Patent Application
20080184755
This invention pertains to hot forming of magnesium alloy workpiece materials where a heated workpiece is stretched or drawn against a forming surface on a tool. The invention is particularly applicable to sheet materials but is also useful in the forming of tubes, bars, and other shapes. An integral, adherent layer of magnesium hydroxide is formed on magnesium alloy sheet material, prior to hot forming, as a low friction surface layer for sliding contact with the forming tool as the sheet metal workpiece is deformed and shaped.
Many articles of manufacture are made by deforming sheet metal workpieces at elevated forming temperatures into shaped panels and other objects. Other metal shapes such as tubes, solid extrusions, and strips are also deformed at elevated temperatures. Often the forming tools for the workpiece shapes are made of strong tool steel compositions having suitable surface hardness and wear resistance for repetitive forming operations required in high volume production.
In the automobile industry, for example, inner and outer body panels are made from suitably formable steel and aluminum alloy sheet materials. While some steel and aluminum alloy panels can be stamped between complementary forming tools at ambient temperatures, some sheet metals, especially some aluminum alloys, may lack suitable formability to be deformed into more complex panel shapes without being heated. A variety of forming processes have been developed for alloys like aluminum in which the sheet material is heated to a suitable temperature and softened for stretching or drawing between heated forming tools.
Aluminum alloy sheet materials have been developed with different compositions and microstructures; some for hot forming or stamping at temperatures above about 200° C., and others for hot stretch forming operations at temperatures above about 400° C. Usually lubricant coatings are applied to the steel forming tools and/or the tool-contacting surfaces of the aluminum sheet material to prevent tearing or other damage to the thin sheet material as it is being deformed and shaped. For example, particles of boron nitride and/or magnesium oxide are suspended in water (or other liquid) and the slurry applied to forming surfaces of the blank sheet material. The wet coating is dried to an adherent film and the blank heated to its forming temperature. The lubricant has its material costs and processing costs since it usually must be removed from the formed sheet and prevented from accumulating on the forming tool.
As lighter weight vehicle body panels are sought, attention is turning to the possibility of using magnesium alloys to make shaped panels for automotive vehicles. There is also a need to form other magnesium alloy workpiece shapes into useful vehicle components. Again, elevated temperature forming processes will be required to make some panels and other parts of the magnesium sheet materials. Magnesium alloys for metal products are presently more expensive than steel and aluminum alloys. There is a need to devise low-cost forming practices for making products from magnesium alloy workpieces.
It is contemplated that a suitably shaped blank of magnesium alloy workpiece material will be heated to a forming temperature and formed, by bending, stretching, drawing, or the like into conformance with a hard, durable forming surface into the shape of a desired article. It is contemplated that one application for the practice of the invention will be in the forming of magnesium sheet metal alloys into body panels or other sheet metal articles. It is necessary to provide a protective boundary layer between the forming surface of the forming tool or die and the facing surface of the magnesium alloy workpiece. In accordance with this invention, such a boundary layer comprises an adherent layer of magnesium hydroxide integrally formed on the surface of the magnesium alloy workpiece shape. The magnesium hydroxide layer is formed at a suitable time in the preparation of the workpiece prior to the hot forming operation.
The adherent coating may be formed by exposure of magnesium alloy workpiece material to an aqueous solution containing one or more dissolved salts, such as NaCl, for the purpose of oxidizing the surface and producing an adherent layer of Mg(OH)2. A trace of NaCl is usually present in the magnesium hydroxide layer. The exposure would be done at a temperature and oxidation potential to produce an optimized magnesium hydroxide layer for a selected magnesium alloy and forming operation. For example, the workpiece material may be immersed in a ten weight percent solution of table salt in water at about 70° C. Depending on the desired thickness (in micrometers) of magnesium hydroxide coating, the time of immersion on the hot salt water may be for up to about twenty minutes. X-ray diffraction analysis of the surface of magnesium alloy sheet materials following their immersion in hot aqueous sodium chloride confirms that magnesium hydroxide is formed as the major surface phase with trace amounts of sodium chloride. Obviously other surface oxidation treatments producing magnesium hydroxide could be used, but the use of aqueous sodium chloride is effective and inexpensive. The treated magnesium is then cleared of excess solution and dried, suitably with a flow of warm air.
