Patent Application
20120294756
The present invention relates to shots that are used for a process by projecting (by blowing) them, such as shot blasting or shot peening. The shot blasting is utilized to remove fins or burrs (below, “deflashing”) in nonferrous metallic parts, such as aluminum die-castings and aluminum castings, to strip sand from castings, to remove burnt deposits of coatings and parting agents, or to remove scales or flow lines (below, “grinding and cleaning”). The shot peening is utilized to improve the fatigue strength of nonferrous metallic parts or welded parts of nonferrous metallic parts.
Herein, the wording “Vickers hardness” means the value measured in accordance with JIS Z 2244 under the condition that the applied force is 0.4093 N and the time for applying the force is 10 to 15 sec. It is expressed as “◯◯◯ HV 0.05,” abbreviated as “◯◯◯ HV.”
The mark “%,” which is used to denote the contents of an alloy, means “mass %,” unless otherwise indicated.
Conventionally, die-cast parts made from nonferrous metals, such as aluminum-based alloys, zinc-based alloys, and magnesium-based alloys, which are used in automobiles or like, are widely used for shot blasting for treating surfaces. Shot blasting is a process to shoot small balls onto a workpiece at high speed for removing the fins of, or grinding and cleaning, castings. The small balls are called shots.
Recently, as a surface treatment for improving fatigue strength of the nonferrous metallic parts or welded parts of nonferrous metallic parts, shot peening is widely used. Shot peening is a process to shoot small balls onto a workpiece at high speed, which is similar to shot blasting.
Shots made from aluminum-based alloys, stainless steels, or zinc-based alloys are usually used for the shot blasting.
Shots made from aluminum-based alloys have a low specific weight. Thus the grinding and cleaning abilities are not sufficient. Because of the characteristics of aluminum, a cloud of dust caused by breakages of the shots during shot blasting can easily burst. Further, the minimum concentration to burst is low. Thus additional controls for work safety are required.
Shots made from stainless steels contain Ni (No. 231 in the Japanese government regulations) and Cr (No. 68 in the Japanese government regulations) that are designated by “Pollutant Release and Transfer Register” (PRTR) regulations. Thus in view of work safety and environmental preservation, their use tends to be restricted.
Dust from shots made from zinc-based alloys cannot easily burst because of the breakages of the shots and because it takes a higher concentration to burst than those made from aluminum-based alloys and stainless steels. Thus in view of safety they are the shots most widely used for shot blasting and for shot peening for die-cast parts made from nonferrous metals.
Patent Literature 1 to 5 are prior-art publications that relate to shots made from zinc-based alloys, though they do not affect the patentability of the present invention.
Prior-Art Publications (Patent Literature)
When treating surfaces for deflashing or for grinding and cleaning by shot blasting and to improve fatigue strength by shot peening, it is preferable to use shots that are suitable for the hardness of the surface of the workpiece.
For example, to treat the surfaces of die-cast parts made of aluminum that have a Vickers hardness of 90 to 110 HV, it is preferable to use shots that have a hardness that is close to, or more than, that hardness.
However, generally no shots made from zinc-based alloys that have a toughness that is more than that toughness are available. This is because the toughness of the shots made from zinc-based alloys generally decreases in proportion to the increase of their hardness.
Patent Literature 2 discloses an invention to add Mn (the Mn amount of 0.3 to 0.5%) to improve the durability of the shots. However, the Mn is also designated by the PRTR regulations (No. 311 in the Japanese government regulations). Thus it has the same problem that stainless steels do.
In view of the recent growth in the interest in the environment, the development of technologies has been awaited to improve the durability of shots wherein no element that is designated by the PRTR regulations, e.g., Mn, is added.
The inventors had engaged in research to solve the problems involving these shots made from zinc-based alloys. Finally, they found that the shots made from zinc-based alloys that have specific components that were gained by adding Al and Cu to Zn have a Vickers hardness of about 100 HV and high toughness, without adding any element that is listed in the PRTR regulations. Thus they conceived the present invention, which is described below.
