Creep resistant, ductile magnesium alloys for die casting

Abstract

The invention provides magnesium alloys for high temperature applications that combine excellent castability with superior corrosion resistance, and with good creep resistance, ductility, impact strength, and thermal conductivity. The alloys contain mainly Al, La, Ce, and Mn, and are particularly useful for high-pressure die casting process.

Claims

1. A creep resistant ductile magnesium alloy maintaining mechanical properties at high working temperatures, said mechanical properties including tensile yield strength and ultimate yield strength at 150 c. of at least 118 and 165 mpa, respectively, said alloy consisting of 2.7 to 3.5 wt. % lanthanum (la), 2.6 to 5.5 wt. % aluminum (al), 0.1 to 1.6 wt. % cerium (ce), 0.14 to 0.50 wt. % manganese (mn), 0.0003 to 0.0020 wt. % beryllium (be), 0.05 to 0.25 wt. % zinc (zn), 0.02 to 0.38 wt. % tin (sn), 0.00 to 0.20 wt. % neodymium (nd), and 0.00 to 0.10 wt. % praseodymium (pr), and the balance being magnesium and unavoidable impurities, wherein said alloy is suitable for die casting and maintains elongation of more than 10% even after aging at a temperature of 150 c. for 2000 hours.

2. An article produced by casting a magnesium alloy of claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and other characteristics and advantages of the invention will be more readily apparent through the following examples, and with reference to the appended tables, wherein:

(2) FIG. 1. is Table 1, showing chemical compositions of alloys according to the invention and of comparative alloys;

(3) FIG. 2. shows the casting shot used for evaluation of susceptibility to hot cracking;

(4) FIG. 3. is Table 2 showing die casting parameters used at evaluation of susceptibility to hot cracking;

(5) FIG. 4. is table 3 showing percentage of crack free junctions for different alloys and die casting parameters;

(6) FIG. 5. is Table 4, showing bearing, shear, tensile and impact strength properties of the alloys;

(7) FIG. 6. is Table 5, showing the creep behavior, bolt load retention properties, corrosion resistance, and thermal conductivity of the alloys; and

(8) FIG. 7. is Table 6, showing variation of tensile properties depending on aging conditions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(9) It has been found that magnesium alloys exhibiting a superior combination of castability, mechanical and corrosion properties as well as thermal conductivity are obtained at affordable cost, when comprising certain elements as explained below. The present invention provides a family of magnesium based alloys comprising from 2.6 to 5.5 wt. % aluminum (Al), from 2.7 to 3.5 wt. % Lanthanum (La), from 0.1 to 1.6 wt. % Cerium (Ce); from 0.14 to 0.50% Manganese (Mn), from 0.0003 to 0.0020 wt. % Beryllium (Be), and optionally up to 0.35 wt. % Zinc (Zn); up to 0.40 wt. % Tin (Sn), up to 0.20 wt. % Neodymium (Nd), and up to 0.10 wt. % Praseodymium (Pr). The alloys of the invention may comprise incidental impurities that are normally present in magnesium alloys. Said alloys may comprise up to 0.004 wt. % Fe, up to 0.002 wt. % Ni, up to 0.08% Si and up to 0.01 Wt. % Cu.

(10) The invention is directed to an article produced by casting a magnesium alloy comprising from 2.6 to 5.5 wt. % Al, from 2.7 to 3.5 wt. % La, 0.1 to 1.6 wt. % Ce, from 0.14 to 0.50% Mn, from 0.0003 to 0.0020 wt. % Be; and optionally up to 0.35 wt. % Zn, up to 0.40 wt. % Sn, up to 0.20 wt. % Nd and up to 0.10% Pr. Said casting is preferably high-pressure die casting, however it may be also thixomolding, semisolid casting, squeeze casting, and gravity casting as well as low-pressure casting.

