relationship between the solidification range and the microstructure. On the horizontal axis the solidification rate in ° C / sec is shown, on the left hand vertical axis the secondary dendritic arm spacing in μm is shown, while on the right hand vertical axis The particle diameter in μm is shown. Line 30 shows the resulting particle size, while line 31 is the value obtained for the spacing of the secondary dendritic arm. When die casting is used, grain refinement can be achieved depending on the cooling rate. As described above, a cooling rate in the range of 10 to 1000 ° C./second is usually obtained. This typically results in a particle size in the range of 5-100 μm.

  It is well known that fine grain size is beneficial for alloy ductility. This relationship is illustrated in the attached FIG. 3, which shows the relationship between grain size and relative elongation. The abscissa shows the adjusted particle size in μm, while the ordinate gives the relative elongation in%. The graph shows two different compositions, a first pure Mg line 35 and a Mg alloy line 36 labeled AZ91.

  It is also well known that fine grain size is beneficial to the tensile yield strength of the alloy. This relationship is shown in the accompanying FIG. 4 (Hall-Petch). The horizontal axis indicates the diameter of the grains expressed in μm and expressed as d (−0.5), and the vertical axis indicates the tensile yield strength expressed in MPa.

  Thus, it is clear that a fine grain size given by the very high cooling rate facilitated by the die casting process is necessary to obtain tensile strength and ductility.

The term castability describes the ability to cast an alloy into a final product with the required functionality and properties. Generally includes three categories.
(1) Ability to form parts with all desired geometric features and dimensions (2) Ability to produce dense parts with desired characteristics, and (3) Efficiency of die casting tools, casting equipment, and die casting processes Effect on

  German patent application 2122148 describes alloys of the Mg-Al-RE class, mainly Mg-Al-RE alloys with an RE content of less than 3% by weight, although alloys with a higher RE content are also included. It is stated. Alloy AE42 (4% Al, 2-3% RE) is known to present significant improvements in high temperature and corrosion properties. Although the slight addition of RE to the Mg-Al alloy results in a significant improvement in corrosion properties, it has been experienced that mold adhesion problems occur more frequently and castability is reduced. In the attached FIG. 5, the excellent, poor and very poor castability regions in the Mg—Al—Re system are shown. The horizontal axis shows the amount of Al expressed in mass%, while the vertical axis shows the amount of RE expressed in mass%. Line 40 is a line that displays the solubility of the RE at 680 ° C., while line 41 displays the solubility of the RE at 640 ° C. Region (dark) 42 represents a composition with very poor castability. Region (intermediate) 43 represents a composition having poor castability, and region (bright) 44 represents a composition having excellent castability. As illustrated in FIG. 5, the castability deteriorates as the RE content of the alloy increases. However, as shown in FIG. 5, the high-pressure mold castability is excellent, the RE exceeding 3.5 mass% (the upper limit is limited by the solubility of RE), the range of 2.5% to 5.0% Al, and there is a region represented by a ratio of RE% / Al% greater than 0.8.

  Accordingly, it is an object of the present invention to provide a relatively low cost magnesium based alloy with improved high temperature performance and improved castability.

Due to the generation of the AlxREy dispersoid phase, the composition of the present invention minimizes the volume fraction of the brittle Mg ₁₇ Al ₁₂ phase (the RE / Al ratio of the dispersoid phase is the RE% / Al% content in the alloy). Increases as the number increases). Since the eutectic Mg ₁₇ Al ₁₂ phase melts around 420 ° C., conventional Mg—Al alloys such as AM50, AM60, and AZ91 have a solidification range of approximately 200 ° C., as shown in FIG. Have FIG. 6 shows the solid fraction (expressed in mass%) on the horizontal axis and the temperature (° C.) on the vertical axis with respect to several alloys. The Mg-Al-RE alloy with the RE% / Al% ratio specified in the present invention is completely solidified around 570 ° C, so the solidification range is only about 50 ° C.