The present invention (the first invention) relates to shots made from zinc-based alloys that contains three components that include Al of 0.5-6.5%, and Cu of 0.5-4.5%, as additional elements. It is characterized in that a ratio of the mass of the Al to Cu (Al/Cu) is 1.0 to 13.0, that their total amount to be added (Al+Cu) is 1.5 to 10.5%, and that their Vickers hardness is 90 to 190 HV.
In the shots made from zinc-based alloys of the present invention, Al is added, as an element of the alloys (an essential element), to increase their resistance to impact. That is the mechanical property that significantly affects the amount used of the shots. Al has the effects to increase the resistance to impact (toughness), mechanical strength, and Vickers hardness of zinc-based alloys. If the amount of Al is less than 0.5% (the total amount is set as 100%, as the case may be), the effects cannot be achieved. If it exceeds 6.5%, the resistance to impact tends to decrease. The amount of Al that is preferable to increase the resistance to impact is 3.0 to 6.0 mass %, preferably about 3.0 to 5.0%, though it depends on the amount of Cu, which is another element to be added.
Cu is added to increase the Vickers hardness of shots made from zinc-based alloys. Cu has effects to increase the mechanical strength and Vickers hardness of zinc-based alloys. If the amount of Cu is less than 0.5%, the effects cannot be achieved. If it exceeds 4.5% or if the total amount of Al and Cu exceeds 10.5%, the resistance to impact tends to decrease (the toughness decreases) while the mechanical strength and Vickers hardness increase. For the shots made from zinc-based alloys of the present invention that have a Vickers hardness of 140 HV, which is much higher than that of a Vickers hardness of 100 HV of die-cast parts made of aluminum, the most preferable amount of Cu is about 1.0 to 3.0%.
As discussed above, if the Vickers hardness of the shots made from zinc-based alloys is lower than 90 HV, the abilities for deflashing and for grinding and cleaning the surfaces are not sufficient. However, if it exceeds 190 HV, shots made from zinc-based alloys are likely to break or be lost when deflashing, grinding and cleaning, or processing by shot-peening. Thus the loss of the shots increases to where they are impractical. This is caused by the low toughness of the shots made from zinc-based alloys. Thus, shots are arbitrarily selected, based on workpieces (products) or purposes for the process, among the shots that have sufficient abilities for deflashing, grinding and cleaning, or processing by shot-peening, and have a Vickers hardness of 90 to 190 HV, preferably 130 to 154 HV, by which the consumption of shots is small (high toughness).
The amount of the elements (non-essential elements) other than the three components (Zn, Al, and Cu) in the shots that are discussed above is 0.5% or less. The amount of Fe is preferably 0.3% or less.
The non-essential elements include, for example, Pb, Fe, Cd, Sn, Si, Ti, Mn, As, Sb, Bi, and S. If the total amount of these non-essential elements exceeds 0.5%, the shots made from zinc-based alloys become brittle and have low toughness. Especially, Fe adversely affects toughness. If the amount of Fe in the shots exceeds 0.3%, the loss of the shots increases to be impractical (see Comparative Examples 1-6 and 2-3).
The purity of each additional element, i.e., Al and Cu, is preferably 99.9% or more in the components of the present invention. Further, the amount of all the non-essential elements is preferably 0.02% or less. By so doing, the decrease in toughness that is caused by inclusion of the non-essential elements and their oxides into grain boundaries can be reduced as much as possible (see Examples 1-6 and 2-3).
Specifically, the special first-class aluminum ingot (99.90% or more) in JIS H 2102 or the special refined aluminum ingot (99.995% or more), the first-class aluminum ingot (99.990% or more), or the second-class aluminum ingot (99.95% or more) in JIS H 2111 (or ICS 77.120.11), may be used for the raw material (an ingot) for Al. The electrolytic cathode copper (99.96% or more) in JIS H 2121 may be used for the raw material (an ingot) of Cu.
The raw material (an ingot) of Zn as the base metal is not limited. Any grade specified by JIS H 2107 (or ISO 725: 1981) can be used. In view of the stable quality of the shots, highly refined zinc, such as the normal-class zinc ingot (99.97% or more), such as the most refined zinc ingot (99.995% or more), and such as the special zinc ingot (99.99% or more), is preferably used.