(11) The alloy of the invention exhibits superior bearing and shear properties both at room and elevated temperatures. The alloy also has excellent castability combined with superior corrosion resistance and impact strength properties, excellent creep performance and bolt load retention properties as well as exceptionally good ductility, impact strength properties and thermal conductivity. Alloying with Lanthanum and Cerium leads to the formation of stable intermetallics at grain boundaries of MgAl solid solution. Enhanced stability of these intermetallics at elevated temperatures results in superior alloy performance at service temperatures of up to at least 175 C. The alloys of the present invention further display low susceptibility to hot tearing and are not prone to die sticking and soldering over high-pressure die casting process, thixomolding and other casting processes. They also have excellent fluidity and are not prone to oxidation and burning.

(12) An alloy of the present invention exhibits exceptionally good impact strength, bearing strength and shear strength in combination with excellent creep and bolt load retention properties at temperatures up to 200 C. For the new alloys, the creep strength to produce 0.2% strain for 200 h is varied between 97 MPa to 108 MPa at testing temperature of 150 C., and between 80 MPa to 88 MPa at testing temperature of 175 C. An alloy according to the invention exhibits excellent Bearing Yield Strength (BYS) that is typically 320 MPa or more, said BYS values being preferably 330 MPa or more at room temperature. At 150 C., BYS values are typically more than 264 MPa, such as 270 MPa or more. An alloy according to the invention shows exceptionally good combination of tensile yield strength, ultimate tensile strength, elongation and impact strength properties. These alloys are not prone to embrittlement over long-term aging at 150 C. that simulates to a large extent the service conditions. Impact strength of the alloys is typically about 20 J while elongation is typically about 15%. Shear strength of the alloys is typically about 160 MPa or more at ambient temperature, and typically about 130 MPa or more at 150 C.; said shear strength values being in some embodiments 165 MPa or more at ambient temperature and 135 MPa or more at 150 C. Thermal conductivity of the alloys is typically about 85 W/K.Math.m or more. The alloys according to the invention combine excellent bearing and shear properties with exceptionally good ductility, creep behavior and bold load retention properties. These alloys also have better corrosion resistance than comparative alloys.

(13) Magnesium-based casting alloys, which have chemical compositions according to the present invention, as noted hereinbefore outperform the prior art alloys in mechanical, technological, and corrosion properties. These properties include excellent molten metal behavior and castability combined with improved bearing, shear, tensile and impact strength properties, and as well as excellent corrosion and creep resistance, ductility, and bolt load retention properties. The alloys of the present invention contain aluminum, lanthanum, cerium, manganese, and beryllium. As discussed below they may also contain other elements as additional ingredients, or incidental impurities.

(14) The magnesium-based alloy of the present invention comprises 2.6 to 5.5 wt. % aluminum. If the aluminum concentration is less than 2.6 wt. %, the alloy will exhibit poor castability properties, particularly low fluidity, insufficient strength properties, and remarkable tendency to shrinkage formation on top surface of ingots that in some cases may lead even to cracks formation. On the other hand, aluminum concentration higher than 5.5 wt. % leads to significantly lower susceptibility to hot cracking, deterioration of ductility, impact strength properties, bearing strength, creep resistance, bolt load retention properties and thermal conductivity.

(15) The preferred ranges for Lanthanum and Cerium are 2.7 to 3.5 wt. %, and 0.1 to 1.6 wt. %, respectively. The above two elements form with aluminum stable eutectic intermetallic compounds that impede grain sliding. In addition, alloying with La and Ce leads to prevention of formation of brittle Mg.sub.17Al.sub.12, intermetallic compounds. Both these factors improve creep resistance. Furthermore, it was unexpectedly found that when La is dominating alloying element, the main intermetallic compound is Al.sub.11(La,Ce).sub.3. This phase is much preferable than Al.sub.2(Ce, La) intermetallic phase which is mainly formed in alloys enriched in Ce. This is related to the fact that in the Al.sub.11(La,Ce).sub.3 intermetallic phase more than 3.5 aluminum atoms are bound to one RE elements atom, while in the Al.sub.2(Ce, La) intermetallic phase just two Al atoms are bound to one RE elements atom. Thus, once the Al.sub.11(La,Ce).sub.3 eutectic intermetallic compound is formed, lower concentration of RE elements is required to suppress the formation of Mg.sub.17Al.sub.12 intermetallics, harmful for creep resistance. On the other hand, at the same concentrations of La and Ce, more eutectic phase is formed in the case of Al.sub.11(La,Ce).sub.3 intermetallics than in the case of Al.sub.2(Ce, La) intermetallics. This in turn leads to shortening the freezing range and lower susceptibility to hot cracking.

