The object of the invention is to reduce core losses in soft magnetic amorphous materials by applying mechanical scribing in-line during the processing of amorphous laminations. The amorphous laminations can be formed into wound core shapes that are utilized in many power conditioning devices and the primary application is for loss reduction in high efficiency distribution transformers.
Domain refinement is a common technique used to reduce the core loss of conventional Si-steel laminations, and the mechanism has been well documented. Pinning locations of the magnetic domains ease reversal, and are applied to the laminations in the direction perpendicular to the casting direction. This can be done through many processes in conventional Si-steel laminations.
For instance, U.S. Pat. No. 4,685,980, the contents of which are incorporated by reference in their entirety, teaches a method of applying pinning locations to the Si-steel laminations through laser treatment of the ribbon surface. Laser scribing is a common method of core loss reduction and is taught through multiple patents. This generally involves using laser heating to locally recrystallize Si-steel laminations. U.S. Patent Publication No. 2003/0121566, the contents of which are incorporated by reference in their entirety, uses mechanical contact methods that include introducing strain in the ribbon through lateral grooves applied to the lamination either during the rolling stages of production or afterwards in a separate processing step. These grooves then help to orient the magnetic domains during crystal growth through heat treating stages. U.S. Pat. No. 5,013,373, the contents of which are incorporated by reference in their entirety, uses a chemical etching process to introduce mechanical grooves into Si-steel laminations.
Production of amorphous laminations is different than that of Si-steel as amorphous foil requires relatively high cooling rates to suppress crystallization. These high cooling rates limit the thickness of the laminations to less than ˜100 microns and more often to 15-30 microns in thickness. U.S. Pat. No. 4,331,739, the contents of which are incorporated by reference in their entirety, teaches a planar-flow melt spinning (PFMS) process, which is the currently preferred method to produce amorphous foil. PFMS commonly occurs at casting speeds of 15-45 m/s, where the foil is cast and synchronously wound, making it very difficult to achieve any type of scribing in-line during production.
Laser scribing of amorphous laminations has been described in U.S. Pat. Nos. 4,915,750; 4,724,015; and 9,290,831, the contents of each of which are incorporated by reference in their entirety, where the laser pattern is applied after the initial production step. Laser scribing requires the laminations to be individually processed, and the throughput for nominally 25 microns thick amorphous lamination material is lower than Si-steel laminations which can be 10-50 times thicker than amorphous foil. The additional cost associated with processing thin laminations is one of the main reasons that laser scribing has not been widely adopted for amorphous material. Mechanical scribing of amorphous laminations has also not been widely commercialized as the additional processing costs results in the material being prohibitively expensive.
In-line methods for scribing amorphous laminations are challenging due to the processing speeds being typically in the range of 20 to 30 m/s. U.S. Pat. No. 10,468,182, the contents of which are incorporated by reference in their entirety, discusses methods to introduce mechanical scribing during processing that utilize either template patterns on the casting substrate surface through scratching the substrate with a wire brush during processing or introducing a wave-like undulation in the ribbon surface by controlling the temperature distribution in the melt nozzle.
The scribing of the present invention can be achieved in amorphous material in-line during the production of the foil through control of the capillary vibrations of the molten metal puddle feeding material onto the quenching substrate resulting in a cast-in mechanical pattern in the amorphous lamination. This pattern refines the magnetic domains and reduces core losses. The pattern in the foil is a localized reduction in thickness that is captured during the capillary vibration. Under controlled conditions this pattern covers the width direction of the foil completely and has a uniform spacing.
The casting conditions that allow for production of amorphous foil have fundamental stability limits. The basics of the PFMS process are that molten metal must flow onto a rotating chill wheel substrate to rapidly quench into a continuous foil. The linear wheel speed, the applied pressure and metallo-static pressure applied to the molten metal and the gap spacing between the nozzle and the wheel are the primary control parameters for the PFMS process. Wheel speeds that are too slow cause the ribbon thickness to be too high to form an amorphous ribbon and wheel speeds that are too fast impede solidification and prevent a fully quenched foil from being formed. Applied pressures to the molten metal flow that are too high cause the process to overflow and fail to form ribbon. Similarly, applied pressures that are too low do not feed enough molten metal to form a complete ribbon. The gap spacing between the nozzle that feeds the molten metal and the chill wheel is also an important control parameter as the gap provides hydrodynamic resistance to the molten metal flow and allows the flow to form a stable sheet in the width direction. A gap spacing too large does not effectively restrict the flow and a gap spacing too small restricts the flow to the point that the metal freezes in the nozzle slot rather than flowing onto the casting wheel. The process can operate within these fundamental stability limits. However, it has been determined that under select process conditions a capillary vibration is induced in the molten metal and controlled at a specific frequency to the point that a uniform scribed pattern is formed in the amorphous lamination.