Another present invention (the second invention) relates to the shots made from zinc-based alloys. It contains four components that include Al of 0.5-6.5%, Cu of 0.5-4.5%, and Mg of 0.01-0.2%, as additive elements. It is characterized in that a ratio of the mass of the Al to Cu (Al/Cu) is 1.0 to 13.0, their total amount to be added (Al+Cu) is 1.5 to 8.0%, and their Vickers hardness is 90 to 190 HV, preferably 140 to 150 HV.
By the second invention, a small quantity of Mg is used as an additional element under the condition of the total amount of Al and Cu (Al+Cu) being 8.0% or less, the same as the condition for the first invention, to prevent the mechanical strength and Vickers hardness of the shots from decreasing due to re-crystallization of the metallic structure. The re-crystallization occurs when the shots made from zinc-based alloys of the first invention are repeatedly used. Mg has effects to prevent re-crystallization by having Mg compounds precipitated in the grain boundaries of zinc-based alloys. It also has effects to increase the mechanical strength and Vickers hardness. If the amount of Mg is less than 0.01%, the effects to prevent re-crystallization are not achieved. If it exceeds 0.2%, the effects by the addition of Al or Cu to increase the resistance to impact may be disturbed. For the shots made from zinc-based alloys having a Vickers hardness of about 140 HV that contain Al at 3.0-5.0% and Cu at 1.0-3.0%, the amount of Mg is preferably 0.01 to 0.2%, more preferably 0.03 to 0.08%. A Vickers hardness of about 140 HV is good for processing non-ferrous workpieces by shot blasting and shot peening.
The reasons for limiting the amount of the additional elements (Al and Cu) other than Mg are the same as those for the first invention.
The amount of all non-essential elements other than the four components is preferably 0.5% or less, the same as that of the first invention. The amount of Fe is also preferably 0.3% or less. The reasons for limiting the amount of the non-essential elements are the same as those for the first invention.
In the second invention the purities of the additional elements, i.e., Al, Cu, and Mg, are preferably 99.9 mass % or more, the same as those of the first invention. The reasons for limiting the purities are the same as those for the first invention.
Specifically, the raw materials (ingots) for Al, Cu, and Zn are the same as those discussed above. The first-class magnesium ingot (99.90% or more) in JIS H 2150 (or ISO 8287: 2000) may be used for the raw material (an ingot) of Mg.
The mean diameter of the shots made from zinc-based alloys by the first invention and second invention is generally 0.1 to 3.0 mm, preferably 0.3 to 2.0 mm, though it depends on the strength of a workpiece and the purpose for the process.
If the mean diameter is too small, the abilities for deflashing and for grinding and cleaning, and the effects by shot peening (e.g., imparting a compressive residual stress) cannot be obtained. However, if it is too large, the workpiece may be damaged when deflashing, grinding and cleaning, or being processed by shot peening, or the specified surface roughness may not be achieved.
If the mean diameter is 0.1 to 3.0 mm, preferably 0.3 to 2.0 mm, a surface treatment such as deflashing of a workpiece, can be done during a short period by means of high performances for grinding and cleaning. Further, if it is 0.3 to 0.6 mm, a clean surface that has a very small rough surface can be obtained.
The shots made from the zinc-based alloys of the first and second inventions can be manufactured by the steps of dropping molten metal into a cooling medium such as water, granulating the dropped molten metal to be deposited in the cooling medium, drying the granulated and deposited metal, and then classifying the granulated metal.
Since the molten metal is rapidly cooled by being dropped into the cooling medium, the metallic structure becomes finer and more uniform than that of normal metal that is cast. When being used for shot blasting or shot peening, the shots made from zinc-based alloys are subject to extremely large forces. By making the metallic structure fine and uniform, the mechanical properties, such as resistance to impact and tensile strength, are improved. Thus the granulated metal that has that structure is suitably used for the shots made from zinc-based alloys.
Since the shots of the present invention are made from zinc-based alloys, a cloud of dust caused by shots breaking cannot be easily burst and the minimum concentration to burst is high. Thus shots made from zinc-based alloys that are safe can be provided.
