Patent Application: US-201113699402-A

Abstract:
an improved magnesium - based alloy for wrought applications is disclosed , including a method of fabricating alloy sheet from said alloy . the improved magnesium - based alloy consists of : 0 . 5 to 4 . 0 % by weight zinc ; 0 . 02 to 0 . 70 % by weight a rare earth element , or mixture of the same including gadolinium ; and incidental impurities . the rare earth clement in some embodiments may be yttrium and / or gadolinium . in some embodiments the magnesium - based alloy may also consist of a grain refiner and in some embodiments the grain refiner may be zirconium . in combination , the inclusion of zinc and a rare earth element , into the magnesium alloy may have enhanced capacity for rolling workability , deep drawing at low temperatures and stretch formability at room temperature . the improved alloy may also exhibit increased tensile strength and formability while evincing a reduced tendency for tearing during preparation .

Description:
the mg — zn based alloy system is considered a suitable candidate for wrought alloy development because both the strength and ductility of the alloy can be increased by increasing the zinc content up to a certain amount . ductility of the mg — zn system will increase with zinc until a maximum of 3 wt % is reached , and starts to decrease with further increase in zinc content . however , the strength of the alloy will increase until a maximum of 6 wt % is reached . as per the mg — zn binary phase diagram of reference 5 , the amount of zinc in solid solution at 340 ° c . is 6 . 2 wt % and at room temperature is close to 1 . 8 wt %. an alloy containing zinc above 1 . 5 wt % will start to form second phase along the grain boundary , the extent of which will increase with increasing zinc content . the small grain size achieved by the trc process and the small amount of second phase formed with zinc contents below 3wt %, allow the sheet to be rolled easily . the small grain size can be achieved by the addition of zirconium to a dc cast billet . although alloys containing zinc above 3 wt % can be cast via the twin - roll casting or dc casting route , the amount of second phase formed along the grain boundary will be much higher . this alloy will require longer homogenisation time to take the grain boundary phase into solution . further the higher zinc content will reduce the ductility of the alloy . for such an alloy to be successfully hot rolled , the percentage reduction per pass will have to be in the range of 10 - 15 % compared to 30 - 35 % achieved for alloys containing zinc below 3 wt %. this will increase the number of roll passes required to achieve the final thickness for an alloy containing zinc above 3 wt % compared to an alloy with zinc below 3 wt %, thus making the system economically less attractive . the magnesium alloy of described embodiments was formed by melting requisite quantities of mg , zn and a rare earth element . two embodiments of the alloy in accordance with the invention were formed comprising magnesium , zn and master alloys of yttrium or gadolinium ( mg with 27 wt . % y and mg with 40 wt . % gd master alloys for example but not restricted to ), respectively , in appropriate amounts were added in an 80 kg furnace ( with about 10 to 15 % excess amount of rare - earth element to account for losses ) to make up 50 kg of the alloy . in each case , the purity of the mg component is about 99 . 95 %, whereas the purity of the zinc component is about 99 . 9 %. the alloy formed is suitable for magnesium billet , sheet or slab production as well as extrusion to form a desired shape . fig1 illustrates a flow chart depicting a method of fabricating a magnesium alloy sheet . at step 105 a magnesium alloy melt is provided according to the composition described herein . at step 110 , the respective alloys were cast using trc or by sand casting with chill plates on the two faces of the casting to provide a faster cooling rate . sand casting , whilst not used extensively in commercial applications , is capable of simulating the effects which would be derived from continuous and semi - continuous casting like direct chill ( dc ) casting . alternatively , any other casting processes like dc casting may be used for this step . dc casting can be performed as described in any of references 1 to 3 , the contents of which are incorporated herein by reference in their entirety . the strip or slab could also be made from a dc cast billet which has been subsequently extruded to a slab or strip such as described in reference 4 , the contents of which are incorporated herein by reference in its entirety . in one embodiment alloys were cast using trc to produce strips approximately 150 mm wide and with two different thicknesses : 3 . 