  In general, increasing the aluminum content of the Mg-Al die cast alloy improves the moldability of the mold. This is due to the fact that the Mg-Al alloy has a wide solidification range, which makes casting essentially difficult if there is not a sufficient amount of eutectic at the end of solidification. This can explain the good castability of AZ91D consistent with the cooling curve shown in FIG. In AM60, AM50, and AM20, decreasing the Al content to 6%, 5%, and 2%, respectively, reduces the remaining eutectic to a level that makes it difficult to supply in the final stage of solidification. For thick parts, this means that there can be micropores and even larger voids. For thin-walled parts, the capacity to supply in the final stage is not so important because volume shrinkage is partially absorbed by thickness reduction due to shrinkage from the mold wall (alloy fluidity is important) It becomes a factor). The AE44 alloy and the AE35 alloy exhibit very different cooling characteristics from the Mg—Al alloy. The interval between coagulations is rather small, suggesting that concentrated coagulation pores can be reduced during coagulation. These alloys have good fluidity when filling the mold, and therefore can be easily cast to the final product with fewer casting defects. The castability of AE44 and AE35 is relatively equivalent to that of AZ91D.

  A further problem with narrow solidification intervals is that the commonly observed reverse segregation that occurs in AM alloys as well as AZ91D does not occur. This is illustrated by the fact that AE alloys with high RE content have a glossy surface without separation of the Mg—Al eutectic phase. The surface layer solidifies during and immediately after mold filling, and the temperature quickly drops below the solid phase temperature, thereby forcing molten metal towards the mold surface when shrinkage begins. Is prevented. This is beneficial in preventing reaction between the mold wall and molten metal that can result in mold adhesion.

  An example with different microstructures in AE44 with a wall thickness of about 3 mm showing 3 layers is shown in FIG. The surface layer has a thickness of about 50 μm and consists of equiaxed grains having a size of about 10 μm. This is a fairly small particle size and can be explained by the quenching conditions on the mold wall. The intermediate layer has a thickness of about 100 μm and is extremely fine granular. This form is different from the former, and DAS in the range of 2 to 4 μm is observed. This observation may be explained by the change in equilibrium melting point with pressure. As the metal is pressurized, the equilibrium melting point increases, i.e., the metal suddenly becomes supercooled. Theoretically, this is the same for all Mg alloys, but among the alloys, there are still significant differences in solidification characteristics. The core consists of equiaxed grains of about 20 μm. The solidification of the core is limited by the heat flowing out of the core into the mold. Both heat transport through the already solidified layer and heat transfer across the casting / mold interface provides a slower cooling rate than the skin, thus producing a coarser microstructure.

  If the RE content is low, or the ratio of RE% / Al% is low, such as AE42 or AE63, there may be eutectic Mg-Al that can be fractionated on the surface to cause adhesion. This can explain why AE42 exhibits poorer castability.

  In FIG. 8, the box mold is shown in the (upper) portion of the figure. Example micrograph from node 3 (near the gate) for alloys AM60, AM40, AE63, AE44, and AE35 as shown below. Hot cracks are observed in AM40 and AE63.

  FIG. 8 demonstrates that AE44 and AE35 are less susceptible to hot cracking than AM alloys. This is explained by the fairly rapid solidification of the surface layer resulting in a relatively fine granular structure as described above.

Due to the fine grain structure in part and the absence of the brittle Mg ₁₇ Al ₁₂ phase in part, this layer becomes very ductile and therefore deforms when thermal strain occurs during solidification. be able to. Surface layers with coarser grains and / or layers rich in Mg ₁₇ Al ₁₂ , such as typically appearing in alloys with larger solidification intervals, have much lower ductility and are hot to crack than deform. It will be prone to cracking.

  Tests on large (about 1.5 m) thin parts (thickness about 3 mm) show that the mold filling characteristics of AE44 and AE35 are excellent, and as mentioned above, Because no supply is required, this alloy is expected to be a viable alternative for these types of components where mold filling is most important.