Further, since the shots made from zinc-based alloys have a high hardness (a Vickers hardness of 90 HV or more), the abilities for deflashing and for grinding and cleaning by shot blasting are high, so as to process workpieces over a short period. Thus the productivity is high. Since the shots have a high toughness that cannot be achieved by the conventional shots, the consumption of shots decreases and the amount of dust generated by shots breaking decreases.
Similarly, for the use for shot peening, the shots of the present invention, which have high hardness and toughness, effectively cause the surface layers of workpieces to plastically deform, to thereby generate a compressive residual stress. Similar to the use for shot blasting, the amount of dust generated by shots breaking decreases.
For the shots made from zinc-based alloys of the present invention the decrease in the mechanical strength caused by re-crystallization is relatively small, so that the Vickers hardness of the shots is stable during their use. Hence, after shot blasting or shot peening, workpieces are finished with little variation so that the quality of surface treatments is constant.
By the shots made from zinc-based alloys of the present invention, the dust contains no element that is listed in the PRTR regulations. Further, there is just a small amount of dust. Thus the shots made from zinc-based alloys are preferable in view of environmental preservation and work safety.
Below, manufacturing the shots made from zinc-based alloys of the present invention is described by using a method of manufacturing granules by dropping molten metal (see
First, ingots (raw materials) 12 of the base element (Zn) and the additive elements (Al, Cu, and Mg) are weighed to comply with the predetermined contents of the alloy. Then they are thrown into a crucible 14.
Next, by heating the crucible 14 by a heating means 15 the ingots that have been thrown and mixed are melted to obtain molten metal 16. The temperature for melting them is generally 550 to 700° C., preferably 580 to 600° C., though it depends on the composition of the alloys and the manufacturing capacities. The melting point of each element is as follows.
Zn: 419.6° C.; Cu: 1,083.4° C.; Al: 660° C.; Mg: 648° C.
Next, the molten metal 16 is poured into a vessel 18 for holding molten metal. The vessel 18 for holding the molten metal is equipped with a heating means (electric resistance heating) 20 so as not to excessively cool the molten metal 16 when manufacturing the shots made from zinc-based alloys. The temperature for holding the molten metal is generally 500 to 600° C., preferably 520 to 550° C., though it depends on the composition of alloys and manufacturing capacities.
The vessel 18 for holding the molten metal is equipped with a nozzle 22 for dropping the molten metal at its bottom. Under the direction of the nozzle 22, a cooling vessel 28 is provided that includes a cooling means (a cooling tower) 26, to which a cooling medium 24, such as water, is supplied. The cooling medium 24 may be oil or the like.
The molten metal 16 in the vessel 18 for holding the molten metal is dropped from the nozzle 22. Thus it contacts air when it passes through the air that exists between the nozzle 22 and cooling medium 24. As it is cooled by contacting the cooling medium 24, it becomes almost spherical by being affected by a surface tension effect.
When the molten metal 16 is dropped from the nozzle 22, its shape is not that of a perfect sphere, but a distorted sphere or an ellipsoid that is elongated along the direction to be dropped. Thus, the shapes of the spherical bodies 30 that are obtained, i.e., particles for shots, are slightly distorted spheres, shapes of a rotating ellipse, or a cylinder with rounded corners. The longitudinal length of the projected figure of these shots is called “a” and the maximum diameter in the direction perpendicular to the longitudinal axis is called “b.” The ratio a/b of 60% or more of the shots is preferably in a range of 1.0 to 1.2. Since the shape of the shots is approximately spherical, and their variation is small, uniform effects by grinding and cleaning may be obtained. The projected figures of these shots are obtainable by using any publicly-known means, such as an observation by a microscope or an image taken by a camera.
The cooling medium 24 is heated by contacting the molten metal 24 that is dropped. This may prevent the molten metal 24 that is dropped from being rapidly cooled. Thus, the temperature of the cooling medium 24 is kept at a predetermined temperature by a cooling means 26. The predetermined temperature is, for example, generally 60° C. or below (preferably 30 to 40° C.) for water. If it exceeds 60° C., water that contacts the molten metal that is dropped boils, then vaporizes on the boundary surfaces. Thus it is difficult to obtain the ability to rapidly cool dropped molten metal.