The invention will be more fully understood and further advantages will become apparent when reference is made to the following detailed description of the embodiments and the accompanying drawing in which:
As defined herein, a “magnetic domain” is a region in which the magnetic fields of atoms are grouped together and aligned. “Domain refinement” refers to techniques that reduce the core loss of the lamination materials. The term “applied pressure” used here refers to the combination of metallo-static head and any additional gas pressure applied in the crucible. “Free side” of the foil refers to the side that is not in contact with the chill wheel substrate during processing. Unless otherwise stated herein, the description of the properties of the scribed pattern on the foil, including the wavelength, depth, width, etc., refers to the properties observed on the free side of the foil. “Frequency scaling” used herein refers to the resonant frequency at which the molten metal puddle is most susceptible to vibrations. “Scribing” includes techniques used to create small distortions on surface of the lamination materials that result in domain refinement. As described above, PFMS is a rapid solidification process used in the manufacture of thin, metallic ribbon, and foil. “Gap height” is the spacing between the nozzle and the chill wheel surface where the molten metal puddle forms during processing. As used herein, “capillary vibration” refers to the vibration of the molten metal puddle caused by capillary forces during the PFMS process.
In a preferred embodiment, a controlled capillary oscillation of the molten metal puddle during the PFMS process is disclosed.
A close-up schematic of the contact zone between the nozzle and the wheel is shown in
f˜(σ/ρ*G3)1/2,
where ρ is the density of the molten metal, G is the gap height and σ is the molten metal surface tension. Physically this is the ratio of inertial to capillary forces within the puddle. Viscous forces are typically low in molten metals; thus there is little vibrational dampening and vibrations can freely resonate.
A feature of this vibrational frequency scaling is the nonlinearity of the gap height, which implies that controlling the gap height is important. Under optimum processing conditions the puddle vibration freely oscillates and a mechanical pattern gets captured for each period of the vibration in the amorphous foil during processing.
λ=C*U*(ρ*G3/σ)1/2.
In one embodiment, under the experimental conditions described herein, C is a geometric constant related to the resonant vibration mode that is experimentally found to be ˜0.5. The method described herein may be applied for PFMS machining operations with any suitable alloys and any suitable casting temperatures.
Thermal expansion of the quench wheel may occur during casting due to the high heat flow rates through the PFMS process. Variables such as the quench wheel thickness, the quench wheel internal cooling design, the quench wheel thermal conductivity, the linear casting speed and many others may impact the amount of thermal expansion that occurs. It was determined that the expansion of the casting wheel is typically symmetric across the width of the foil with most expansion occurring in the center of the rim as shown in
A solution to the thermal expansion is to modify the nozzle gap height in the width direction of the foil as shown in
Maintaining a uniform gap height across the width of the ribbon allows for the control of the capillary vibration to scribe a wavelength that is uniform across the ribbon.
A broad range of soft magnetic compositions may utilize this scribing method. Alloys generally follow the formula Fe100-v-w-x-y-zSivBwPxCyMz in atomic percentages, where Si, B, P and C are nonmetals included in the alloy to help form the amorphous structure, and M can preferably be some combination of Co, Nb, Cu, Mo, Cr, Ni or any transition metals belonging to Groups IV to XI, in addition to unavoidable impurities. One embodiment includes an alloy where v=0-15.2 atomic percent, w=0-20.3 atomic percent, x=0-15.9 atomic percent, y=0-2 atomic percent, z=0-66.8 atomic percent and 15<v+w+x+y<30. In other embodiments, the alloy for producing the foil consists essentially of Fe100-v-w-x-y-zSivBwPxCyMz in atomic percentages, wherein Fe is from 78-84, Si is from 0-10, B is from 11-18, and C is from 0-0.5. Table 1 lists representative examples of chemistries that exhibit the scribing pattern with wavelength λ in the amorphous foil and the associated induction level, B, when driven under an applied field of 800 A/m.
For example, the scribing pattern may be applied to foil in widths from 10 mm to 1 meter. The width of the foil may be limited by the nozzle and casting wheel dimensions, as well as the ability to apply a contour to the nozzle such that the gap height is held constant where the scribing pattern is uniformly applied. In one embodiment, the method described herein may be used to scribe ribbons in widths ranging from 10 mm to 260 mm, for example, the ribbon may be 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 250, and 260 mm, and any variation within those widths. The ability to cast ribbons at these widths with the scribed pattern depends on the dimensions of the nozzle and casting wheel and the contour applied to the nozzle such that the gap height is held constant and the scribing pattern is uniformly applied. This scribing pattern may be applied to foils that are 13 to 75 microns thick. In one embodiment, the ribbons are about 13-40 microns thick, more preferably the ribbons are about 13-30 microns thick. In one embodiment, λ is observed to vary between 1 to 5 mm depending on the alloy and processing conditions. The scribing pattern may be seen to cover anywhere from 10 to 100% of the surface of the foil. In one embodiment, the scribed pattern covers from 10 to 100% of the ribbon including both edge to edge and head to tail on the spool of the ribbon. The induction levels of the foil may vary between 0.6 to 1.8 T depending on the alloy chemistry.