00 mm and 4 . 35 mm . it should be noted that the alloy can be cast wider using trc depending on the size of the commercial trc machine . the method of trc of magnesium alloys as substantially described in pct / au2003 / 001097 , assigned to the commonwealth scientific and industrial research organisation , and incorporated herein by reference in its entirety . in an alternative embodiment , alloys were cast using sand casting to provide slabs approximately 195 mm in length , 115 mm wide and 29 mm thick . at step 115 , the cast strip or slab is homogenised , or preheated , at a selected temperature and for a selected period of time . homogenisation or preheating is employed to reduce the interdendritic segregation and compositional differences associated with the casting process . a suitable commercial practice is to choose a temperature , usually 5 to 10 ° c ., below the non - equilibrium solidus . given that magnesium and zinc are the major constituents in the alloys , a temperature range of 335 ° c . to 345 ° c . (± 5 ° c .) is preferable . for the present examples a temperature of approximately 345 ° c . (± 5 ° c .) was chosen from the mg — zn binary phase diagram depicted in reference 5 . for dc casting generally temperatures between 450 ° c . to 500 ° c . are commonly used . the time required for the homogenisation step is dictated by the size of the cast strip or slab . for trc strip a time of 2 to 4 hrs is sufficient , while for sand cast slab or direct - chill cast slab up to 24 hrs will be required . the homogenised strips or slabs were hot rolled at a suitable temperature , step 120 . the rolls themselves are generally warm with temperatures of 80 ° c . to 120 ° c ., however cold rolls may also be used . depending on the cast material different rolling steps are used . for alloy slabs with a thickness above 25 mm produced by sand casting , dc casting or any other type of casting , a break - down rolling step is used . techniques described in either of references 1 or 6 may be employed . the content of reference 6 is incorporated herein by reference in its entirety . the aim of this step is to reduce the thickness , as well as to refine and remove the cast structure . the temperature for this step is dependent on the furnace available at the rolling facility , but usually a temperature between 450 to 500 ° c . is employed . once a thickness of 5 mm or lower is reached , rolling is performed at a temperature between 250 ° c . to 450 ° c . for alloy strips produced by trc , rolling is performed at a temperature between 250 ° c . to 450 ° c . without the need of a break - down rolling step . after each pass the strip or slab may be re - heated for about 10 to 15 minutes to bring the temperature up before the next pass . a few cold passes with a percentage reduction per pass of 10 % may also be used as a final rolling or sizing operation . this process is continued until the final thickness ( within the set tolerances ) is achieved , at step 125 . at step 130 , the hot rolled sheets were then annealed at a suitable temperature and time . annealing is a heat treatment process designed to restore the ductility to an alloy that has been severely strain - hardened by rolling . there are three stages to an annealing heat treatment — recovery , re - crystallisation and grain growth . during recovery the physical properties of the alloy like electrical conductivity is restored , while during recrystallisation the cold worked structure is replaced by new set of strain - free grains . recrystallisation can be recognised by metallographic methods and confirmed by a decrease in hardness or strength and an increase in ductility . grain growth will occur if the new strain - free grains are heated at a temperature above that required for recrystallisation resulting in significant reduction in strength and should be avoided . recrystallisation temperature is dependent on the alloy composition , initial grain size and amount of prior deformation among others ; hence , it is not a fixed temperature . for practical purposes , it may be defined as the temperature at which a highly strain - hardened ( cold worked ) alloy recrystallises completely in 1 hour . the optimum annealing temperature for each alloy and condition is identified by measuring the hardness after exposing the alloy at different temperatures for 1 hr , and establishing an annealing