In the bottom of the cooling medium 24 the spherical bodies 30 of zinc-based alloys pile up. They are collected, dried by a dryer (a rotating dryer) 32, and then classified by a classifier (shaking sieve) 34 to obtain the shots made from zinc-based alloys. Classifying is performed so as to obtain the predetermined diameter of the shots made from zinc-based alloys that complies with the intended use.
The method of manufacturing the shots made from zinc-based alloys is not limited to the method of manufacturing granules by dropping molten metal that is discussed above. Any publicly known method, such as gas atomization, centrifugal atomization, and water atomization, may be selectively used for the intended shape, size, etc., of the shots made from zinc-based alloys.
Below, the working examples are described with comparative examples to verify the effects of the first and second inventions.
Working Examples 1-1 to 1-8 and Comparative Examples 1-1 to 1-6 correspond to the first invention. Working Examples 2-1 to 2-3 and Comparative Examples 2-1 to 2-3 correspond to the second invention.
The raw materials for each element for the alloys are the following ingots. The purities (the lower limits) of each ingot for the alloys that are specified by JIS are shown with their amounts of Fe (allowable maximum contents). Here, the Fe amount of the “first class recycle material of copper wires” is an estimated value.
The total amount of raw materials of the elements for the alloy: 200 kg
Used Ingot A (for working and comparative examples except for Working Examples 1-6 and 2-3)
Used Ingot B (for Working Examples 1-6 and 2-3)
Shots of each alloy were manufactured by the method (dropping molten metal) as shown in
The temperature for melting metal: about 600° C.
The temperature for keeping the molten metal: about 550° C.
The temperature for keeping the cooling medium (water): about 40° C.
The total amount of non-essential elements (impurities) and Fe amount in each working and comparative example are shown in Table 1 (corresponding to the first invention) and Table 2 (corresponding to the second invention). These amounts were obtained from the indicated contents by the respective JIS standards. The Fe amounts in Comparative Examples 1-6 and 2-3 are adjusted to be 0.35% by adding Fe.
As can be seen, the total amounts of impurities are 0.014 to 0.092% in Working Examples 1-1 to 1-8 and 2-1 to 2-3, while they are 0.032 to 0.378% in Comparative Examples 1-1 to 1-6 and 2-1 to 2-3.
In addition, Conventional Example 1 was prepared by using ingots having purities of 99.9% or more for each element and by adding Al and Mn at 0.01% and 1.9%, respectively, and adjusting the Vickers hardness to be 88 HV. Conventional Example 2 was also prepared by adding Al and Mn at 0.05% and 4.5%, respectively, and adjusting the Vickers hardness to be 129 HV
The Vickers hardness of the shots that were so manufactured was measured. The results are shown in Tables 3 and 4, which are given below.
The shots made from zinc-based alloys that were so manufactured were tested in for their performance for (1) shot blasting and (2) shot peening.
(1) Performance Tests for Shot Blasting 50 kg of the shots made from zinc-based alloys (the mean diameter at 1.0 mm, a/b at 1.2 or less for 70% or more of the shots) were prepared. They were shots for shot blasting by using “Centrifugal blasting Machine Type DZB (5HP)” (available from Sintokogio, Ltd.) at a shooting speed of 45 m/s onto a die-cast product of an aluminum alloy (the hardness on the surface at 100 HV). By doing so the performance in shot blasting was tested. For all of the shots, preliminary shooting was performed for eight hours so as to have the metallic structures of the shots made from zinc-based alloys to be stable because of re-crystallization. Thus, the shots made from zinc-based alloys were adjusted to have the same conditions as those in practical use.
Items to be evaluated were “lost amounts of the shots,” which correspond to toughness (their resistance to impact), “abilities for deflashing,” and “abilities for grinding and cleaning,” which correspond to abilities for shot blasting. They are tested in the following manner.
After shot blasting by using the shots made from zinc-based alloys was performed for eight hours, the amounts that had been lost as fine particles were measured as “the lost amounts of the shots.” They were evaluated under the following criteria.
⊚: 0.06 kg/(h·HP) or less
◯: over 0.06 kg/(h·HP), and 0.08 kg/(h·HP) or less
Δ: over 0.08 kg/(h·HP), and 0.10 kg/(h·HP) or less
x: over 0.10 kg/(h·HP)
They were evaluated under the following criteria.
