Patent Application
20210140014
The present invention relates to a zinc wrought alloy with improved machinability as compared to known wrought alloys, as well as semifinished products, forgings, turned parts, locks, screw connections, locking cylinders, sleeves, fittings, pressed parts, pneumatic parts, hydraulic parts, mountings, valves and ball valves that comprise a zinc wrought alloy according to the invention.
A wide variety of copper-zinc alloys (brass alloys) are known in the prior art. When objects are prepared from these alloys, they are processed with shaping, for example, by machining. Thus, machinability, i.e., the property of a material to be processable by machining, is an important characteristic of corresponding materials. To improve their machinability, brass alloys are often alloyed with lead, such as free-machining brass, CuZn39Pb3.
In recent years, the conditions to be met for the protection of health and environment have been tightened up significantly by the legislation in many fields. This affects, in particular, a ban or a drastic reduction of lead as an alloy component in copper alloys (see, for example, [1] Verordnung zur Novellierung der Trinkwasserverordnung vom 21. Mai 2001, German Federal Law Gazette, Issue 2001, Part I No. 24, issued in Bonn on May 28, 2001; [2] Directive 2000/53/EC of the European Parliament and of the Council of Sep. 18, 2000, on end-of-life vehicles, Official Journal of the European Communities, L 269/34, DE, Oct. 21, 2000; or [3] Directive 2002/95/EC of the European Parliament and of the Council of Jan. 27, 2003, on the restriction of the use of certain hazardous substances in electrical and electronic equipment, Official Journal of the European Union, Feb. 13, 2003, DE, L 37/19). Therefore, in view of these conditions to be met, low-lead brass alloys were developed, but which can still contain up to 0.25% lead.
In addition to brass alloys, zinc alloys have also been described in the prior art. Herein, low lead or even lead-free alloys are increasingly being developed. As an example, EP 2675971 A—“Accessory consisting of a lock accessory” may be mentioned. It discloses a zinc alloy with an Al content of from 13 to 25%, a Cu content of from 0.2 to 3.5%, and an Mg content of less than 0.1%, which is employed for lock accessories.
EP 2 385 148 A—“Zinc alloy with high creep resistance” relates to a zinc-aluminum alloy with an Al content of 10 to <25%, a Cu content of 0.05 to 3%, an Mg content of from 0.001 to 0.1%, an Mn content of 0.05% to 1.0% and an Si content of from 0.05 to 1%. The disclosed alloy has a high creeping resistance and is suitable for the furnace brazing and normal brazing of heat exchangers.
U.S. Pat. No. 3,734,785—“Zinc forging alloy” claims a zinc-based alloy with an Al content of 9 to 22%, a Cu content of 0.5 to 1.5%, and an Mg content of 0.01 to 0.03%, which is particularly suitable for hot formability.
U.S. Pat. No. 3,880,679—“Method of forming zinc-aluminum alloys with good machinability” describes zinc-aluminum alloys with an Al content of 22 to 27%, a Cu content of 0 to 10%, an Mg content of 0.01 to 1%, and a Bi content of 0.01 to 3%.
EP 0 679 198 A—“Method for producing Zn—Al—Cu alloy articles by centrifugal or die casting” describes a zinc alloy with an Al content of 6.0 to 8.0%, a Cu content of 3.2 to 4.3%, for preparing articles by centrifugal casting in a rubber mold, or pressure die-casting in a metal mold.
Also widely known are zinc pressure die-casting alloys, also referred to as ZAMAK®. These consist of zinc-aluminum-copper-magnesium alloys, which cannot have the corresponding strength properties, however. Further, machining is clearly more problematic in zinc pressure die-casting alloys because of the higher porosity structure.
Proceeding from this prior art, it has been the object of the present invention to provide a zinc-based wrought alloy having an improved machinability as compared to the prior art. At the same time, the mechanical properties should not be adversely affected. The improved machinability is to be achieved without including lead in the alloy. Surprisingly, it has been found that titanium enables an improved machinability of zinc wrought alloys. “Alloying” basically means the preparation of an alloy by melting a metal together with at least one other metal or non-metal. If in the present application it is referred to the fact that a metal or non-metal is not alloyed to an alloy, this means that the metal or non-metal in question is not actively added.
