Patent Application
20130118657
This invention pertains to methods of processing hot-rolled and annealed magnesium-based alloy sheet materials to obtain enhanced room temperature ductility in the material without reduction of its tensile strength.
There is continuing interest in reducing vehicle weight. One approach to weight reduction has been the substitution of higher strength-to-weight sheet materials for the steels and aluminum alloys now commonly used in vehicle construction.
The low density and reasonable strength of magnesium and alloys containing, for example, 85% by weight or more of magnesium, make them attractive candidates for such material substitution. However, magnesium alloys are not as ductile as steel and aluminum and so stamping or shaping such sheet magnesium alloys into the complex shapes of components in common use may be challenging. While the ductility of magnesium alloys may be enhanced by deformation at temperatures appreciably above room temperature, say 250° C. or higher, such practices complicate and slow the forming process.
What is required is a method of enhancing the room temperature formability or ductility of magnesium to expand and extend the use of magnesium sheet alloys in high volume vehicle production.
In general, practices of this invention start with a magnesium-based alloy sheet that has been annealed or otherwise prepared with a microstructure that is substantially free of strain (e.g., containing fewer than about 108 dislocations per square centimeter). A magnesium alloy sheet is lightly deformed to introduce small local strained volumes or ‘islands’ distributed and embedded in an otherwise substantially strain free matrix. The sheet, with its distribution of strained and unstrained sheet regions, is then annealed, for example by holding for about 30 minutes at about 350° C. After such processing, the sheet exhibits significantly enhanced ductility, relative to the original sheet, but is of comparable strength.
In an embodiment a substantially strain free magnesium alloy sheet is deformed, without heating, by rolling the sheet between roughened rolls adjusted so that only the high points of the rolls contact and deform the opposing surfaces of the sheet. By appropriately adjusting the height and distribution of high points on the rolls a suitable distribution and scaling of deformed and substantially undeformed regions may be imparted to the sheet to achieve a distribution of strained ‘islands’ in an otherwise substantially strain free sheet volume. In a second embodiment a similarly substantially strain free magnesium alloy sheet is deformed, without heating from its ambient room temperature, by uniformly reducing its thickness by up to about 4% by rolling between smooth rolls. This practice may likewise be effective in introducing the desired distribution of strained ‘islands’ in an otherwise substantially strain free sheet volume due to the tendency of magnesium to deform inhomogeneously.
The effectiveness of the procedure may be understood by consideration of the following example. A sample of AZ31 magnesium alloy (nominally 3% by weight aluminum, 1% by weight zinc, balance magnesium and common impurities) when tested in tension at room temperature, about 25° C., using sub-size, 25.4 millimeter gage length tensile samples, exhibited a measured total elongation of about 17% and a tensile strength was about 275 MPa. Another sample of AZ31 alloy of like initial microstructure, after the above-described ductility-enhancing processing comprising introducing regions of local deformation followed by annealing, exhibited a total elongation of about 28% and a tensile strength of about 275 MPa.
Some appreciation of how such methods may be effective in enhancing the ductility of magnesium sheet may best be understood by consideration of the microstructure of the sheet.
In general, structures fabricated of metals and alloys consist of space-filling assemblages of individual crystals, or grains, each of which is oriented differently from its neighboring grains. Typically the grains are much smaller than any of the dimensions of the structure and so the detail of the structure, at the level of the grains, is referred to as its microstructure. The microstructure may be modified or manipulated and any modifications to the microstructure will affect the properties of the metal or alloy.
Magnesium alloy sheet products are most frequently prepared from cast ingots or continuously cast slabs which are progressively reduced in thickness by rolling at temperatures above about 315° C. so that the sheet anneals as it cools down. In addition to imparting the desired thickness change, rolling has the beneficial effect breaking down and refining the as-cast microstructure and reducing the grain size. Often the grains in the rolled and annealed sheet are generally uniformly sized and equiaxed, that is, when viewed in section, their extent does not depend in any systematic way on the orientation of the measurement. A magnesium alloy with a microstructure consisting of generally small, about 20 micrometers or so, substantially uniformly-sized equiaxed grains, may be transformed, by application of the practices of the invention, to form a different microstructure which conveys improved ductility. The strength of the two structures however is only imperceptibly different. This is a remarkable result since most metallurgical techniques for enhancing ductility also reduce strength.
