Patent Application
20100096046
The present invention relates generally to metal forming processes and more particularly to a method for improving the formability of magnesium and magnesium alloy tubes by grain size refinement using mechanical deformation and recrystallization.
Because of the increasing emphasis on reducing the weight of structural members in environments such as automobiles, aircraft and other transportation vehicles, substantial effort has been directed towards the development of parts formed from magnesium and magnesium alloys that are suited for such applications. A number of such parts are tubular in construction formed from a magnesium or magnesium alloys. These tubular structures may be shaped by techniques such as applying internal pressure in a heated mold so as to selectively expand the tube at positions as may be desired. By way of example only, and not limitation, suitable formation practices may include elevated temperature forming techniques such as hot blow forming and the like.
As will be appreciated, it may be desirable to form relatively complex shapes incorporating tight curvature radius segments. Magnesium and magnesium alloy tubes may be difficult to form into such parts due to limited formability. This limited formability may result in splits or tears during forming that prevent the desired geometry from being formed. Accordingly, it may be desirable to improve the formability of the magnesium or magnesium tubes prior to shaping. At the same time, it may be undesirable to change the basic chemistry of such materials which provides the desired strength to weight characteristics. Likewise it is generally undesirable to change features of the formation tooling due to the extreme cost.
The present invention is believed to provide advantages and/or alternatives over prior practices by providing a method that improves the formability of magnesium and magnesium alloy tube stock without altering the chemistry of such material or requiring changes to formation tooling.
According to potentially preferred features of the instant invention, magnesium or magnesium alloy sheet stock is subjected to at least one cycle of progressive roll-forming deformation to produce a curved cross section followed by reflattening and roll forming in a reverse direction prior to completion of a circular profile. The roll forming deformation may be reversed multiple times if desired such that multiple cycles of bending in a first direction and then bending in the opposite direction are carried out. The introduction of controlled plastic deformation gives rise to the development of dislocations and twins within the metal matrix which, in turn, cause a refinement in grain size when the metal is subjected to subsequent annealing or elevated temperature formation. The application of such pre-formation working preferably facilitates the use of lower cost strip cast sheet stock without the need for intermediate rolling treatment.
The accompanying drawings which are incorporated in and which constitute a portion of this specification illustrate exemplary practices according to the invention which, together with the general description above and the detailed description set forth below will serve to explain the principals of the invention wherein:
While potentially preferred procedures and practices have been illustrated and generally described above, it is to be understood and appreciated that in no event is the invention to be limited to such embodiments and procedures as may be specifically described herein. On the contrary, it is intended that the present invention shall extend to all alternatives and modifications as may embrace the broad principals of the invention within the true spirit and scope thereof.
For ease of reference and understanding, the following description is set forth with respect to a simplified exemplary process. However, it is to be understood that the process according to the present invention is in no way limited to a particular forming practice as may be illustrated and described. Rather it is intended that any number of treatment practices applying pre-formation deformation and recovery may be utilized.
Referring to
Following initial formation, the sheet of magnesium or magnesium alloy is thereafter subject to a roll forming process wherein the sheet is gradually formed into a curved, generally trough-shaped, cross-sectional geometry. Thereafter, the curved cross-section is substantially reflattened and curved in a reverse direction by roll forming. This process may be reversed and repeated multiple times if desired. After a desired number of roll forming bending cycles has been completed, the roll forming is carried out in a given direction to a degree such that a substantially circular cross-section is developed. Thereafter, an appropriate joining procedure such as seam welding or the like is used to join the free edges together and form a tube. The tube is thereafter heated above the recrystallization temperature as will be described further hereinafter.
Referring to
By way of example only, and not limitation,
As illustrated, the sheet undergoes a gradual bending from the substantially flat planar configuration illustrated in
During the roll forming operation the sheet 20 is preferably maintained at a temperature of about 25 degrees Celsius to about 200 degrees Celsius. Such a temperature is believed to facilitate the efficiency of the roll-forming process while nonetheless being below the temperature that provides substantial recrystallization of the magnesium or magnesium alloy.
Referring back to
By way of example only, and not limitation, an exemplary elevated formation process is illustrated in
Without being limited to a particular theory, it is believed that the progressive cyclical bending of the sheet 20 has the effect of introducing atomic level dislocations and so-called twins within the metal lattice structure. Such dislocations and twins tend to reduce the grain size when the sheet is subjected to recrystallization temperatures. This grain size refinement is believed to promote more uniform performance across the material when high temperature forming processes are applied thereby enhancing formability during the forming process.
It is to be understood that while the present invention has been illustrated and described in relation to potentially preferred procedures, that such procedures are illustrative only and that the invention is in no way limited thereto. Rather, it is contemplated that modifications and variations embodying the principals of the invention will no doubt occur to those of skill in the art. It is therefore contemplated and intended that the invention shall extend to all such modifications and variations as may incorporate the broad aspects of the invention within the true spirit and scope thereof.