Therefore, in a first embodiment, the object of the present invention is achieved by a zinc wrought alloy having an Al content of from 5% by weight to 18% by weight, a Cu content of from 0.1% by weight to 4% by weight, an Mg content of from 0.001% by weight to 0.05% by weight, a Ti content of from 0.01% by weight to 1% by weight, wherein Zn is the balance to 100%, and wherein the alloy may contain impurities at a proportion of 0.07% by weight or less.
This may be achieved, for example, by purposefully alloying with titanium in zinc-aluminum-copper-magnesium alloys, which may then be used in the preparation of a wide variety of semifinished products and articles, such as forgings, turned parts, locks, screw connections, locking cylinders, sleeves, fittings, pressed parts, pneumatic parts, hydraulic parts, mountings, valves or ball valves. Titanium is an extremely effective alloy element, strongly affecting the microstructure already in the ppm range because of its lattice structure. With it, a better machinability can be achieved. At the same time, the mechanical properties of the alloy are not adversely affected. Further, the zinc alloy according to the invention has very good hot formability properties.
If percentages are stated with respect to components contained in the alloy in the present application, they are percent by weight unless explicitly stated otherwise, respectively based on the total weight of the alloy. In particular, no further metals in addition to the metals mentioned are alloyed with the alloy when it is prepared. More preferably, the alloy according to the invention is free of zirconium.
Surprisingly, it has been found that an improved machinability, in particular, is achieved by selectively alloying titanium. Other zinc wrought alloys (as disclosed, for example, in EP 2675971—“Accessory consisting of a lock accessory”) have a poorer machinability (chip formation). Surprisingly, the machining index could be brought into the reference range of free-machining brass (CuZn39Pb3) by alloying with titanium, but without having to alloy with lead. In addition, very good drilling, milling and broaching properties are obtained. Further, processing may be dry or wet. Alloying with lead is not necessary. Preferably, lead is not alloyed. Preferred is a Pb content in the alloy according to the invention of 0.003% by weight, especially <0.003% by weight, which is present in the alloy as an impurity of zinc, in particular, but not as an additional alloy component.
In addition, surprisingly, the microstructure (fineness of grain) can be influenced by alloying with titanium so that the forgeability is significantly improved. The proportion of titanium (Ti) in the alloy according to the invention is preferably from 0.01% to 1% by weight, especially from 0.03% to 1% by weight, specifically from 0.05% to 1% by weight, preferably from 0.06% to 1% by weight. It has been found that these proportions of titanium are sufficient to achieve the improved properties. Larger amounts are not necessary and also can be introduced only with difficulty without adversely affecting the microstructure of the alloy. Particularly preferred is a Ti content of from 0.05% to 1% by weight.
In addition to the mentioned components (Zn, Al, Cu, Mg, Ti), the alloy according to the invention may also comprise impurities resulting from the fact that these components (Zn, Al, Cu, Mg, Ti) are derived from recycling. However, for the usual sources of the components (Zn, Al, Cu, Mg, Ti), these are not critical. Common impurities are Cd, Pb, Sn and/or Fe. Preferably, these impurities are contained only in very small amounts, so that they do not affect the properties of the alloy according to the invention. Therefore, preferred is a Pb content of <0.003% by weight, and/or a Cd content of <0.003% by weight, especially <0.0005% by weight, and/or an Sn content of <0.001% by weight, especially of <0.0005% by weight, and/or an Fe content of <0.05% by weight. Preferably, the content of all stated impurities is below the mentioned values. Preferably, the content of all impurities is 0.07% by weight or less.
The alloys according to the invention are suitable for surface treatments (for example, electroplating, PVD, CVD, passivation, painting, cathodic dip painting/coating, powder coating).