The properties of various AE alloys are explained by the observation that Al alone provides solid solution strengthening while RE combines with Al to form a dispersoid phase in the grain boundary region. In alloys AE44 and AE35, the dispersoid phase (mainly Al ₂ RE) constitutes a continuous three-dimensional network structure and effectively prevents creep resulting from thermal activation and grain boundary sliding. This is shown in FIG. 9, which is a SEM-BEC (backscattered electron composition) image showing the die-cast microstructures of AE44, AE35, and AE63 (from left to right). Al alone provides solid solution strengthening, while RE combines with Al to form a dispersoid phase in the grain boundary region.

A further magnification of the SEM-BEC image for AE44 is shown in FIG. 10 and also shows the lamellar structure of the AlxREy phase of AE44. As can be seen from FIG. 10, the dispersoid AlxREy phase of the AE alloy has a very fine lamellar structure. This submicron lamellar structure strengthens the grain boundaries and thereby prevents creep. On the other hand, these lamellae are not brittle (or not as brittle as eutectic Mg-Al) so that die cast AE44 alloy exhibits ductility similar to AE42. In AE63, the network structure (mainly Al ₁₁ RE ₃ ) is shattered and the grain boundary regions are probably affected by a substantial amount of eutectic Mg—Al, reducing ductility and creep properties. Also, in AE42, there is probably a significant amount of eutectic Mg-Al that limits the creep properties. Alloy AE35 is slightly lower than AE44 but still has higher ductility than AE63.

  A number of examples of mechanical properties such as ductility, tensile strength, creep resistance and corrosion properties of AE alloys are given below. A unique combination of creep resistance and ductility compared to existing alloys is illustrated in FIG. In FIG. 11, the ductility (horizontal axis) is shown in contrast to the creep resistance of some known Mg alloys. Zone 50 includes an AM alloy, zone 51 includes an AE alloy, zone 52 includes an AZ91 alloy, and zone 53 includes another high temperature alloy. The AE alloy of the present invention is the only die casting alloy that combines ductility and high temperature properties in this manner, thus presenting many new and unexplored opportunities for constructors and designers, particularly in the automotive industry.

  A more specific objective is to provide a relatively low cost die-cast magnesium-aluminum-rare earth alloy with excellent castability, particularly good creep resistance, tensile yield strength, and bolt load retention at high temperatures of at least 150 ° C. Is to provide.

  Accordingly, the present invention provides the following: the alloy is cast in a mold whose temperature is controlled in the range of 180-340 ° C., said mold having a time expressed in milliseconds of 5 Filled in a time equal to the product of a value between ˜500 and the average part thickness in millimeters, the static metal pressure is maintained between 20 and 70 MPa during casting and then increased to below 180 MPa .

  By using a combination of specific Mg-Al-RE alloys and special casting methods, products with excellent creep resistance at high temperatures, high ductility, and generally good mechanical properties as well as corrosion properties Can be obtained.

  In general, many RE metals can be used, such as, for example, Ce, La, Nd, and / or Pr, and mixtures thereof. However, it is preferred to use a sufficient amount of cerium, as this metal gives the best mechanical properties. Mn is added to improve corrosion resistance, but its addition is limited due to limited solubility.

  Preferably, the aluminum content is 2.0 to 600 mass%, more preferably 2.60 to 4.50 mass%.

If higher amounts of aluminum are present, this can easily lead to the formation of Mg ₁₇ Al ₁₂ phase, which is detrimental to the creep properties. Too little Al is negative for castability. Regarding the RE metal, the RE content is preferably 3.50 to 7.00 mass%, and the upper limit is limited by the solubility of RE in the Mg-Al-RE system as shown in FIG. .

  If there is more than 3.50% by weight of RE, this gives a considerable improvement in creep properties. Above 7.00% by weight is impractical due to the limited solubility of RE metal in liquid magnesium-aluminum alloys.

  Furthermore, the RE / Al ratio is preferably greater than 0.9.

  For specific applications, the composition of the alloy is selected such that the aluminum content is 3.6-4.5% by mass and the RE content is 3.6-4.5% by mass, and further RE / There is an additional constraint that the Al ratio is greater than 0.9.