curve to identify the approximate temperature at which re - crystallisation ends and grain growth begins . this temperature may also be identified as the inflection point of the hardness - annealing temperature curve , as described in reference 7 , the content of which is incorporated herein by reference in its entirety . although this technique is used for non - ferrous alloys , this has not been applied before to hot rolled magnesium alloys . in order to ascertain the most suitable annealing temperature this technique was used for the present investigation . accordingly , approximate annealing temperature for each magnesium alloy was chosen using an annealing curve as demonstrated in the examples which follow and with reference to fig2 to 4 . this technique allows achieving the optimum temperature easily and reasonably accurately . a series of experiments were undertaken to test the relative merit of the described alloy embodiments , and to establish the low temperature formability of the alloys having been fabricated to form a sheet product . two examples of the alloy in accordance with the embodiments were tested . in the first embodiment the rare earth component was yttrium . the alloy contained 2 . 0 % by weight zinc , 0 . 3 % by weight of yttrium ( nominal compositions ) with the remainder being magnesium . this alloy is referred to as mg - 2zn - 0 . 3y . in the second embodiment the rare earth component was gadolinium . this alloy contained 2 . 0 % by weight zinc , 0 . 3 % by weight of gadolinium ( nominal compositions ) with the remainder being magnesium . this alloy is referred to as mg - 2zn - 0 . 3gd . conventional az31b was further tested . in addition comparisons were referenced against existing alloys : mg - 1 . 5zn - 0 . 2y and mg - 1 . 5zn - 0 . 8y , as described in reference 8 ; and mg - 1 . 2zn - 0 . 79gd and mg - 2 . 26zn - 0 . 74gd , as described in reference 9 . the improved rollability of the alloys is demonstrated by comparing them to the conventional alloy az31b . in the first instance , the results from the trc strips are presented followed by sand castings . all the rolling work was performed in a two - high rolling mill with un - heated rolls ( rolls at room temperature ). the sheet dimensions , pre - rolling treatment and process parameters are detailed in table 1 . the roll settings for each pass and the sheet thickness after each pass , etc ., are given in table 2 . as evident in the table , six passes were required to reduce 3 mm thick az31b strip to a final thickness of 0 . 73 mm . the annealing temperature shown in table 1 is used in practice . this annealing step could be performed at 200 ° c . for trc strips . this alloy was rolled at two different temperatures , 420 ° c . and 350 ° c ., to demonstrate that the alloy not only has improved rollability when compared to az31b but can also be rolled at a lower temperature . the sheet dimensions , pre - rolling treatment and process parameters are detailed in table 3 and 5 , respectively , for the two rolling temperatures . as evident from table 4 and 6 , that details the roll settings for each pass , sheet thickness after each pass , etc ., only three passes are required to reduce the 3 mm thick strip to a final thickness of 0 . 74 mm or 0 . 77 mm , respectively . the annealing temperature in table 3 and 5 is chosen from the annealing curve shown in fig2 . fig2 depicts the three stages of an annealing heat treatment previously mentioned , those being recovery , re - crystallisation and grain growth the sheet dimensions , pre - rolling treatment and process parameters are detailed in table 7 for this alloy . in this example the sheet thickness is about 1 . 2 mm more than that of az31b and mg - 2zn - 0 . 3y presented above ( or ˜ 40 %). as evident from table 8 it took only six passes to roll this alloy strip from an initial thickness of 4 . 25 mm to a final thickness of 0 . 84 mm at a rolling temperature of 350 ° c . this confirms the superior rollability of the mg - 2zn - 0 . 3gd alloy compared to az31b . the annealing temperature in table 7 was chosen from the annealing curve shown in fig3 . rollability of the sand castings of conventional alloy az31b and mg - 2zn - 0 . 3gd are presented in this section . the slabs were initially rolled length wise and once the slab reached 300 mm , was rotated 90 ° and rolled until the final pass . this rotation is identified in the tables showing the rolling schedule as cross - rolled . as described before , higher homogenisation temperature and time as well as breakdown rolling is necessary for sand castings . the slab dimensions and process variables are given in table 9 , while the rolling schedule is given in table 10 . a total of 11 passes was required to reduce the thickness of the slab from an initial thickness of 26 mm to a final thickness of 0 . 