This ductility-enhancing microstructure also contains equiaxed grains, but the grain size distribution is bimodal, that is it contains a generally narrowly distributed mixture of larger grains with an average size and smaller grains, also generally narrowly distributed, of a lesser average size. Local, substantially uniformly spaced, regions of large grain size are intimately mixed with, and surrounded by, regions of predominantly small grain size. The large grain size regions are spaced apart by about 500 micrometers or so within a larger volume of smaller grains. In a suitable proportion, the volume of small grains may about twice the volume of large grains
In the exemplary application of the invention summarized previously, the AZ31 magnesium alloy which exhibited a total elongation of about 17% had a generally uniform grain size of about 20 micrometers. The second sample of the AZ31 alloy, processed in accordance with the practices of the invention, and exhibiting a total elongation of about 28%, had a bimodal grain distribution consisting of regions of large grain size spaced apart by about 500 micrometers in a larger volume of smaller grains. About 50% of the grains had a grain size of between about 5 and 25 micrometers and about 25% of the grains ranged in size from about 70 micrometers to about 100 micrometers with the remainder generally uniformly distributed in the range from 25 to 70 micrometers.
Conventional processing schemes for metals and alloys in general, and for magnesium in particular, do not produce such a bimodal grain distribution. But such a bimodal grain size distribution may be obtained by lightly deforming only selected regions of the magnesium alloy sheet, preferably to a critical strain of from between about 3% to about 7% strain while leaving the remaining portions of the sheet substantially undeformed and then, annealing the sheet. A typical annealing time is about 30 minutes and annealing temperatures of between 350° C. and 515° C. have resulted in a bimodal grain size distribution and resulted in improved ductility. The greatest ductility improvement, achieved without significant (less than 2% or so) loss of tensile strength, resulted from using an annealing temperature of 350° C. Higher annealing temperatures of 450° C. and 515° C. yielded lesser, but comparable, ductility improvements but these ductility improvements were accompanied by a loss in tensile strength of about 10% or so.
This preferred distribution of bimodal grains is achieved by a sequential two-step process. First, locally deforming some regions of the volume of sheet material, but not others, and then annealing the sheet. Early in the annealing process the microstructure in the locally deformed regions is first transformed into small strain free grains, or nuclei. With continued annealing some of these nuclei then grow into large grains by absorbing other nuclei. In the undeformed regions of the sheet the effect of annealing is to modestly increase the grain size but the grains remain equiaxed and of generally uniform size. The final microstructure thus consists of local regions of large grains distributed within a fine-grained microstructure. Since the strained regions are the source of the large-grained regions, in general, the scale and distribution of the large-grained regions will substantially mirror the scale and distribution of the local deformations. Suitably, the large grained regions may be generally uniformly dispersed in the fine-grained microstructure and spaced about 500 micrometers or so apart.
The local or inhomogeneous deformation may be imposed by any convenient means provided it leads to dispersed deformed regions embedded in substantially deformation-free regions, and separated by about 500 micrometers or so. Deformation should also be conducted at a low temperature, generally below about 160° C. or so to inhibit recrystallization. Examples of suitable deformation processes include the smooth roll or roughened roll rolling procedures already described. Yet further processes for inducing inhomogeneous deformation include shot blasting or peening, and flex rolling or reverse bending, as well as combinations of such approaches.
The practices of the invention may be applied to single phase magnesium-based alloys and to alloys containing second phases. If the second phases are present as discrete particles, these particles may themselves promote the development of the desired inhomogeneous distribution of strain in the alloy.
Annealing after deformation may be conducted in a batch or continuous manner, in a reducing atmosphere or under vacuum.
These and other aspects of the invention are described below, while still others will be readily apparent to those skilled in the art based on the descriptions provided in this specification.
Application of stamped magnesium alloy sheet components in vehicle bodies may offer opportunity for reducing the mass of such bodies, but the broad application of magnesium alloy stampings is hampered by their limited ductility and formability at low temperatures, or below about 160° C. This reduced low temperature formability, compared to that of the steel and aluminum alloys currently in broad use, limits the complexity of shapes which may be stamped from such alloys.