(16) If the Lanthanum content is less than 2.7 wt. %, it does not gives rise to the formation of sufficient amount of Al.sub.11(La,Ce).sub.3 intermetallics, thereby leading to the deterioration of creep resistance and to increased tendency to hot cracking. It should be noted that the Al.sub.11(La,Ce).sub.3 intermetallic compound, which is enriched in La is more stable than that one enriched in Ce. On the other hand, the La content higher than 3.5% results in reduced fluidity, excessive oxidation and melt loss, necessity of additional stirring at the die casting furnace and unnecessarily further increase of the alloy cost because La is more expensive than Mg. The effect of La is more remarkable in combination with Ce. The Ce content less than 0.1% insignificantly affects the formation of Al.sub.11(La,Ce).sub.3 intermetallics. The Ce concentration higher than 1.6% results in intensive formation of less desirable AL.sub.2(La,Ce) intermetallic phase at the expense of Al.sub.11(La,Ce).sub.3 intermetallics. In addition, it also leads to decreasing the alloy fluidity, increasing the melt loss without stirring at the die casting shop and unnecessarily further increase of the alloy cost. Beryllium is added into alloys of this invention in the amount of 0.0003 to 0.0020 wt. % in order to prevent burning, and to reduce dross and sludge formation. The Be content less than 0.0003% does not provide effective protection against oxidation. The Be content higher than 0.0020 leads to contamination by non-metallic inclusions and unreasonable increase of an alloy cost.

(17) It was also unexpectedly found that small additions of Zn in the range of up to 0.35 wt. %, such as between 0.05 and 0.25 wt %, may improve castability and creep resistance. On the other hand, the Zn content higher than 0.35% results in increased tendency to die sticking and deterioration of creep resistance. This positive effect of Zn is more remarkable in the presence of Sn in the range of up to 0.40 wt. %. The Sn content higher than 0.40 wt. % may result in the deterioration of creep resistance and in unjustified increase of the alloy cost. The alloys of the present invention contain minimal amounts of iron, copper and nickel, to maintain a low corrosion rate. There is preferably less than 0.004 wt. % iron, and more preferably less than 0.003 wt. % iron. A low iron content can be obtained by adding manganese. The iron content of less than 0.003 wt. % can be achieved at minimal residual manganese content 0.14 wt. % in the alloy. Adding Mn in amounts higher than 0.50 wt. % leads to reduction of ductility and impact strength, unjustified increase of the alloy cost and to excessive sludge formation over ingots remelting and melt holding prior to the high-pressure die casting process. Optionally, the alloys of the present invention may also contain up to 0.20 wt % Nd, and up to 0.10% Pr.

(18) The magnesium alloys of the instant invention exhibit high impact strength, bearing strength and shear strength, as well as enhanced ductility combined with excellent creep resistance and bolt load retention properties. They also have excellent castability and corrosion resistance.

(19) The invention will be further described and illustrated in the following examples.

EXAMPLES

General Procedures

(20) Series of experiments were contacted using the electric resistant furnace with 120 liter crucibles made of low carbon steel. During melting and holding, the melt was protected under a gas mixture of CO.sub.2+0.5% HFC134a

(21) The experimental alloys were prepared using different starting materials: pure Mg of grade 9980A as well as Magnesium alloys of AM and AZ alloying systems comprising 0.001-10.5 wt. % of Aluminum, 0.05-2.5 wt. % of Manganese and 0.001-1.5% Zn (for example, M2, AM20, AM50 AM60, AM100, AZ91D). The above alloys were used in the form of ingots or as a clean die casting scrap. The alloying procedure was performed in the temperature range of 670-730 C.

(22) Manganesean AlMn master alloy containing 60-90% Mn, compacted Mn powder and M2 magnesium alloy containing about 2% Mn were used for alloying with Mn. The above materials were added to molten metal at a melt temperatures from 700 C. to 740 C., depending on the manganese concentration in the master alloy.

