SE544427C2 - A Magnesium Alloy and a High Performance Magnesium Cylinder made from the Magnesium Alloy - Google Patents

Abstract

A magnesium alloy contains:Al: 1.8 - 4.2 wt %La Ce: 3.2 - 5.8 wt %Nd: 0.05 - 2.0 wt %Zn: 0.1 - 0.8 wt %Mn: 0.1 - 0.5 wt %the balance being Mg, and incidental elements in an amount in the interval 0.0 - 0.5 wt %. Advantages include that the weight is reduces, the castability is improved, the thermal conductivity is improved and the mechanical properties at the working temperature of the cylinder are excellent. There is further provided a cylinder for an internal combustion engine where the advantages of the alloy are utilized. A method for manufacture of a cylinder is also provided.

Description

A Magnesium Alloy and a High Performance Magnesium Cylinder made from the Magnesium Alloy Technical fieldThe present disclosure relates to a magnesium alloy. The present disclosure furtherrelates to a cylinder for a combustion engine manufactured by said magnesium alloy.

The present disclosure further relates to a method for manufacturing said cylinder.

Background art Handheld power tools, such as chainsaws, clearing saws and power cutters are typicallydriven by combustion engines, such as two-stroke engines. The cylinder is the spacethrough Which the piston travels, propelled by the energy generated from thecombustion of the air/fuel mixture in the combustion Chamber. The material around thespace in Which the piston travels is also is referred to as the cylinder. In an air-cooled,the outer Walls of the cylinder (s) are exposed to the airflow, to provide the primarymethod of cooling to the engine. Most air-cooled engines have cooling fins on theoutside of the cylinder(s) and each cylinder has typically a separate case in order to maximise the surface area available for cooling.

The cylinder in a two-stroke petrol handheld product is a heavy component and Weightreduction in handheld products is crucial. There is thereby a need for a lightweightcylinder material. Magnesium is a lightweight metal and is used as material in certain components to reduce Weight.

WO2006/ 105594 discloses a magnesium alloy, Which is intended to be used for cylinderblocks for engines of vehicles. The alloy demonstrated excellent high temperature creep properties but have proven somewhat difficult to die cast.

WO2009/0865 85 discloses a magnesium alloy, Which is intended to be used for cylinder blocks for engines of vehicles. The alloy is optimized for achieving excellent creep- strength in the cylinder blocks in combination with good castability of the alloy. Toachieve this, the alloy comprises balanced amounts of the rare-earth metals cerium andlanthanum, which provides increased creep-strength and improved castability.Aluminum is included in the alloy of WO2009/0865 85 in small amounts to increase thecreep-strength further.

WO2020/ 171758 discloses a magnesium alloy with good mechanical properties interms of tensile strength at elevated temperatures, such as up to 400 °C. The alloy isintended to be used for a piston of a combustion engine. It is taught that an increased addition of Al gives impaired mechanical properties.

Cylinders have in general a more complex form with ports and cooling fins and so oncompared to pistons. Thus, it is important that the castability is good for an alloy intended for a cylinder.

It is still a problem in the prior art to provide an alloy for a cylinder with low weight, good castability and with satisfactory mechanical properties.

Summary of InventionMany known magnesium alloys suffers from poor die-castability, which makes themunsuitable for large scale casting production methods for cylinders with their complex shape.

A material for a cylinder requires good mechanical properties at elevated temperatures.The inventors have realized that not only the properties at high temperatures areimportant, but also the therrnal conductivity. The inventors have realized that byincreasing the therrnal conductivity, the temperature in the material of the cylinder willbe lower during operation and then the mechanical properties at the highesttemperatures are not that important, since the working temperature of the cylinder willbe lower due to the increased therrnal conductivity. Thus, one idea behind the invention is to improve the therrnal conductivity so that the temperature of the cylinder becomes lower during operation and optimize the mechanical properties of the alloy for the slightly lower working temperature, while keeping the weight as low as possible.

It is an object of the present disclosure to provide an improved lightweight alloy for cylinders for combustion engines.