In accordance with one embodiment of the invention the magnesium hydroxide coating is used in the hot forming of magnesium alloy sheet metal blanks into vehicle parts such as body panels. The magnesium hydroxide layer is formed on the blanks at a desired time prior to forming. For example, the surface oxidation treatment might be applied by the sheet manufacturer after the last hot rolling pass, in which case the layer would help prevent welding during the coiling operation. Alternatively, it could be applied after the coil is uncoiled, or before or after blanking, or just before hot forming. The surface treatment may also be performed in the preheating stage of a warm stamping operation or a hot stretch forming operation for shaping the blanked magnesium sheet material into a desired article of manufacture.
The adherent magnesium hydroxide coating reduces the coefficient of friction of an untreated magnesium alloy surface. And the coating provides a thin protective barrier between the base metal and the forming tool, thereby reducing metal adhesion and transfer, and facilitating part release from the tool. The thickness of the coating is suitably in the range of about one micrometer to about thirty micrometers. It is contemplated that many elevated temperature forming operations may be performed using the adherent magnesium hydroxide layer as the sole barrier/lubrication layer. However, it is recognized that in some forming operations or with some magnesium alloy workpiece materials, a practitioner may choose to apply a forming lubricant to the tool or over the magnesium hydroxide layer on the magnesium alloy workpiece.
Other objects and advantages of the invention will be apparent from descriptions of preferred embodiments which follow in this specification.
The need for weight reduction and improved automobile fuel economy has prompted the use of magnesium alloys in vehicle components. Previously, automotive applications of magnesium alloys have been in die castings because of the development of castable alloys and the efficiency of the die casting process. Magnesium based sheet alloys (typically including about ninety percent by weight magnesium or higher) have not been readily formable at room temperature and it is necessary to develop higher temperature forming processes for high volume production of panels and other sheet products for vehicle applications.
A commercially available magnesium based alloy sheet material is that designated as AZ31B. The composition of AZ31B in weight percent is nominally 3.0% aluminum, 1.0% zinc, 0.2% manganese, and the balance substantially all magnesium. It is suitable for elevated temperature forming of certain sheet products. The sheet material is usually formed by casting a billet of suitable size and subjecting the billet to a series of hot rolling steps to reduce the billet to a long sheet of desired width. The hot-rolled sheet may be subjected to one or more passes through cold rollers to achieve a desired uniform final thickness and a desired surface finish for the intended product. The cold rolled sheet is usually given a temper heat treatment to increase its formability.
Ultimately, blanks of suitable plan view are cut from the sheet for successive transfer to a forming press or a succession of forming presses. The purpose of this invention is to prepare the sheet for forming against a forming tool surface, especially when the sheet and tool are heated to an elevated forming temperature, above ambient temperature, and usually above about 200° C. One or both surfaces of the sheet are provided with an integral adherent coating of magnesium hydroxide.
The magnesium hydroxide coating may be formed on the surface of a magnesium alloy based sheet material by suitable contact of the surface with an aqueous solution comprising one or more dissolved salts, such as sodium chloride, that oxidize the magnesium surface. The dissolved salt promotes oxidation of the surface of the magnesium sheet yielding an adherent layer on the sheet comprising magnesium hydroxide. A trace amount of residual salt may be found in the hydroxide coating. The temperature of the aqueous solution, the agitation of the solution, the oxygen potential of the solution, and the time the sheet is exposed to the aqueous solution may all be varied to produce an optimized adherent layer. These processing parameters may be determined experimentally for a magnesium alloy workpiece and the forming operation to which it is to be subjected. After exposure to the aqueous solution the sheet may be rinsed or cleaned of excess solution and dried, for example, by a flow of warm air.