(23) Aluminumcommercially pure Al containing less than 0.2% impurities was used in some cases for the chemical composition correction.

(24) Rare earth elementsa lanthanum based mischmetal comprising 70-80% La+20-30% Ce and a cerium based mischmetal comprising 65% Ce+35% La were mainly used. In addition, pure La, pure Nd and pure Pr were partially used along with a cerium based mischmetal comprising 50% Ce+25% La+20% Nd+5% Pr.

(25) Tinpure tin containing less than 0.5% impurities was used.

(26) Zincpure zinc containing less than 0.3% impurities was used.

(27) Berylliumup to 20 ppm of beryllium were added to the new alloys in the form of a master alloy Al-1% Be, following settling the melt at temperatures of 650-690 C. prior to casting.

(28) After obtaining the required compositions, the alloys were cast into the 12 kg ingots. Neither burning nor oxidation was observed on the surface of all the experimental ingots.

(29) On the second stage, the above experiments were carried out in the industrial conditions using alloying furnace with the capacity of 2 tons. In the above experiments pure Mg or Mg alloys were transferred to the alloying furnace in the molten state from the continues refining furnace with the capacity of 20 tons. After alloys preparation and settling, the molten metal was cast into ingots with weights varied between 6 to 23 kg in different experiments.

(30) Chemical analyses were conducted using spark emission spectrometer.

(31) The die casting trials were carried out using an IDRA OL-320 cold chamber die casting machine with a 345 ton locking force.

(32) Die lubrication (Acheson cp-593 lubricant) and metal ladling were performed manually. The mixture of CO.sub.2+0.5% HFC134a with flow rate of 20 l/min was used as a protective gas.

(33) The casting temperature was varied in the range of 660-720 C. while the die temperature was varied between 100 and 340 C. for different compositions and experiments. The die was filled in a time between 5 and 250 milliseconds. The shot sleeve filling ratio was varied in the range of 15-65%. The static metal pressures that was maintained during casting varied between 15 and 120 MPa. The dwell time of the molten metal in the die was varied between 3 and 15 seconds.

(34) Experiments for evaluation of alloy susceptibility to hot cracking were performed using a specially designed test-part schematically shown in FIG. 2.

(35) Prior to experimental casting, the main HPDC process parameters, such as injection profile, melt temperature and die temperature, were optimized for the test-part shown in FIG. 2 according to the physical properties of each alloy. The part is divided into several sections. Each section contains a junction between different wall thicknesses. The impact strength specimens are designated for evaluation of properties homogeneity throughout the test-part and were not addressed in the present invention.

(36) All HPDC samples were X-rayed using SIEFERT ERSCO 200 MF constant potential X-ray tube. Table 1 presents the process parameters that were examined. The second phase velocity, different intensification pressure and molten metal temperature were used as variable parameters for each alloy tested. These parameters were selected in order to generate solidification shrinkage which in turn causes hot cracking during solidification of the casting. For each of the 24 variants listed in Table 2, ten components were die cast in order to obtain representative results.

(37) As can be seen in FIG. 2, the hot cracking evaluation part was designed with different thicknesses in order to provide different solidification time. Each wall section has different thickness and therefore it solidifies differently. The shrinkage between the wall sections causes hot cracking formation. The parts were inspected in terms of hot cracking appearance, and then the results obtained at different junctions were averaged. This procedure was performed for ten parts that were cast at the same casting conditions (temperature, pressure, plunger velocity) with subsequent averaging of results obtained on all parts.

(38) Corrosion performance was evaluated by SAE J2334 cyclic corrosion test, which is considered as showing the best correlation with car exploitation conditions. According to the above standard, each cycle required a 6 hours dwell in 100% RH atmosphere at 50 C., a 17.4 hours dry stage in 50% RH atmosphere at 60 C. Between the main stages a 15 minutes dip in an aqueous solution (0.5% NaCl, 0.1% CaCl.sub.2, 0.07% NaHCO.sub.3) was performed. At weekends and holidays the test was ran on the dry mode. The test duration was 80 cycles that corresponds to 5 years of car exploitation. The tests were performed on plates with dimensions of 140+100+3 mm. The plates were degreased in acetone and weighed prior to the immersion in the test solution. Five replicates of each alloy were tested. At the end of the test, the corrosion products were stripped in a chromic acid solution (180 g CrO.sub.3 per liter solution) at 80 C. about three minutes and the weight loss was determined. Then the weight loss was used to calculate the average corrosion rate in mils per year (MPY) over the 80 days period.