9 mm . the slab dimensions and process variables are given in table 11 , while the rolling schedule is given in table 12 . it took a total of 9 passes to reduce the thickness of the slab from an initial thickness of 26 mm to a final thickness of 0 . 9 mm . the reduction in the number of passes demonstrates the improved rollability of the mg - 2zn - 0 . 3gd alloy . the annealing temperature is selected from the annealing curve shown in fig4 , established for the sand cast alloy . tensile properties of the rolled and annealed sheets ( the finished product ) at room temperature were measured using a screw driven instron tensile testing machine . tensile specimens from both the longitudinal , direction ( also referred to as rolling direction or 0 ° orientation ) and transverse direction ( 90 ° to the rolling direction or 90 ° orientation ) were punched from the sheet for testing . the specimens were 6 mm wide and the gauge length was 25 mm . the results for the alloys are the average of six samples tested for each case . in magnesium alloys the basal planes of the hcp crystal structure tends to orient approximately parallel to the surface during rolling . a sheet with this preferred orientation will have the tensile properties higher in the 90 ° orientation compared to 0 ° orientation . tensile properties of trc and sand cast az31b is shown in table 13 . as expected for magnesium alloys the tensile properties of the specimens , especially the proof stress and the ultimate tensile stress , from the 0 ° orientation is lower than that of the specimens from the 90 ° orientation . the table also shows the tensile properties of the trc az31b after annealing at the optimum temperature of 200 ° c . for 1 hr ( highlighted with an astrix ). the tensile properties are certainly , higher than that achieved after annealing at 350 ° c . tensile properties of the trc mg - 2zn - 0 . 3y are presented in table 14 along with the properties of two similar alloys published in the literature . as expected the proof stress and ultimate tensile stress of the specimens from the 0 ° orientation is lower than that of the specimens from the 90 ° orientation for the trc sheet , while this is not the case for the two alloys in the published literature . the proof stress of these alloys is higher for the specimens from the 0 ° orientation compared to the specimens from the 90 ° orientation . similar results were observed for the trc sheet as shown in table 15 . however , by carefully choosing the process conditions , especially the homogenisation temperature and rolling temperature , it was possible to achieve higher proof stress on both orientations . this is very important as a sheet supplier because when an end user specifies a minimum proof stress , it is expected that the sheet meets that minimum value in all the orientations . tensile properties from specimens taken from the trc and sand cast sheets are shown in table 16 along with the properties of two similar alloys published in the literature . the proof stress and ultimate tensile strength of the specimens from the 90 ° orientation is higher than that of the specimens from the 0 ° orientation . this was not the case with the alloys published in the literature . as described in the section for mg - 2zn - 0 . 3y alloy , by carefully choosing the homogenisation and rolling temperatures it was possible to achieve higher values for both orientations . tensile properties , in three orientations , from specimens taken from the trc are shown in table 17 along with their respective percentage elongation . the proof stress and ultimate tensile strength of the specimens from the 90 ° orientation are higher than that of the specimens from the 0 ° orientation , except for the mg - 1zn - 0 . 65gd alloy . a series of tests were undertaken to ascertain the degree of formability of trc mg - 2zn - 0 . 3y and trc mg - 2zn - 0 . 3gd with trc az31b as a reference material . formability or workability is defined as the amount of deformation that can be given to a specimen without fracture in a given process . the tests , referred to below , included a swift cup test for deep drawing and an erichsen test to measure the stretch formability of the respective sheet metal . deep drawing tests using the hot rolled and annealed sheets of mg - 2zn - 0 . 3y , mg - 2zn - 0 . 