⊚: Deflashed by shot blasting for 30 seconds with excellent results.
◯: Deflashed by shot blasting for 60 seconds with good results.
Δ: Deflashed by shot blasting for 90 seconds with fair results.
x: Not deflashed by shot blasting for 90 seconds, and thus the results are no good.
They were evaluated under the following criteria.
⊚: The surfaces of workpieces after shot blasting shine with a silver-white color.
◯: The surfaces of workpieces after shot blasting shine, but turned slightly dark.
Δ: The surfaces of workpieces after shot blasting shine, but turned dark.
These results in the evaluations are shown in Table 3.
Next, the range of the Al amount (0.5 to 6.5%) in the first invention is discussed.
In Comparative Example 1-1, where the Al amount is too low (0.5%), though the lost amounts of the shots are small and evaluated as “◯,” the Vickers hardness is low, thus having a poor ability for deflashing and for grinding and cleaning. Thus, the overall evaluation is “Δ,” i.e., fair.
In Comparative Examples 1-2 and 1-3, where the Al amounts of the shots made from zinc-based alloys are too high (over 6.5%), their resistances to impact (toughness) decrease because of excessive Al amounts. Thus the lost amounts of the shots are “Δ,” i.e., fair. Further, since too much Al is added in Comparative Examples 1-2 and 1-3, the specific gravity of the shots made from zinc-based alloys become light. Thus, though the Vickers hardness increases, the forces to impact a workpiece decrease. So the abilities for deflashing and for grinding and cleaning decrease. Their overall evaluations are“Δ,” i.e., fair.
For the shots made from zinc-based alloys in Working Examples 1-1 to 1-3, where the Al amounts are within the range of 0.5 to 6.5% in the first invention, their resistances to impact are high and the lost amounts of the shots are “⊚,” i.e., very little. Especially, the shots of Working Examples 1-2 and 1-3, where the Vickers hardness is 130 HV or more, have high abilities for deflashing and for grinding and cleaning. Their overall evaluations are “⊚,” i.e., excellent.
Below, the range of the Cu amount (0.5 to 4.0%) in the first invention is discussed.
About the shots made from the zinc-based alloy in Comparative Example 1-4, where the Cu amount is too low (less than 0.5%), the lost amounts of the shots are “⊚,” i.e., very little, but the Vickers hardness is low, and so the ability for grinding and cleaning is reduced. Thus the overall evaluation of the shots is “Δ,” i.e., fair.
The shots made from the zinc-based alloy in Comparative Example 1-5, where the Cu amount is too high (over 4.5%), have a very high hardness, i.e., 190 HV, in zinc-based alloys. Their abilities for deflashing and those for grinding and cleaning are evaluated as “⊚.” However, since excessive Cu is added, the toughness decreases, and thereby the lost amounts of the shots are increased and evaluated as “Δ.” This is caused by the decrease of toughness of the shots made from the zinc-based alloy as the Cu amount increases.
In the first invention, the shots in Working Example 1-1 (a Vickers hardness of 92 HV) and Working Example 1-2 (a Vickers hardness of 130 HV) have almost the same hardness as those in Conventional Example 1 (a Vickers hardness of 88 HV) and Conventional Example 2 (a Vickers hardness of 129 HV), respectively. However, the shots in the present invention have better abilities for deflashing and for grinding and cleaning.
A comparison of the shots of Working Example 1-4 where an added ingot with a relatively low purity is used to the shots of Working Example 1-6 where an added ingot with a high purity is used shows that the shots of Working Example 1-6 have a slightly lower Vickers hardness and slightly smaller lost amounts of the shots, and thereby have a better evaluation. The shots of Working Example 1-6 use an ingot with a high purity.
A comparison of the shots (Working Example 1-4) where the Fe amount is low to the shots (Comparative Example 1-6) where the Fe amount is high shows that the shots of Comparative Example 1-6, which contains excessive Fe, have a lower Vickers hardness, larger lost amounts of the shots, lower abilities for deflashing, and lower abilities for grinding and cleaning.