To illustrate a preferred embodiment, a magnesium alloy sheet, such as a sheet of AZ31B composition, may be exposed to an aqueous solution at 70° C. comprising about 10% by weight sodium chloride. The time of exposure may vary depending on the desired thickness, but usually ranges from about 30 seconds to about 30 minutes. Alternatively, the sheet may be treated with the salt solution by spraying or roller coating.
In many elevated temperature forming operations the magnesium hydroxide coating alone on the magnesium alloy sheet workpiece will markedly improve the forming of the sheet and the surface quality of the formed panel or part. However, there may be some forming operations and/or some magnesium alloys in which it may be preferred to apply an additional lubricant film over the magnesium hydroxide coating to obtain a particular product shape. For example, a thin coating of boron nitride over the magnesium hydroxide layer usually further reduces the coefficient of friction between the sheet surface and the forming tool at the elevated temperature of the forming operation.
While a specific coefficient of friction value between workpiece and tool is not the sole factor in successfully achieving a desired sheet metal product shape, surface lubricity contributes to reducing defects as the metal is stretched or drawn under a loading in sliding frictional contact over a forming tool surface. Forming tools for shaping sheet metal alloys are often machined from a cast block of P20 steel, a pre-hardened tool steel. A representative composition of P20 steel for a mold body, by weight, includes about 0.4% carbon, 0.45% silicon, 1.4% manganese, 1.8% chromium, 0.5% molybdenum, 0.035% max sulfur, and the balance mainly iron. The composition presents a hard surface that can be polished to a smooth finish for the shaping of sheet metals and other materials. In order to assess the formability of a representative magnesium sheet (AZ31B) tribological tests were conducted.
Friction and adhesion between a sheet metal material and a tool material may be studied using a sliding loading system provided by a tribology test machine, such as a commercial Plint machine. In this work, the Plint machine was used to evaluate friction and wear properties of the materials in dry and lubricated reciprocating sliding contact conditions using flat-on-flat surfaces. In this testing magnesium sheet samples (coated and uncoated) were successively fastened to a steel plate. The steel plate was clamped to a heater block on a flat horizontal bed of the machine and the temperature of the block and magnesium sheet were monitored by a thermocouple pressed against the back side of the steel plate. The tool specimen of AISI P20 steel was made in the form of a rectangular block with a flat surface for sliding against the magnesium sheet sample. The area contact surface of the P20 specimen was polished to a surface finish of 1 μm.
Prior to testing, the Mg sheet samples were cleaned and dried in air for a few minutes prior to being coated with BN. Some magnesium sheet samples were tested without any coating. Some were tested with a magnesium hydroxide coating prepared in accordance with this invention. Some were tested with a boron nitride coating over bare magnesium alloy and some were tested with a boron nitride coating over an adherent layer of magnesium hydroxide. Tribological testing was conducted at incrementally varying temperatures in the range of from room temperature (24° C.) to 450° C. Several tests were conducted at the higher temperatures to evaluate friction properties between the magnesium sheet material and the P20 tool material at temperatures used in elevated temperature sheet forming operations.
After both the magnesium sheet and the P20 steel specimen were properly setup for face-to-face sliding contact under load, the temperature of the heater block was quickly raised to 50° C. below a specified temperature. Then, the temperature was slowly increased to the specified temperature. Four loadings were studied; 20N, 50N, 75N, and 100N, but the focus of the study was at 50N (apparent contact pressure=160 kPa.). The travel distance of the P20 specimen for a half cycle was 15 mm. The frequency used in all the experiments was set at 0.11 Hz, close to the actual sliding rate in a sheet forming operation.