Magnesium, and typical magnesium alloys with 85% by weight or more of magnesium, have a microstructure consisting of an assemblage of grains, each with a hexagonal crystal structure. Metals may deform by slip along preferred crystallographic planes and directions, each unique combination of plane and direction being termed a slip system. But metals with a hexagonal crystal structure, like magnesium, are limited in the number of available slip systems and may deform inhomogeneously so that regions of intense deformation are separated by regions of minimal deformation. This behavior is believed responsible for the generally poorer ductility of sheet magnesium alloys compared to steel and aluminum sheet alloys.
But, in an embodiment, inhomogeneous deformation of magnesium and its alloys may be promoted, and then exploited to prepare sheet magnesium alloys with appreciably more ductility than the sheet magnesium alloys currently available.
Rolls 10, 11 are set so that their cylindrical segments 13, 15 are spaced apart by a distance just equal to the thickness h of the incoming sheet 12. Thus the overall thickness of the exiting sheet will also equal h and no global reduction in the thickness of sheet 12 will occur. However, because protrusions 14 extend to a greater radial distance than the cylindrical segments 13, 15, the protrusions 14 will engage the upper 22 and lower 20 surfaces of sheet 12. In engaging the sheet surfaces 20, 22 the protrusions 14 will create indentations 16 on the sheet surfaces 20, 22. Each of indentations 16 will have an associated deformed region 18 surrounding the indentation. The nature and extent of the deformed region will be analogous to that obtained when conducting a hardness test so that the deformation will be of limited extent and, particularly, will not extend to the opposing sheet surface.
It will be appreciated that, in practice, it will be challenging to achieve the set-up shown in
Typically the pattern of protrusions 14 will vary across the width of the roll so that a more or less uniform distribution of spaced-apart deformed regions is formed on the surface(s) of the sheet 12′. A representation of such an array of deformed regions 18′ separated by undeformed surface regions 22′ is shown in
A number of procedures may be employed to impart protrusions to initially smooth rolls. For example, roll roughening may be conducted using lasers, following procedures well known to those skilled in the art. Such laser roll roughening may be applied in a programmed and controlled fashion to create regularly-patterned rolls with peaks and valleys scaled to impart the desired strain levels. More random roll patterns may be introduced by other roll roughening processes such as shot peening to produce a substantially random distribution of deformed regions on the sheet as illustrated in
On shot peened rolls the relative heights of the peaks and valleys may be adjusted by varying the mass of the shot and the slinger velocity. Again, since the objective is to impart a pre-determined strain to the sheet the roughness of the rolls should be scaled or proportioned to the thickness of the sheet.
Other processes for introducing local deformation confined within specific sheet regions may include flex rolling, temper rolling or roller leveling. In application of these process, the deformation may be progressively decreased as the sheet passes through the roll sets. Asymmetric or shear rolling in which the rolls rotate at differing speeds may also be effective.
By annealing outgoing sheet 12′ (
The effect of such a rolling practice on the microstructure in a sheet magnesium AZ31 alloy sample is shown in
The microstructure shown in
Some of the electrons incident on a crystalline sample are back scattered, that is scattered or redirected in a direction opposite the incident electron beam, without appreciable loss in energy. Some of these back scattered electrons are redirected only after they have penetrated into the sample and as these scattered electrons exit the sample they may further interact with the crystal by being diffracted by the atomic planes of the crystal to form an observable diffraction pattern. The less the deformation in a grain the more perfect the diffraction pattern will be. Thus by scanning or stepping the electron beam across a polycrystalline sample and examining the perfection of the diffraction pattern, the extent of deformation in different regions of the sample may be assessed.
If the size of the electron beam is less than the grain size, the deformation associated with a particular grain may be identified and if the beam size is very small, the disordered grain boundaries may also be identified. In the microstructures shown the spatial resolution is 0.5 micrometers or less. By representing undeformed grains as white and deformed grains as black and assigning some number of intermediate intensity levels, an image representing the qualitative levels of deformation in every grain in the sample surface can be constructed as shown in
Annealing the structure of
The structure shown in of
The microstructure shown in
Recrystallization occurs in a number of steps. Initially small new grains or nuclei form at preferred sites in the deformed region and these nuclei then grow, consuming or absorbing the deformed grains. Growth is competitive and when each of the nuclei has consumed the deformed grains surrounding it and encounters or collides with a neighboring growing nucleus the process slows, leaving a space-filling structure of strain free grains. Continued annealing will promote a general coarsening of the grain structure and lead to at least an overall increase in grain size, as well as modifying the size distribution.