Changing the material from aluminum to magnesium can result in a weight reduction ofaround 30%. Changing the material in the cylinder from aluminum to magnesium can reduce the weight of the cylinder with 200-300 g depending on cylinder size.A mix of rare earth metals in the magnesium alloy can improve the mechanicalproperties, the properties regarding fatigue as well as therrnal conductivity to make it possible to manufacture the alloy in magnesium.

According to a first aspect there is provided a magnesium alloy containing: Al: 1.8 - 4.2 wt%La+Ce: 3.2-5.8 wt%Nd: 0.05 - 2.0 wt %Zn: 0.l - 0.8 wt %Mn: 0.l - 0.5 wt % The balance being Mg and incidental elements, wherein the incidental elements are present in an amount in the interval 0.0 - 0.5 wt %.

In a second aspect there is provided a cylinder for a combustion engine, said cylinder manufactured by the magnesium alloy according to the first aspect.

In a third aspect there is provided a method for manufacturing a cylinder for a combustion engine according to the second aspect.

Advantages include that the Weight is reduces, the castability is improved, the therrnalconductivity is improved and the mechanical properties at the working temperature of the cylinder are excellent.

As taught by for instance WO2020/ 171758 an increased amount of Al can impair themechanical properties at high temperatures. However, by improving the therrnalconductivity such high temperatures are not reached during operation. Thus, it hasunexpectedly been found that a higher amount of Al can be tolerated in the alloy and that the alloy will still have the required properties.

Without wishing to be bound by any particular scientific theory, the favorable propertiesof the magnesium alloy of the present disclosure may be explained as follows. In an Alcontaining Mg-matrix, Rare-earth elements (Re) such as La, Ce and Nd, form eutecticAl-Re phase more easily than Mg-Al eutectic phase and thereby suppress the quantity ofMg-Al eutectic phase. The Mg-Al eutectic phase has a negative impact on the high-temperature strength of the alloy because the Mg-Al eutectic phase has a low melting point of 437 °C, and it is unstable at elevated temperatures especially above 175 °C.

La increases therrnal conductivity, and thereby contributes to leading heat away andreducing the maximum temperature during operation. Thereby, by increasing the La content, the need for good mechanical properties at elevated temperatures is lessened.

Lanthanum (La) is a Re-element, which is available at low cost and readily forms stableeutectic phase with magnesium. In addition, La has low solubility and low eutecticcomposition point in magnesium at eutectic temperature. This improves castabilitybecause the solidification temperature range is reduced whereby solidification of thealloy is achieved in short time. The castability may be improved by increased amount ofLa, because this moves the alloy composition closer to the eutectic point and reducesthe solidif1cation range further. As the shape of a cylinder for a combustion engine is more complex than for example a piston, such as the piston manufactured in WO2020/171758, there is an increased need for good castability for the manufacturing of these. This is achieved in part by further increasing the La content.

Cerium (Ce) has similar behavior as La and may therefore replace some or all of the Lain the Mg alloy of the present disclosure. To achieve both good mechanical propertiesand better castability as well as improved therrnal conductivity, La + Ce may be presentin a total amount of 3.2 - 5.8 wt %. La + Ce means that the sum of the amounts of bothmetals are in the interval. The amount can be a combination of La and Ce, but it can also be solely La, or solely Ce.

Aluminum (Al) is added to achieve good mechanical properties at elevated temperaturesin the magnesium alloy according to the present disclosure. Although the detailedmechanism is still not fully understood from a scientific point of view, it has beenshown that small amounts of Al in Mg-Re alloys is benef1cial to the mechanicalproperties at elevated temperatures and thus improves the tensile strength of the alloy. Ithas further been shown in the prior art that the strengthening effect of Al in Mg-Realloys becomes invalid when Al is added in higher amounts; however, the increasedamount of Al in the alloy can be tolerated because the therrnal conductivity is improved.Al increases the therrnal conductivity so that the operating temperature of the cylinderbecomes lower and thereby a higher amount of Al is fine. Norrnally, high additions ofAl are usually avoided, as it is detrimental to the mechanical properties at elevatedtemperature. However, Aluminum increases therrnal conductivity, and therebycontributes to leading heat away and reducing temperature, thereby reducing the needfor good mechanical properties at elevated temperatures, at least for the highesttemperatures. In the present disclosure, the Al content is increased compared toWO2020/ 171758 to reduce the maximum temperature this way. The Al content of theMg-alloy is therefore 1.8 - 4.2 wt %. It was unexpected that the amount of Al and Lacould be increased compared to WO2020/ 171758 since the mechanical properties areimpaired by a high content of Al, but this is compensated by the improved therrnalconductivity which gives a lower operating temperature so that the mechanical properties of the alloy are not impaired at that operating temperature.