A friction force transducer measured the friction force during the sliding, and friction coefficients were obtained by dividing the friction force by the normal load. A potential of about 50 mV was applied across both the steel and Mg specimens. A drop in this voltage (called contact potential) indicated that any barrier film or coating on the magnesium specimen had been eroded away and that metal-to-metal contact was occurring. The temperature was measured with a thermocouple and recorded. The test was stopped manually when severe scars or wear was observed on the sheet surface. The experiment could last from a few seconds to up to 15 minutes.
The friction force output data of the Plint tribology machine was in the form of a rapidly oscillating line due to the reciprocation motion of the steel relative to the magnesium specimen. The oscillating line showed the calculated friction coefficients as computed by dividing the measured friction forces by the applied normal load. The friction coefficients of each test cycle reach a maximum at the start and end of the cycle and approach the minimum at the middle of the half cycle. The temperature was held constant throughout the test and the contact potentials changed according to the amount of metallic contact between the specimens. When there was no metallic contact between the moving steel and Mg sheet, the contact potential stayed at approximately 54 mV. Where there was metallic contact, the contact potential would decrease and reach zero when there was full metal-to-metal contact. After the specimens made contact as seen by the large drop of the contact potential line, the friction coefficients usually rose. The change in the contact potential was considered an important parameter to be measured during the test, indicating the potential removal of the lubricant during sliding.
An important data developed in the tribology tests was the time to contact (TTC), which is useful in determining the durability of the lubricant under different sliding conditions. TTC was extracted from the smoothed contact potential line where it first produces a significant potential decline (between 40-30 mV). A friction coefficient at TTC was measured from the smoothed friction coefficient line at that time to contact.
The tests yielded typical coefficient of friction values for sliding contact (at 0.11 Hz) between P20 steel and AZ31B magnesium alloy sheet in the range of about 0.35 to about 0.45 over the temperature range. Magnesium hydroxide coated AZ31B sheet materials were prepared using aqueous sodium chloride solutions (10 wt % at 70° C.) for immersion periods of one minute, ten minutes and twenty minutes, respectively. Typical COF values in these tests were in the range of about 0.25 to about 0.31.
It was found that the magnesium sheet samples coated with boron nitride films yielded lower values of COF than the oxidized samples at lower temperatures. But the COF values for BN coatings increased substantially at higher testing temperatures. The combined coatings of magnesium hydroxide and BN yielded COF values generally lower than magnesium hydroxide alone.
Typical time-to-contact values were zero seconds for bare magnesium alloy (AZ31B), five seconds for samples having a magnesium hydroxide coating, one hundred seconds for samples having a boron nitride coating, and five hundred seconds for samples having both a magnesium hydroxide coating with an overlying coating of boron nitride.
The use of magnesium hydroxide coatings on magnesium alloy sheets is thus seen to provide a barrier between the surface of the magnesium alloy workpiece and the surface of a forming tool. The magnesium hydroxide coating of this invention is seen as a useful aid to high temperature forming of the magnesium alloy workpieces in loaded, sliding contact against a forming tool surface. And the addition of a lubricant film over the magnesium hydroxide film is considered useful in some workpiece forming operations.
Practices of the invention will be further described with respect to the forming of magnesium alloy sheet materials.
The disclosed methods for treating magnesium based alloy sheet materials may be used with various forming operations, such as stamping, superplastic forming (SPF), quick plastic forming (QPF), warn forming, or any other forming method where a metal or alloy workpiece is forced into conformance and sliding contact with the forming surface of a forming tool. The sheet material may be provided with its adherent magnesium hydroxide coating layer at any time during the manufacturing process which produces the sheet, as well as during the actual forming operation. The coating and forming of a magnesium based alloy sheet material is illustrated below in connection with a hot blow forming operation, more specifically a QPF process as described in commonly assigned U.S. Pat. No. 6,253,588.