In most cases this scribed pattern in the foil is not a desirable feature and prior methods sought to avoid casting alloys with any patterns. However, it was determined that there is an unexpected benefit that this pattern has on the magnetic performance of the foil where the losses are reduced. The capillary vibration method described herein to apply the scribing pattern in the foil allows for the magnetic domain control to be applied in-line during the foil production in a single step.
The loss reduction found in the scribed foil depends in part on the final application. Typically, the amorphous foil properties are reported in the single strip configuration. Each coil of foil is sampled and tested in a flat single strip configuration according to the test methods defined in ASTM International Amorphous Testing standards. The foil is used primarily in either a wound toroid configuration or a laced distribution transformer core application. Each of these has a building factor or destruction factor that adds to the loss when converting from a single sheet to a core configuration. Table 2 illustrates embodiments of the invention including typical sample weights and measured losses of the three configurations for nominal foil and scribed foil of composition Fe81B14.7Si4C0.3. In all cases the scribing condition results in a typical loss reduction of around 30%. The methods described herein may allow for scribing conditions that show loss reduction ranging from 10 to 40%, preferably the conditions result in a loss reduction ranging from 20 to 40%. The single sheet test may include a sample of foil that weighs on a few grams. The toroidal configuration may include a foil that is wound upon itself, most commonly in a cylindrical shape, and may weigh anywhere from a few tens of grams to a few kgs. The distribution transformer cores are much larger in mass and may weigh anywhere from a few kgs to over 1000 kgs depending on the transformer size.
Table 3 shows a list of embodiments of the invention, including sample castings of 213 mm wide foil of composition Fe81Bi4.7Si4C0.3 that utilize the scribing method. The core loss and exciting power measurements are taken at 1.4 T, 60 Hz. Here, the single sheet test results are reported under optimum process conditions and the scribing is further characterized in terms of λ, δ and ω as defined in
In the embodiment shown in Table 3, the amorphous foil core has a reduced core loss of 31% in comparison to the amorphous foil core operating under normal PFMS process conditions when tested at an operation induction level of 1.4 T, at 60 Hz, because the single sheet losses have been reduced from a typical value of 0.125 W/kg to 0.083 W/kg. According to the invention, the controlling of the scribing pattern may affect the core losses by ˜25-40% overall.
A) Alloy Composition Fe81B14.7Si4C0.3
The alloy of Fe81B14.7Si4C0.3, in atomic percentage, is one of the normal chemistries that is commercially produced and multiple finished cores has been formed under normal operating conditions. Table 4 lists the typical process parameter range for important control variables of the process. The nozzle condition for standard production is of a flat bottom, non-contoured type. This results in the scribing pattern being observed when the process conditions align with the conditions in Example 1. However, since the nozzle is not contoured the scribing coverage is rarely in the 75-100% range, the coverage is typically in the 25-50% range.
The scribing pattern had an average wavelength of ˜2.2 mm and a coverage percentage of between 75-100%. Table 5 lists the geometric conditions of the distribution transformer core, the percentage of scribing coverage and the final core loss.
The alloy of Fe81Bi4.7Si4C0.3, in atomic percentage, is one of the normal chemistries that is commercially produced and multiple finished cores has been formed under normal operating conditions. Table 4 lists the typical process parameter range for casting foil with almost none of the scribing present. The nozzle condition here may be either a flat bottom (non-contoured) or contoured type. The gap height is on the very low end of the stable operation condition to prevent any scribing from occurring so perhaps the contouring does very little at this low gap level. This results in the ribbon having a near mirror finish. The coverage for this trial is in the 0-25% range.
(A, B, C, and D are the dimensions of the core as noted in
B) Alloy Composition Fe79B11.6Si9.3C0.1
An alloy of Fe79B11.6Si9.3C0.1, in atomic percentage, was produced utilizing used standard operating conditions and a flat bottom nozzle and non-contoured. The casting conditions were not restricted to the optimum level for scribing but allowed to vary within the operational control limits. Here the scribing pattern was present but had a coverage percentage of between 25 to 50% and a core loss of 0.22 W/kg when measured at 1.3 T, 60 Hz.
An alloy of Fe79B11.6Si9.3C0.1, in atomic percentage, was produced utilizing the in-line scribing method with a nozzle that was contoured to match the wheel profile. This alloy has a saturation induction of 1.56 T. The conditions for optimizing the scribing condition from Table 4 were also applied here. Here the scribing pattern had an average wavelength of ˜2.5 mm and a coverage percentage of between 75-100%. The operating induction of a transformer with this alloy is lower due to the lower saturation induction of the alloy. Therefore, the losses are evaluated at 1.3 T, 60 Hz, and show a core loss of 0.16 W/kg.
The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.