  This type of alloy can be used for applications below 175 ° C. and still exhibiting excellent creep properties and tensile strength. Moreover, this alloy does not show any deterioration of its properties over time and has good castability.

  For applications above 175 ° C, the alloy composition has an aluminum content of 2.6-3.5 wt% and an RE content of greater than 4.6 wt%.

  Apart from excellent creep properties and tensile strength, this alloy shows no deterioration of properties over time.

  Preferably, the RE metal is selected from the group of cerium, lanthanum, neodymium, and praseodymium.

  RE metal contributes to ease of alloying, but also enhances corrosion and creep resistance and improves mechanical properties.

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

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

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

  Preferably, the amount of cerium is more than 50% by weight of the total content of RE metal, preferably 50-55% by weight.

  Calcium and strontium are known to provide improved creep resistance, and the addition of at least 0.5% calcium by weight improves tensile strength.

  However, Ca and Sr should be avoided, even at very low concentrations, because these elements cause considerable adhesion problems, thereby affecting the castability of the alloy.

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

  In order to configure the influence of alloying elements, many Mg alloys were prepared with the compositions shown in Table 1.

For each alloy purpose, a number of test specimens were prepared and tested as described in the following examples. The tests performed are as follows. A 6 mm test piece was prepared in accordance with tensile strength and ductility ASTM, and was as follows.
Test conditions used ・ Instron machine of 10kN ・ From room temperature to 210 ℃ ・ Parallel at each temperature ・ Strain rate Strain of 0.5% or less 1.5mm / min Strain of more than 0.5% 10mm / min Test according to ISO 6892

Tensile creep test For this text, the following test materials are used.
・ Diameter 6mm
・ Gauge length 32.8mm
-Curvature radius 9mm
・ Grip head diameter 12mm
-Total length 125mm test is performed according to ASTM E139.

Stress relaxation test ・ Test material 12mm diameter 6mm length Creep piece from the free end ・ Test according to ASTM E328-86

  Corrosion properties Corrosion is tested according to ASTM 117.

  For many compositions, the strength was measured as a function of temperature. The results are shown in FIG. 12, FIG. 13, and FIG. In these figures, the Y-axis indicates the tensile strength expressed in MPa, while the X-axis indicates the temperature expressed in ° C.

  For many compositions, creep strain was measured as a function of time. The results are shown in FIG. 15 and FIG. In FIG. 15, the measurement was performed at 175 ° C. with a force of 40 MPa, and in FIG. 16, the measurement was performed at 150 ° C. with a force of 90 MPa. In these figures, the Y axis indicates the creep strain expressed as a percentage, while the X axis indicates the time period expressed in time.

  For many compositions according to Table 1, the stress relaxation was clarified and expressed as a residual load over time. The results are shown in FIG. 17, FIG. 18, and FIG. In these figures, the Y-axis indicates the remaining load expressed as a percentage of the initial load, while the X-axis indicates the time period expressed in time.

  For many compositions, the corrosion properties were defined according to ASTM B117. In this test, a large amount of data was taken in order to clarify the effect of RE content on Al content. The results are shown in FIG. In this figure, the Y-axis indicates the RE content expressed in mass%, and the X-axis indicates the Al content expressed in mass%. Borders between zones with different shading show equivalent corrosion resistance lines.

  From these test results, it is clear that a magnesium alloy casting method is provided that provides a product with an excellent combination of high temperature creep properties, ductility and corrosion performance.

It is a figure which shows the die-casting machine of this invention. It is a figure which shows the relationship between a solidification range and a fine structure. It is a figure which shows the relationship between a particle size and relative elongation. It is a figure which shows the tensile yield strength. It is a figure which shows the area | region of the castability of Mg-Al-Re type | system | group. It is a figure which shows the relationship between a solid-phase rate and temperature. It is a figure which shows the example which has a different fine structure in AE44, and has a thickness of about 3 mm which shows three layers. It is a microscope picture which shows the state of some alloys. It is a SEM-BEC image which shows a die-cast fine structure. It is a SEM-BEC image which shows the enlarged part of AE44. It is a figure which shows the creep resistance and ductility about Mg alloy. It is a figure which shows intensity | strength measurement. It is a figure which shows intensity | strength measurement. It is a figure which shows intensity | strength measurement. It is a figure which shows a creep distortion measurement. It is a figure which shows a creep distortion measurement. It is a figure which shows a residual load. It is a figure which shows a residual load. It is a figure which shows a residual load. It is a figure which shows corrosion resistance.