Typically, a high volume manufacturing process will start with a roll of magnesium based alloy sheet material which has been formed by a sequence of hot rolling steps, cold rolling steps, and heat treatment steps to achieve a desired sheet metal thickness, microstructure, and formability. The sheet material is unrolled and blanks of a predetermined size are cut in accordance with design specifications for a particular forming process. The magnesium based sheet material may be treated to form an adherent coating of magnesium hydroxide about one micrometer to about thirty micrometers in thickness at any suitable processing step prior to or at the onset of the forming operation.
In this illustrative example, the magnesium hydroxide coated, magnesium alloy based sheet material blanks are introduced into a forming tool that comprises a lower tool member and an upper tool member. The lower tool member comprises a forming surface of complex three-dimensional curvature that is complementary to, and defines the first side of a formed blank. The magnesium hydroxide coating has been formed on at least the first or tool-facing side of the magnesium alloy blank sheet material. The periphery of the lower tool member is adapted to sealingly engage the lower peripheral portion of the blank to prevent, among other things, movement of the blank during this stretch forming process. The lower tool member may also be hollowed out in one or more regions to reduce its mass and facilitate machining of a plurality of passageways to allow a working gas to be introduced into or vented from underneath the blank. Venting the region below the blank can help shape the blank into strict conformance with the forming surface of the lower tool member.
The upper tool member is complementary in shape, but not identical in shape, to the lower tool member for hot blow forming operations such as superplastic forming or quick plastic forming. The upper tool is provided with a conduit for the introduction of a high pressure working against the side of the metal blank opposite the first side. Nitrogen, air, and argon are examples of commonly used high pressure working gases. The periphery of the upper tool member is adapted to sealingly engage the upper peripheral portion of the metal sheet to prevent the escape the high pressure working gas from escaping. Both the lower tool member and the upper tool member may comprise internal electrical resistance heating elements and temperature sensing devices to controllably heat the tool members to a predetermined temperature.
In order to produce a formed workpiece by a QPF process, the metal blank is usually heated to a predetermined forming temperature, for example, 450° C. The magnesium hydroxide coating formed in accordance with this invention is functional at such a forming temperature.
This heating may occur in an oven or other suitable preheating apparatus. Or it may occur in a first tool capable of heating the blank as well as performing preliminary and simple bending of the blank to a desired preform shape. The flat or preformed sheet may then be positioned between the opened upper tool member and lower tool member. The upper and lower tool members then close and engage the upper and lower peripheral portions of the metal blank. If available, the heating elements in the tool members can help maintain the metal blank at its predetermined forming temperature. The heating elements of the tool members may also heat the blank from ambient temperature to its forming temperature if preliminary heating was not performed on the blank.
After the metal blank has been positioned, clamped, and attained it's forming temperature, a pressurized working gas may be introduced through the conduit in the upper tool member and against the side of the metal blank opposite the magnesium hydroxide coated first side. The working gas gradually forces the metal blank downward and into conformance with the forming surface of the lower tool member at a controlled strain rate. The working gas pressure may be continually increased in accordance with a predetermined pressure schedule. The magnesium hydroxide coated surface of the magnesium alloy sheet material slides under load in frictional contact with the forming surface.
After completion of the forming operation, the upper tool member may be separated from engagement with the upper surface of the formed sheet metal part. The deformed panel or other part may be removed from the lower tool member and a new magnesium alloy blank with an adherent magnesium hydroxide coating may be introduced between the two members, thus restarting the forming process.
The formed part may be in its intended shape or it may be subjected to further forming steps. For example, the other side of the original blank may have been coated with an adherent layer of magnesium hydroxide and the still warm sheet may be brought into contact with a second forming tool.
Magnesium hydroxide coatings are also useful in the elevated temperature forming of other magnesium alloy workpiece shapes. For example, tubular sections of magnesium alloys may be expanded or contracted at elevated forming temperatures by the uses of forming tools and/or fluid pressure or electromagnetic force. And tubes or solid-section shapes may be bent or otherwise shaped using forming tools.
The practice of the invention has been illustrated by a few examples that are only intended to describe the practice of the invention, but not to limit the scope of the invention.