The number and size of the strain-free gains depends primarily on the number of nuclei formed in the early stages of the process. After intense deformation many closely-spaced nuclei form and only limited growth can occur before they encounter one another or collide, leading to a fine-grained structure. Unstrained regions undergo substantially no recrystallization although some grain coarsening may occur. But, at a critical strain, recrystallization will begin from a few, more widely-spaced, nuclei each of which can undergo extensive growth before collision occurs, leading to a much more coarse-grained structure.
So, the desired bimodal grain size distribution shown in
When such a deformed sheet is annealed, no nuclei will form in the undeformed regions and only limited grain growth will result leading to only minor changes in the grain size and distribution in these regions. However, the small number of nuclei formed in the strained regions will grow extensively to produce large grains. These behaviors in the deformed and undeformed regions will promote the desired bimodal grain size. Since the large grains will form only in the originally strained regions the desired grain distribution may be enforced by controlling the size and distribution of the deformed regions.
Review of
Numerous time and temperature combinations may be used to anneal the sheet, but not all annealing processes yield equivalent results.
Rolling the sheet with roughened rolls imposes the desired strain distribution on the sheet geometrically since only some locations on the rolls are capable or imparting deformation. But, magnesium is prone to deform inhomogeneously even under conditions which might be expected to promote uniform deformation, such as smooth roll rolling deformation.
At low reductions in thickness, of say between about 2% and 5%, the magnesium sheet will deform inhomogeneously so that outgoing sheet 12″ contains strained local regions, or islands 48, dispersed in a substantially unstrained volume. Thus strained regions 48 are separated by substantially unstrained regions like that shown at 50. This distribution of strained and unstrained regions will manifest itself across the sheet width in a manner similar to that as illustrated in
To achieve the desired bimodal grain size distribution, the strained regions in which nuclei form, should exhibit strains of between 3 and 7% and be present in suitable number and density. The prescribed overall strain of between 2 and 5% is effective in producing, in the strained regions 48, the required strains of between about 3 and 7% with generally unstrained regions between. The relative fractions of strained and unstrained regions are such as to develop a volume-weighted average strain equal to the imposed average strain of between 1 and 4%. Rolling is a convenient method of achieving the desired average strains but, in principle, any deformation mode, tensile, compressive, bending or shear may be used. Bending, because it inherently introduces a strain gradient in the sheet, may be more forgiving of sample to sample processing variations. The sheet may be processed as individual cut sheets or as continuous coil.
Since the magnesium is in sheet form, the most convenient deformation process would be rolling between either roughened rolls or smooth rolls as previously described. In sheet form, cross-rolling, or rolling across the sheet width, may be employed but continuous coil products may be rolled only along the coil length. In all cases deformation should be carried out at low temperature, here considered to be below about 160° C., a temperature at which recrystallization may occur. The locally-strained regions imparted during rolling may be readily achieved using only a single pass, or a single passage of the sheet through the rolls, and no further benefit is obtained by using multiple passes.
The subsequent annealing process may be performed as a batch or a continuous process, often, to avoid oxidation of the magnesium alloy, under vacuum or inert atmosphere. Short-time high temperature annealing schedules may be most suited for continuous processing while longer-time, lower temperature annealing schedules may best be practiced in batch mode. While the results presented previously should be viewed as illustrative and not limiting it appears than an annealing schedule of about 350° C. for a period of about 30 minutes promotes appreciable improvement in ductility without loss of strength.
In a particular exemplary practice of the invention, commercial AZ31 alloy magnesium sheets were obtained in H24 temper. In this temper the sheet is only partially annealed, and the sheet was first annealed by holding at 350° C. or so for about 15 minutes and air cooling to create an equiaxed fine-grained microstructure. The annealed sheet was then rolled, using roughened rolls, in a single pass, at room temperature to minimally reduce its thickness, by about 1% or so, to ensure that all elevated portions of the rolls engaged the sheet surface. After this treatment the sheet microstructure was representative of that shown in
Magnesium alloy AZ31 has sufficient concentrations of aluminum and zinc that it may be expected to contain particles of Mg17(A1,Zn)12 after annealing. No such particles are evident in the micrographs shown as
The practice of the invention has been illustrated through reference to certain preferred embodiments that are intended to be exemplary and not limiting. The full scope of the invention is to be defined and limited only by the following claims.