The prepared shots made from zinc-based alloys (the mean diameter at 1.0 mm) were shot by using “Centrifugal blasting Machine Type DZB” (available from Sintokogio, Ltd.) at a shooting speed of 60 m/s onto a workpiece of a continuous cast aluminum alloy, i.e., AC4CH, which is described below. The shot peening was performed to achieve the coverage of the surface of the workpiece at 300%.
For all of the sample shots, preliminary shooting was performed for eight hours. Thus, the shots made from zinc-based alloys were adjusted to have the same conditions as those in practical use. Then, the shot peening was performed. The Vickers hardness of the shots after the preliminary shooting is shown in Table 4.
In the test for shot peening, the continuous cast aluminum alloy AC4CH was treated by a solution heat treatment where the alloy was kept at 520° C. for eight hours followed by quenching by water. Then it was left for twelve hours as it was. Then, it was treated by an aging treatment at 160° C. for six hours. Test pieces with a plate-like shape that are 5 mm thick, 15 mm wide, and 17 mm long along a portion having parallel edges were used.
The test for shot peening was evaluated in the following items.
The items to be evaluated are the “improved ratio of compressive residual stresses,” which corresponds to the effect by shot peening, and the “toughness (their resistance to impact).” They were evaluated as follows.
The compressive residual stress at a depth of 0.15 mm from the processed surface at the center of the test pieces was measured. The ratio of the change in the stress from (the residual stress of unprocessed test piece—100 MPa) was evaluated by using the following criteria.
⊚: 250% or more
◯: 200% or more, and less than 250%
Δ: less than 200%
After shot peening by using the shots made from zinc-based alloys was performed for eight hours, the amounts that had been lost as fine particles were measured as the “lost amounts of shots.” They were evaluated under the following criteria.
⊚: 0.06 kg/(h·HP) or less
◯: over 0.06 kg/(h·HP), and 0.08 kg/(h·HP) or less
Δ: over 0.08 kg/(h·HP), and 0.10 kg/(h·HP) or less
x: over 0.10 kg/(h·HP)
The “improved ratio in compressive residual stresses” and “lost amounts of shots” were evaluated overall by denoting “⊚” for an excellent evaluation, “◯” for a good evaluation, “Δ” for a fair evaluation, and “x” for a poor evaluation. The results are shown in Table 4.
The shots made from zinc-based alloys of Working Examples 1-2 to 1-5, which are within the scope of the first invention, had a Vickers hardness of 90 to 97 HV, which decreased at 27 to 45% by the preliminary shooting. If the total amount of added Al and Cu is less than 7.5%, the mechanical strength and Vickers hardness of the shots made from zinc-based alloys is found to decrease because of re-crystallization of the metal structure that is caused by a repeated use of the shots. That is, the shots made from zinc-based alloys that have the total amount of Al and Cu of less than 7.5% are not best suited for shot peening.
Thus, the shots made from zinc-based alloys of Working Examples 1-2 to 1-5 had a Vickers hardness that was lower than that of the workpiece, i.e., 104 HV. They could not cause the surface of the workpiece to plastically deform to a satisfactory extent. They were evaluated as “Δ,” since the “improved ratio in compressive residual stresses” was less than 200%, which means that the effect by peening is low. The overall evaluations of the shots of Working Examples 1-2 to 1-5 were “Δ,” i.e., fair.
The shots made from the zinc-based alloys of Working Examples 1-7 and 1-8, which are within the scope of the first invention and which have a total amount of Al and Cu of 7.5% or more, had a high Vickers hardness of 180 to 183 HV before the preliminary shooting, and a Vickers hardness of 122 to 130 HV after the preliminary shooting, thereby being decreased by 29 to 32% by the preliminary shooting. Since the Vickers hardness was maintained to be higher than that of the workpiece, i.e., 104 HV, the shots of Working Examples 1-7 and 1-8 could cause the surface of the workpiece to plastically deform to a satisfactory extent. They were evaluated as “⊚,” since the “improved ratio in compressive residual stresses” was over 250%, which means that the effect by peening is extremely high. The lost amounts of the shots were evaluated as “◯,” which means it is small. The overall evaluations of the shots of Working Examples 1-7 and 1-8 were “◯,” i.e., good.