In one embodiment of the alloy according to the present disclosure, the sum of theamounts of Al, La, and Ce is in the interval 6.0 - 9.0 Wt %. In one embodiment of the alloy according to the present disclosure, the sum of the amounts of Al, La, and Ce is in the interval 7.0 - 8.0 Wt %. This is to achieve a suitable amount of eutectic Al-Re phase.

In one embodiment of the alloy according to the present disclosure, the amount of Al isin at least one of the intervals selected from the group consisting of 2.0 - 4.0 Wt %, 2.3- 3.7 Wt %, 2.5 - 3.5 Wt %, 2.7 - 3.3 Wt %, and 2.8 - 3.2 Wt %.

In one embodiment of the alloy according to the present disclosure, the amount of La +Ce is in at least one of the intervals selected from the group consisting of 3.4 - 5.6 Wt%, 3.7 - 5.3 Wt %, 3.9 - 5.1 Wt %, 4.0 - 5.0 Wt %, 4.1 - 4.9 Wt %, and 4.2 - 4.8 Wt %.The amount of La + Ce is interpreted so that the sum of the amounts of La and Ce is inthe interval. The amount of La + Ce can either be any combination of La and Ce as Wellas solely La or solely Ce. In one embodiment, the amount of La + Ce is only La. In oneembodiment, the amount of La + Ce is only Ce. In one embodiment according to thepresent disclosure, the alloy comprises La and no Ce. In one embodiment of the alloy according to the present disclosure, the alloy comprises Ce and no La.

Neodymium (Nd), is a rare-earth element that have good solubility in Mg and isincluded in the magnesium alloy according to the present disclosure in order to increasethe amount of Mg-Re eutectic phase and thereby the mechanical strength of the alloy.Nd is present With 0.05-2.0 Wt%. In one altemative embodiment of the alloy according to the present disclosure, the amount of Nd is in the interval 0.2 - 0.8 Wt %.

An advantage of using the particular alloy elements selected from La, Ce and Nd in thealloy of the present disclosure is that these elements are available in form of mixed rareearth metal, so called “mischmetal”. Such mixed rare earth metal is available in specificratios on the market at comparatively low cost and allows thus for production of a cost effective alloy With good mechanical properties and good castability.

Zinc (Zn) is a common element used in Mg alloys because of its benefits in providingimproved mechanical properties at high temperatures, machinability and castability. Inone embodiment of the alloy according to the present disclosure, the amount of Zn is in the interval 0.1 - 0.8 Wt %.

Manganese (Mn) helps to prevent die soldering and improves thus the die releasingcapability of the Mg alloy according to the present disclosure. Mn may further enhancethe strength of the alloy. However, more importantly, Mn contributes to neutralizeimpurities in the alloy. Namely, Mn combines With Fe to alter the morphology of Fe-containing compounds from needles to nodular to reduce the harrnful effect of Fe. Inone embodiment of the alloy according to the present disclosure, the amount of Mn is in the interval 0.1 - 0.5 Wt %.

In one embodiment of the alloy according to the present disclosure, the alloy comprises 2.8 Wt% Al, 4.2 Wt% La, 0.5 Wt% Nd, 0.2 Wt% Zn, and 0.2 Wt% Mn.

In one embodiment of the alloy according to the present disclosure, the alloy comprises 3.2 Wt% Al, 4.8 Wt% La, 0.5 Wt% Nd, 0.2 Wt% Zn, and 0.2 Wt% Mn.