3gd and az31b were performed using a 40 mm flat bottom punch . two sizes of discs were cut from the sheet ( 100 mm and 82 mm in diameters ) to achieve a limiting draw ratio ( ldr ) of 2 . 5 and 2 . 05 . the tests commenced using the 100 mm disc with a die temperature of 225 ° c . if the draw was successful , the next sample was drawn at 25 ° c . lower than the last draw and the process repeated . if , however , the draw was unsuccessful , the temperature was raised by 10 ° c . and tried again until the lowest temperature at which the disc could be drawn successfully was established . the 82 mm disc was then used and the process above repeated until the lowest temperature at which the 82 mm disc could be successfully drawn was identified . the results from the deep drawing test are shown in table 18 . as shown from the test results , the alloys in accordance with various embodiments of the invention can be deep drawn at lower temperatures than that required for az31b . for the limiting draw ratio ( ldr ) of 2 . 05 , the lowest temperature at which the yttrium containing alloy can be successfully deep drawn was 160 ° c ., while for the gadolinium containing alloy it was 135 ° c . both these temperatures are lower than that required for az31b , which could be deep drawn only at 175 ° c . for the same ldr . erichsen tests were performed on the hot rolled annealed sheets of mg - 2zn - 0 . 3y , mg - 2zn - 0 . 3gd and az31b using a hemispherical punch ( 20 mm diameter ) at room temperature . the respective sheets were clamped and the punch was pushed against the sheet until the sheet cracked . the height of the resulting dome on the sheet is the erichsen value , which is a measure of the stretch formability of the sheet . the higher the erichsen value , the better the response of the sheet to stretch formability . the erichsen values achieved for trc az31b , mg - 2zn - 0 . 3y and mg - 2zn - 0 . 3gd at room temperature were 3 . 6 , 8 . 5 and 6 . 3 , respectively . the results confirm that the alloys in accordance with several embodiments also exhibit good stretch formability at room temperature . the erichsen values for each of the two embodiments of the invention exhibit significantly higher values than that returned from the az31b sample . corrosion resistance of the alloys was tested using trc az31b as the reference material . three samples each from the hot rolled annealed sheets of trc az31b , mg - 2zn - 0 . 3y and mg - 2zn - 0 . 3gd were immersed in a non - aerated solution containing 3 . 5 wt . % nacl for 7 days . the respective samples were weighed before and after the immersion process . from weight loss measurements , the corrosion rate was calculated and expressed as a weight ratio to eliminate differences in the sample dimensions . the weight ratio achieved for trc az31b , mg - 2zn - 0 . 3y and mg - 2zn - 0 . 3gd were 0 . 007 , 0 . 038 and 0 . 0083 , respectively . the alloy containing gadolinium as the alloying element , exhibited a corrosion resistance comparable with az31b ( 0 . 0083 , expressed as weight ratio , compared to 0 . 007 ). the alloy containing yttrium as the alloying element was an order of magnitude higher . advantageously , the cost of alloys of the described embodiments were comparable with that of az31b ingots ( based on the cost of alloying elements as of may 2009 ). furthermore , alloys characterised in accordance with the embodiments are able to be deep drawn at significantly lower temperatures whilst exhibiting a good degree of stretch formability at room temperature . furthermore , the alloys in accordance with the embodiments generally exhibit good ductility and rolling workability that equates to 50 % less number of rolling passes compared to the commercially known wrought magnesium alloy , az31b . moreover products formed from alloy sheeting exhibit comparable corrosion properties to products formed from az31b . the alloy , at least in accordance with the above mentioned embodiments is well suited for room temperature applications within the electronic and automotive industries , similar to az31b . it will be appreciated by persons skilled in the art that numerous variations and / or modifications may be made to the described embodiments and examples without departing from the scope of the invention as broadly described . the described embodiments are , therefore , to be considered in all respects as illustrative and not restrictive . 1 . e . f . emley , principles of magnesium technology , ( oxford , london : pergamon press ltd ., 1966 ), 452 - 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