Particularly preferred is a zinc wrought alloy with a content of aluminum (Al) of from 5% by weight to 18% by weight, especially from 8% to 18% by weight, preferably from 10% to 16% by weight, more preferably from 5% to 9% by weight, preferably from 10% to 12% by weight, more preferably from 14% to 16% by weight, especially from 16% to 18% by weight. These ranges are preferred because all alloys are supereutectic therein, and there is a first beta phase in the crystal structure. This beta phase is preferred because it recrystallizes at room temperature very slowly (>10 years), so that the properties of the alloy are retained.
Particularly preferred is a zinc wrought alloy with a content of copper (Cu) of from 0.1% to 2.5% by weight, especially from 0.5 to 1.5% by weight. This range is preferred to achieve the maximum mechanical strength, and to avoid the risk of forming of a brittle epsilon phase in the crystal structure.
Particularly preferred is a zinc wrought alloy with a content of magnesium (Mg) of from 0.003% by weight to 0.05% by weight, especially from 0.003% to 0.03% by weight. This range serves as a precaution to prevent intercrystalline corrosion by the residual traces of impurities.
The titanium content of at most 1% in the zinc alloy is limited by the solubility of titanium.
The zinc wrought alloy according to the invention may further contain silicon as an impurity. If it contains silicon, the content of silicon in the alloy is within a range of from 0.005% by weight to 0.02% by weight, in particular. The silicon content is determined by the selection of Al, because silicon is an impurity in aluminum.
It has been found that an alloy having an Al content of from 10% to 12% by weight, a Cu content of from 0.5% by weight to 1.5% by weight, an Mg content of from 0.003% by weight to 0.05% by weight, a Ti content of from 0.05% to 1% by weight, with zinc as the balance to reach 100% by weight, has particularly good properties with respect to machinability. At the same time, mechanical properties, such as strength or hardness, are not adversely affected. Therefore, such an alloy is preferred.
Particularly preferred according to the invention is a zinc wrought alloy with an Al content of from 14% to 16% by weight, a Cu content of from 0.5% by weight to 1.5% by weight, an Mg content of from 0.003% by weight to 0.05% by weight, a Ti content of from 0.05% to 1% by weight, with zinc as the balance to reach 100% by weight. Corresponding alloys have good properties with respect to machinability and in addition have a good processability. Herein, mechanical properties of the alloy, such as strength or hardness, are not adversely affected.
Further preferred alloys have the following compositions:
The present invention further relates to the use of the titanium-containing zinc wrought alloy according to the invention for preparing semifinished products and articles with improved machining properties. Further included according to the invention is a semifinished product or article obtainable by processing the zinc wrought alloy according to the invention. This semifinished product may be, in particular, a billet, an extruded section, a drawn section, a wire, a strip, a powder, or a pressure die-cast alloy. In particular, the article may be a forging, turned part, lock, screw connection, locking cylinder, sleeve, fitting, pressed part, pneumatic part, hydraulic part, mounting, valve or ball valve.
The semifinished product according to the invention, especially the billet, can be prepared, for example, by casting the zinc wrought alloy according to the invention into a mold. When the alloy according to the invention has been formed into a billet shape, for example, a section can be prepared therefrom by reshaping by means of extrusion. Further, the zinc wrought alloy according to the invention can be processed by different reshaping methods. Such reshaping methods include, in particular, rolling, forging, drawing. The articles according to the invention are excellently suitable for being subjected to machining methods.
The zinc wrought alloy according to the invention and the articles prepared therefrom exhibit an improved machinability as compared to conventional ZnAl/ZnAlCu/ZnAlCuMg alloys.
The requirement for the invention is to enhance the processability of zinc wrought alloys. This object was achieved by alloying with titanium. There may be mentioned, in particular, the machining properties that were significantly improved thereby, so that a machining index within the reference range of free-machining brass (CuZn39Pb3) can be achieved. In the experiments, it is found that the titanium content leads to ideal chip shapes. Surprisingly, increased cutting speeds could be achieved additionally, which significantly enhances productivity.