The Mg alloy according to the present disclosure may further comprise incidentalelements. The incidental elements may be alloy elements that have negligible orinsignif1cant influence on the properties of the Mg-alloy. The incidental elements mayin some instances be considered as impurities. Non-limiting examples of incidentalelements are: Fe<0.2Wt %, Si<0.05Wt %, Dy<0.05Wt %, Ni<0.03Wt %, Er <0.0l Wt %,Ca < 0.05Wt % and Sr < 0.05 Wt %. These incidental elements are not elements, Whichare deliberately added to achieve a certain amount of these elements; on the contrary,they should typically be viewed as impurities. Impurities can be accepted if the amount is not too high.

Typically, the total amount of incidental elements are 0 - 0.5 Wt % in the Mg-alloy.

Magnesium (Mg) constitutes the balance in the Mg alloy. Taking account of theelements in the alloy including 0 - 0.5 wt % incidental elements the amount of Mg is 94.75-86.2 wt%.

In one embodiment, the cylinder is configured for a two-stroke combustion engine. Inone embodiment, the cylinder is for a handheld power tool. In one embodiment, the power tool is a chainsaw or a clearing saw.

In one embodiment at least a part of an outside surface of the cylinder (1) comprisescooling fins. Such a cylinder is typically intended for an air cooled engine. Thus, in oneembodiment the cylinder is an air cooled cylinder, where at least a part of the outsidesurface of the cylinder (1) comprises cooling fins. Such an air cooled cylinder can be forboth a two-stroke and a four-stroke engine. Water cooled cylinders in general have aless complex shape compared to air cooled cylinders. The alloy according to the presentdisclosure with its improved castability is particularly suitable for casting cylinders with complex shapes such as a cylinder comprising cooling fins.

In one embodiment of the method for manufacturing a cylinder for a combustionengine, the method comprises the steps of: a) providing (1000) a magnesium alloy according to the present disclosure; b) melting (2000) the magnesium alloy; c) casting (3 000) the magnesium alloy into a mold cavity def1ning the shape of acylinder; d) solidifying (4000) the magnesium alloy in the mold cavity; e) removing (5000) the solidified cylinder from the mold cavity.

In one embodiment of the method for manufacturing a cylinder for a combustionengine, the step c) of casting (3000) the magnesium alloy is made by High Pressure DieCasting.

Further embodiments are defined in the appended dependent claims, which are explicitly incorporated herein.

Brief description of the drawingsAspects and embodiments will be described with reference to the following drawings in which: Figure 1 shows a simplified schematic drawing of a two stroke combustion engine witha cylinder (1), piston (2), spark plug (3) and a connecting rod (4). The outside of thecylinder is equipped with cooling fins. The cylinder (1) defines the space in which the piston (2) moves.

Figure 2 shows a flowchart showing schematically the steps of a method according to the present disclosure.

Detailed description of embodiments Figure 1 shows schematically a cylinder 1 according to the present disclosure for acombustion engine. Here exemplified as a cylinder for a two-stroke engine for a hand-held motor tool. The cylinder 1 comprises, i.e. is manufactured from, the magnesium alloy according to the first aspect of the present disclosure.

The cylinder is in one embodiment by the following method. The steps of the method may be followed in figureThus, in a first step 1000 of the method, a magnesium alloy according to the presentdisclosure is provided. Typically, the magnesium alloy is provided in form of pre-manufactured solid pieces such as ingots. In a second step 2000, the magnesium alloy ismelted such that it assumes a liquid state. Melting is performed by heating themagnesium alloy above its melting point. Typically, the magnesium alloy may therebybe heated to a temperature of 720°C or above. In a third step 3000, the molten magnesium alloy is cast, i.e. poured into a mold having a mold cavity, which defines the shape of a cylinder for a combustion engine. For example, the mold cavity def1nes theshape of a cylinder for a two-stroke combustion engine. In a fourth step 4000 the moltenmagnesium alloy is allowed to solidify for a predeterrnined time in the mold cavity. Thesolidif1cation time depends on dimensions of the cylinder and casting conditions andmay be deterrnined in advance by e. g. practical trials. In a f1fth step 5000, the cylinder isremoved, from the mold cavity. The mold may thereby comprise two mold halveswhich may are movable away from each other to allow access to the mold cavity and the solidified cylinder.