In Comparative Example 2-1, which relates to the second invention, the added amount of Mg was 0.001%, which is too small (Mg of less than 0.01%). Though Mg was added, no effect to prevent re-crystallization was obtained. The Vickers hardness after the preliminary shooting decreased to 92 HV (decreased by 36%).
Thus, since the Vickers hardness of the shots of Comparative Example 2-1 was lower than that of the workpiece, i.e., 104 HV, the “improved ratio of compressive residual stresses” was evaluated as “Δ,” i.e., less than 200%, which means that the effect by peening is low. The overall evaluation of these shots made from zinc-based alloy was “Δ,” i.e., fair.
The shots made from the zinc-based alloy of Comparative Example 2-2 have an excessive amount (over 0.2%) of Mg, i.e., 0.3%. The addition of Mg causes Mg compounds to be precipitated in the grain boundaries of zinc-based alloys. Thus, it was estimated that re-crystallization was prevented, since the decrease of the Vickers hardness after the preliminary shooting was suppressed, to 3%. However, their resistance to impact decreased and the “lost amounts of the shots” were evaluated as “x,” that is, the lost amounts are great. The overall evaluation of these shots made of zinc-based alloys was “x,” i.e., poor.
In the shots made from the zinc-based alloys of Working Examples 2-1, 2-2, and 2-3, where the total amount of added Al and Cu is 7.5% or less and the amount of Mg is at a range of 0.01 to 0.2%, i.e., within the scope of the present invention, the re-crystallization of the metallic structure that is caused by repeated use was suppressed so that the decrease of the Vickers hardness after the preliminary shooting was only 6 to 12%. Further, the absolute values of the Vickers hardness after the preliminary shooting were 123 to 141 HV. High hardness was maintained, to thereby be significantly higher than the Vickers hardness of the workpiece, i.e., 104 HV.
Thus, the shots of Working Examples 2-1 to 2-3 could cause the surface layer of the workpiece to plastically deform to a satisfactory extent. The “improved ratio of compressive residual stresses” was evaluated as “⊚,” i.e., over 250%, which means that the effect by peening is extremely high. Since the added Mg was within a range where the decrease of their resistance to impact can be controlled, the “lost amounts of shots” was evaluated as “⊚,” i.e., very small (for Working Examples 2-1 and 2-3), and “◯,” i.e., small (for Working Example 2-2). The overall evaluations of these shots made from zinc-based alloys were “⊚,” i.e., excellent (for Working Example 2-3), and “◯,” i.e., good (for Working Examples 2-1 and 2-2).
As can be seen from the working examples, it was verified that the shots made from zinc-based alloys of the working examples of the first and second invention have both high hardness (100 HV or more) and toughness, which could not have been achieved by the conventional shots made from zinc-based alloys.
The performance in deflashing and grinding and cleaning by shot blasting as well as the ability to provide compressive residual stresses by shot peening can be significantly improved for practical uses. Further, the reduction of the cost by decreasing the lost amounts of the shots and improvement in the working environment by reducing the generation of fine particles can be simultaneously achieved. In addition, the shots contain no element that is listed in the PRTR regulations, such as Mn.
A comparison of the case where an element that is added to alloys has relatively low purity (Working Example 2-1) to the case where it has high purity (Working Example 2-3) shows that the latter case has a slightly higher Vickers hardness, a slightly higher Vickers hardness after the preliminary shooting, and a slightly higher compressive residual stress, and smaller lost amounts of the shot. Thus, a better result was achieved by the latter case.
A comparison of the case where the amount of the Fe in the shots made from zinc-based alloys is small (Working Example 2-1) to the case where it is large (Comparative Example 2-3) shows that Comparative Example 2-3, which contains much Fe has a lower Vickers hardness, a lower Vickers hardness after the preliminary shooting, and a lower compressive residual stress. The overall evaluations of it were “x.”
The compressive residual stress in Comparative Example 2-3, where the added amount of Mg is excessive (0.2%), was slightly larger than that in Working Example 2-2. However, the lost amounts of the shots were smaller. The reason for these results is estimated to be that the toughness of the shots made from zinc-based alloys decreased because of too much Mg being added.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-250356 | Oct 2009 | JP | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2010/064443 | Aug 2010 | US |
| Child | 13435525 | US |