The prepared products made of the titanium-containing zinc wrought alloy according to the invention are more cost-effective than comparably lead-free brass materials. This results from a lower density and an excellent processability caused by the optimum composition of zinc, aluminum, copper, magnesium and titanium.
In the following Examples, the present invention is further explained in a non-limiting way, and advantages over the prior art are pointed out.
The zinc wrought alloy according to the invention was compared with the following materials:
A zinc wrought alloy as described in EP 2 675 971 was used as a comparative material (information in column “zinc alloy” in Table 1).
The qualification of the zinc wrought alloy according to the invention is based on four methods delimited from one another, which are set forth in the following. They are the basis of the determination of the claimed composition boundaries. If one of the compositions showed defects, this led to exclusion.
From a zinc alloy as described in Table 1 as well as from the following alloys according to the invention, billets having a diameter of 135 mm were prepared, which served as a starting point for the qualification:
Both the billets/alloys according to the invention and the comparative alloys/billets were analyzed by the following methods relating to different mechanical properties as well as machinability (qualification):
Method 1 (Reshaping Method):
The billet was heated at 250° C. in an oven. Thereafter, the billet was extruded into a round section. Further, the extruded round rod was drawn to a final dimension of 26 mm. The testing requirements were considered to be met if no signs of surface cracks or blisters have formed.
Method 2 (Tensile Test):
As the second method, a tensile test was performed. The exact realization, the definition of the measurable characteristics and the specimen shape are defined in DIN EN ISO 6892-1:2017.
A section of the drawn round rod having a diameter of 26 mm was lathe-turned into a specimen for tensile testing as shown in
Method 3 (Creep Tendency):
Further, the creep tendency or creep strength according to DIN EN ISO 204:2009 was tested as a third method. A specimen as shown in
Method 4 (Shape of Chip):
In the fourth method, a section of the drawn round rod was clamped into a turning machine. A turned part having rotational symmetry with five recessed grooves having widths of 3 mm and depths of 3 mm was prepared therefrom. The testing requirements were considered to be met if the chip shape corresponds to industrial custom.
Results:
At first, the different alloy ranges were tested with respect to aluminum content, because the latter represents the major alloy component. In the following Table 3, the mechanical characteristics of the alloys according to the invention (samples 1 to 4 according to Table 2) are shown. They were determined by the above described methods 2 and 3.
Surprisingly, the selective alloying with titanium did not have a negative impact on the mechanical characteristics. In the comparison with the comparative material (zinc alloy from Table 1), no significant differences could be seen.
Results of Method 4 (Shapes of Chips)—Specimens 1, 2, 3 and 4:
a) Specimens 1 to 4 According to the Invention were Processed as Described Above Under Method 4 with the Following Parameters:
Photographs of the chip shapes and the specimens that were processed are shown in
b) Specimens 1 to 4 According to the Invention were Processed as Described Above Under Method 4 with the Following Parameters:
Photographs of the chip shapes and the specimens that were processed are shown in
Other alloys according to the invention—specimens 3a to 3d
Further, different titanium contents were tested for determining a preferred composition by means of specimen 3:
Results of Methods 2 and 3—Specimens 3a, 3b and 3c:
The following Table 7 shows the results of the mechanical properties of the specimens (results of methods 2 and 3).
Surprisingly, the titanium content in different amounts does not show any negative impact on the mechanical properties.
Results of Method 4 (Shapes of Chips)—Specimens 3a, 3b, 3c and 3d:
The machining parameters of Tables 4 and 5 remained identical. Photographs of the chip shapes and the specimens that were processed are shown in
In the comparison in
The results from
In the last step, the alloy 3 according to the invention was compared with the comparative material from EP 2 657 971.
Results of Method 4 (Shapes of Chips)—Specimen 3 vs. Zinc Alloy According to EP 2 675 971:
The machining parameters of Tables 4 and 5 remained identical. Photographs of the chip shapes and the specimen that was processed are shown in
In the comparison in
Also at lower cutting speeds, as shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 17179643.6 | Jul 2017 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2018/067808 | 7/2/2018 | WO | 00 |