Explanation of symbols

10 Mold 11 Hydraulic Damping System 12 Shot Cylinder 13 Piston 20 Mold 21 Hydraulic Damping System 22 Shot Cylinder 23 Piston 35 Pure Mg Line 36 Mg Alloy Line 40 Solubility of RE at 680 ° C 41 RE of 640 ° C Solubility 42 area (dark)
43 area (middle)
44 area (bright)

Claims (18)

2.0 to 6.00 mass% aluminum,
3.50 to 8.00 mass% rare earth metal (RE metal), and less than 0.5 mass% (including 0%) manganese,
Zinc of less than 1.00% by mass (including 0%),
Calcium of less than 0.01% by mass (including 0%),
Less than 0.01% by mass (including 0%) of strontium, and
Consisting of magnesium and the remainder being an inevitable impurity,
Here, the ratio of the amount of RE metal to the amount of aluminum in mass% is greater than 0.8, and at least 40 mass% of the RE metal is cerium,
A method for casting a magnesium alloy comprising a total level of unavoidable impurities of less than 0.1% by weight, wherein the alloy is cast in a mold whose temperature is controlled in the range of 170 to 390 ° C. The mold is filled in a time equal to the product of a value between 5 and 500 in milliseconds and an average part thickness in millimeters, and a static metal pressure between 20 and 20 during casting. A process that is maintained at 70 MPa and then increased to 180 MPa or less.   The method according to claim 1, wherein the temperature of the mold is controlled at 180 to 340 ° C.   The method according to claim 1 or 2, wherein the filling time of the mold expressed in milliseconds is a time equal to a product of a value between 8 and 200 and an average part thickness expressed in millimeters.   4. A method according to any one of claims 1 to 3, wherein the static metal pressure is maintained at 30 to 70 MPa throughout casting.   The method according to any one of claims 1 to 4, wherein a cooling rate after casting is 10 to 1000 ° C / second.   The method of any one of Claims 1-5 whose aluminum content rate is 2.50-5.50 mass%.   The method according to any one of claims 1 to 6, wherein the RE content is 3.50 to 7.00 mass%.   The aluminum content is 3.6 to 4.5 mass%, the RE content is 3.6 to 4.5 mass%, and the ratio RE / Al is greater than 0.9. The method according to any one of the above.   The method according to claim 1, wherein the aluminum content is 2.6 to 3.5% by mass and the RE content is greater than 4.6% by mass.   The method according to any one of claims 1 to 9, wherein the RE metal is selected from the group of cerium, lanthanum, neodymium and praseodymium.   The method of claim 10, wherein the amount of lanthanum is at least 15% by weight of the total content of RE metal.   The method according to claim 10 or 11, wherein the amount of lanthanum is less than 35% by mass of the total content of RE metal.   The method according to any one of claims 10 to 12, wherein the amount of neodymium is at least 7% by weight of the total content of RE metal.   The method according to any one of claims 10 to 13, wherein the amount of neodymium is less than 20 mass% of the total content of RE metal.   15. A method according to any one of claims 10 to 14, wherein the amount of praseodymium is at least 2% by weight of the total content of RE metal.   The method according to any one of claims 10 to 15, wherein the amount of praseodymium is less than 10% by mass of the total content of RE metal.   The method according to any one of claims 10 to 16, wherein the amount of cerium is more than 50 mass% of the total content of RE metal.   The method according to any one of claims 10 to 17, wherein the amount of calcium and / or strontium is 0.01% by mass or less.

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