Casting of the cylinder is in one embodiment made by High Pressure Die Casting(HPDC). In this process, molten metal is inj ected under Velocity and high pressure intoa forrning cavity that is formed between two mold halves that are clamped together. TheHPDC process allows for fast production of components with high dimensional accuracy due to that the forrning cavity is rapidly filled with molten metal.

The steps of melting of the magnesium alloy and the step of removing the solidif1ed cylinder may be comprised in the High Pressure Die Casting equipment.

After removal of the solidif1ed cylinder, in an optional sixth step 6000, the cylinder maybe subj ected to a machining operation, such a drilling and or tuming into final shape.It is appreciated that the cylinder may have any suitable dimensions for its intended application.

It is further appreciated the cylinder may be configured for four-stroke engines.Moreover, casting of the magnesium alloy may be achieved by other suitable castingprocesses. For example, sand casting, low-pressure die-casting, semi-solid metal processing or permanent mold gravity die-casting.

Examples Pure magnesium and zinc ingots, Mg-30Wt.%La, Mg-30Wt.%Ce, Mg-25Wt.%Nd andMg-5Wt.%Mn master alloys Were used as starting materials to make the desired alloyingcompositions. During alloy making, a electrical resistant fumace Was used to melt the materials in a steel crucible under protection of Nz + SA batch of 12 kg alloy Was melted at a temperature of 720 °C each time. After the meltWas homogenised in the crucible, a mushroom sample With <|>60x10 mm testing part forcomposition analysis Was made by casting melt directly into a steel mould. The compositions of the experimental alloys Were listed in Table 1 and TableCasting samples for mechanical properties and therrnal conductivity tests Were made bya 4500 kN cold chamber high pressure die casting machine. The die and pouringtemperatures Were controlled at 250 °C and 700 °C, respectively. Samples formechanical properties and therrnal conductiVity tests had a casting Wall thickness of 6mm, and mechanical properties and therrnal conductivity Were measured following thestandard methods defined by ASTM. For comparison, the industrially Widely usedAE44 die-cast magnesium alloy for elevated applications Was casted and tested, underthe same conditions. The test results of therrnal conductiVity and mechanical properties of the experimental alloys Were shown in Table 1 and Table 2, respectiVely. mThermalSample Alloy composition (Wt. %) conductivity(20°C, W/m-K)1 Mg-2.6Al-3.6La-0.3Nd-0.1Zn-0.1Mn 1282 Mg-2.6Al-2.0Ce-1.6La-0.5Nd-0.3Zn-0.3Mn 1243 Mg-2.8Al-3 .9La-0.3Nd-0.2Zn-0.2Mn 1264 Mg-2.8Al-2.0Ce-1.9La-0.5Nd-0.2Zn-0.2Mn 1255 Mg-3 .0Al-4.2La-0.3Nd-0.2Zn-0.2Mn 1246 Mg-3.0Al-3.0Ce-1.2La-0.5Nd-0.2Zn-0.2Mn 1217 Mg-3.2Al-4.5La-0.3Nd-0.3Zn-0.3Mn8 Mg-3.2A1-3.0Ce-1.5La-0.5Nd-0.3Zn-0.3Mn9 AE44 (Reference) 859Tensile properties (20°C)Sample Alloy cornposition (Wt. %) YS UTS El (MPa) (MPa) (%)1 Mg-2.6A1-3.6La-0.3Nd-0.1Zn-0.1Mn 132 242 9.22 Mg-2.6A1-2.0Ce-1.6La-0.5Nd-0.3Zn-0.3Mn 134 240 9.03 Mg-2.8A1-3 .9La-0.3Nd-0.2Zn-0.2Mn 136 240 8.84 Mg-2.8A1-2.0Ce-1.9La-0.5Nd-0.2Zn-0.2Mn 135 241 9.05 Mg-3 .0A1-4.2La-0.3Nd-0.2Zn-0.2Mn 140 243 8.76 Mg-3.0A1-3.0Ce-1.2La-0.5Nd-0.2Zn-0.2Mn 141 245 8.57 Mg-3.2A1-4.5La-0.3Nd-0.3Zn-0.3Mn 143 245 8.68 Mg-3.2A1-3.0Ce-1.5La-0.5Nd-0.3Zn-0.3Mn 145 247 8.39 AE44 (Reference) 128 242Note: YS-Yield strength; UTS-Ultirnate tensile strength; El-Elongation