(39) Tensile testing at ambient and elevated temperatures was performed using an Instron 4483 machine equipped with an elevated temperature cabinet as per ASTM standards B557M. Tensile yield strength (TYS), Ultimate Tensile Strength (UTS) and percent elongation (% E) were measured. The Shear Strength was measured as per ASTM B565 standard using cylindrical samples with a 6 mm diameter excised from the gage area of tensile samples. The Bearing yield strength was measured as per ASTM E 238-84(08) standard using the corrosion plates with dimensions of 1001403 mm having a hole for pin with 8 mm diameter. Edge distance of 2 mm was used. Bearing Yield Strength was calculated as offset equal to 2% of the pin diameter. The impact strength properties were tested on Charpy hammer. Un-notched specimens with dimensions of 10 mm10 mm55 mm were used.

(40) The SATEC Model M-3 machine was used for creep testing. Creep tests were performed at 150 C. and 175 C. for 200 hrs under a stresses in the range of 40 to 110 MPa in order to determine the creep strengths at the above temperatures. Furthermore, bolt load retention was measured. This parameter is used to simulate the relaxation that may occur in service conditions under a compressive loading. The cylindrical samples with outside diameter of 17 mm containing whole with a 10 mm diameter and having height of 18 mm were used. These specimens were loaded to certain stress using hardened 440C stainless still washers and a high strength M8 bolt instrumented with strain gages. The change in load over 200 h at 150 C. and 175 C. was measured continuously. The ratio of two loads, namely the load at the completion of the test after returning at ambient condition to the initial load at room temperature is a measure of the bolt load retention behavior of an alloy.

Examples of Alloys

(41) Tables 1 to 6 present chemical compositions and properties of alloys according to the invention and alloys of comparative examples. The chemical compositions of 12 novel alloys along with 8 comparative examples are listed in table 1.

(42) Table 3 demonstrates that new alloys exhibit lower susceptibility to hot cracking than comparative alloys at all second phase piston velocities and intensification pressures estimated by percentage of crack free junctions as it is shown in FIG. 2.

(43) Table 4 shows the bearing, shear, impact strength and tensile properties of new alloys along with those of the comparative alloys. The alloys of the present invention exhibit significantly higher Bearing Yield Strength (BYS) and Impact Strength than those of comparative alloys. Furthermore, Shear Strength, Tensile Yield Strength (TYS) and Ultimate Tensile Strength (UTS) of new alloys also surpass those properties of comparative alloys both at ambient temperature and at 150 C. The main difference is also seen in elongation values of new alloys of present invention and comparative alloys.

(44) Table 5 demonstrates creep behavior, bolt load retention properties and corrosion resistance of new alloys along with those properties of comparative alloys. Corrosion resistance of new alloys evaluated under SAE J2334 cycling outperforms that of the alloys of Comparative Examples. As can be seen from Table 5, the alloys of the present invention are superior to the comparative alloys in creep resistance and bolt load retention properties. One of important requirements to creep-resistant alloys is their ability to maintain mechanical properties over exploitation period. Since creep resistant magnesium alloys should serve in the temperature range of 120-170 C. the ability of the alloys to maintain their properties can be evaluated by comparison the properties of as cast material and after long-term aging for 2000 h at the temperature of 150 C. (Table 6). This table clearly demonstrates that the alloys of present invention have much more stable properties than comparative alloys. This is most remarkable for elongation. This property after aging at 150 C. for 2000 h experiences reduction of 7-15% for the alloys of instant invention while the elongation of comparative alloys undergoes the reduction in the range of 25-44% under the same test.

(45) While this invention has been described in terms of some specific embodiments, it will be appreciated that other forms can readily be adapted by one skilled in the art. It is therefore understood that within the scope of the appended claims, the invention may be realized otherwise than as specifically described.