Claims (18)

1. ClaimsA magnesium alloy containing: Al: 1.8 - 4.2 Wt %La + Ce: 3.2 - 5.8 Wt %Nd: 0.05 - 2.0 Wt %Zn: 0.1 - 0.8 Wt %Mn: 0.1 - 0.5 Wt % the balance being Mg and incidental elements, Wherein the incidental elements are present in an amount in the interval 0.0 - 0.5 Wt %. The magnesium alloy according to claim 1 Wherein the sum of the amounts of Al, La, and Ce is in the interval 6.0 - 9.0 Wt%. The magnesium alloy according to claim 1, Wherein the sum of the amounts of Al, La, and Ce is in the interval 7.0 - 8.0 Wt%. The magnesium alloy according to any one of claims 1-3, Wherein the amount ofAl is in at least one of the intervals selected from the group consisting of 2.0 - 4.0 Wt %, 2.3 - 3.7 Wt %,2.5 - 3.5 Wt %,2.7 - 3.3 Wt %, and2.8 - 3.2 Wt %. The magnesium alloy according to any one of claims 1 - 4, Wherein the amountof La + Ce is in at least one of the intervals selected from the group consisting of3.4 - 5.6 Wt %, 3.7 - 5.3 Wt %, 3.9 - 5.1 Wt %, 4.0 - 5.0 Wt %,4.1 - 4.9 Wt %, and4.2 - 4.8 Wt %. The magnesium a11oy according to any one of c1aims 1 - 5, Wherein the a11oy comprises La and no Ce. The magnesium a11oy according to any one of c1aims 1 - 5, Wherein the a11oy comprises Ce and no La. The magnesium a11oy according to anyone of c1aims 1 - 7, Wherein the amount of Nd is in the interva1 0.2 - 0.8 Wt %. The magnesium a11oy according to any one of c1aims 1 - 8, Wherein the amount of Zn is in the interva1 0.1 - 0.3 Wt %. The magnesium a11oy according to any one of c1aims 1 - 9, Wherein the amount of Mn is in the interva1 0.1 - 0.3 Wt %. The magnesium a11oy according to c1aim 1, Wherein the a11oy comprises:2.8 Wt% A1, 4.2 Wt% La, 0.5 Wt% Nd, 0.2 Wt% Zn, and 0.2 Wt% Mn. The magnesium a11oy according to c1aim 1, Wherein the a11oy comprises:3.2 Wt% A1, 4.8 Wt% La, 0.5 Wt% Nd, 0.2 Wt% Zn, and 0.

2. Wt% Mn. 1

3. A cylinder (1) for a combustion engine characterized in that the cylinder (1) is manufactured from a magnesium alloy according to any one of claims 1 -1

4. The cylinder (1) according to claim 13, Wherein the cylinder (1) is configured for a two-stroke engine. 1

5. The cylinder (1) according to any one of claims 13-14, Wherein the cylinder (1) is for a hand-held power tool. 1

6. The cylinder (1) according to any one of claims 13-15, Wherein at least a part of an outside surface of the cylinder (1) comprises cooling fins. 1

7. A method for manufacturing a cylinder (1) for a combustion engine comprisingthe steps:a) providing (1000) a magnesium alloy according to any one of claims 1 -12;b) melting (2000) the magnesium alloy;c) casting (3000) the magnesium alloy into a mold cavity defining the shapeof a cylinder (1);d) solidifying (4000) the magnesium alloy in the mold cavity;e) removing (5000) the solidified cylinder (1) from the mold cavity. 1

8. The method according to claim 17, Wherein the step c) of casting (3000) the magnesium alloy is made by High Pressure Die Casting.