KR20190004235A - Substantially pb-free aluminum alloy composition - Google Patents

Abstract

A substantially Pb-free aluminum alloy consisting essentially of (wt%): Si < 0.40; Fe < 0.70; Cu 5.0 - 6.0; Zn < 0.30; Bi 0.20 - 0.80; Sn 0.10 - 0.50, and remaining aluminum and unavoidable impurities. In one aspect with respect to applications sensitive to cracking attributable to stress occurring during machining, the Bi/Sn ratio (based on wt%) is less than 1.32/1 and is manufactured on a T8 temper. In another aspect with respect to applications that are not sensitive to cracking attributable to stress during machining but can benefit from a smaller machine chip size and a more robust material removal rate, the aluminum alloy is manufactured by using a T6 temper. The substantially Pb-free aluminum alloy has mechanical properties including a maximum tensile strength of 45.0 KSI/311 MPa or more, a yield strength of 38.0 KSI/262 MPa or more, and an elongation percentage (%) of 10% or more.

Description

A substantially Pb-free aluminum alloy composition {SUBSTANTIALLY PB-FREE ALUMINUM ALLOY COMPOSITION}

The present invention relates to a substantially Pb-free aluminum alloy composition, and to a process for making said alloy composition while achieving the machinability characteristics of its Pb-containing counterpart.

Historically, Pb-containing aluminum alloys such as 2011 and 6262 (registered in the Aluminum Association in 1954 and 1960, respectively) have been used in demanding machining applications. These applications require alloys that can be machined at high material removal rates while maintaining good machined surface finishes and producing machine chips that are small and easily removed from the work area to prevent clogging of the machine tool. Pb-containing aluminum alloys have met this need by providing a metal interstice that acts as a chip breaker in materials that enable faster material removal rates, smaller mechanical chips, and better machined surfaces. Pb provides an effective solution, but it is considered a heavy metal and a hazardous material.

In an effort to reduce the negative health effects and environmental hazards that these alloys can cause, alternative Pb-free aluminum alloys are desired that can exhibit similar machinability performance. There have been several attempts to develop free-cutting / Pb-free alloys for years, including alloys 2012, 2111, 6020 and 6040. These alloys use Bi and / or Sn instead of Pb. Although many of these alloys have been successful in terms of machined chip size and machined surface finishes, many producers of complex parts of thin walls have found that the material removal rate achieved with Pb-bearing conventional alloys, as the components tend to crack, Could not be achieved. Many of these alloys have therefore been warned of consumers to exit the market or limit the rate of material removal for some applications. This is problematic considering that many of the applications for Pb-bearing aluminum alloys are sold through distribution channels, so that the final machining application is unknown to the material producer.

To avoid potential failures resulting from such cracking tendencies, the Pb-free alternative alloys that are still available are often limited in their solubility and often have limitations in machining parameters that do not achieve the same level of performance as the Pb- . As a result, there is still a market demand for products that meet the machining characteristics of Pb-containing alloys while meeting strength requirements. Typically, for example, the Pb-containing alloy 2011-T3 has a minimum yield strength of 38 KSI / 262 MPa.

The substantially Pb-free aluminum alloy composition of the present invention is free machining to achieve the same or superior as conventional Pb-containing prior compositions, high material removal rates, machined chip size and machined surface finish ) &Lt; / RTI &gt; product.

The substantially Pb-free aluminum alloy composition of the present invention does not cause cracking to thin walls under severe material removal conditions and is not susceptible to complicated machining. This is an important difference that has not been achieved in other inventions attempting to solve the above mentioned technical problems. Substances sensitive to these cracking conditions make the material removal rate considerably low to ensure the integrity of the final part or irrelevant to the machining performance by rejecting all of the material.

The substantially Pb-free aluminum alloy composition of the present invention substantially meets or exceeds the material property requirements of current free-cutting materials. Specifically, in a preferred embodiment, the substantially Pb-free aluminum alloy composition has an Ultimate Tensile Strength of at least 45.0 KSI / 311 MPa, a yield strength of at least 38.0 KSI / 262 MPa, and an elongation minimum of at least 10% ) (%). &Lt; / RTI &gt;

The substantially Pb-free aluminum alloy composition comprises or consists essentially of the following components (wt.%): Si <0.40; Fe <0.70; Cu 5.0 - 6.0; Zn <0.30; Bi 0.20 - 0.80; Sn 0.10 - 0.50, the remainder being aluminum and incidental impurities. In a preferred embodiment, the substantially Pb-free aluminum alloy composition maintains a Bi / Sn ratio of less than 1.32 / 1 (based on weight percent; 1.32 / 1 is the eutectic ratio of Bi-Sn) The fabrication of materials at T8 tempering offers particular advantages in machining applications that are sensitive to machining cracks due to their high material removal rate and thin wall geometry. Conversely, certain machining applications that are not susceptible to machining cracks due to more robust part geometry, but that can benefit from much higher material removal rates, can be produced on a T6 temper.

BRIEF DESCRIPTION OF THE DRAWINGS The features and advantages of the present invention will become apparent from the following detailed description of preferred embodiments thereof, together with the accompanying drawings, in which:
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram showing the sequence of operations of a substantially Pb-free aluminum alloy composition produced in various embodiments in accordance with the present invention; FIG.
Figure 2 is a conceptual drawing of a representative part used to evaluate machinability in terms of chip size of a substantially Pb-free aluminum alloy composition according to the present invention;
3 is a graph showing the machinability of the alloy / temper combination evaluated in Example 1, measured in chips / gram;
4 is a conceptual view of a machined crack susceptibility test part;
Figure 5 shows an observation photograph taken from a mechanical crack susceptibility test showing four classes used;
6 is a graph showing the results of the machining crack susceptibility test of Example 1 measured in% without tear or blowout;
Figure 7 is a graph showing the machinability results of Example 2 measured in chips / gram;
8 is a graph showing the results of the machining crack susceptibility test of Example 2 measured in%, without wrinkles, tears or blowouts;
9 is a graph showing the machinability results of Example 3 measured in chips / gram;
10 is a graph showing the machinability results of Example 3 measured in chips per gram for a 2.000 " diameter rod;
11 is a Bi-Sn phase balance diagram.

The substantially Pb-free aluminum alloy composition comprises or consists essentially of the following components (wt.%): Si <0.40; Fe <0.70; Cu 5.0 - 6.0; Zn <0.30; Bi 0.20 - 0.80; Sn 0.10 - 0.50, the remainder being aluminum and incidental impurities. In a preferred embodiment, Si, Fe, Cu, Zn, Bi, and Sn are the only components that are intentionally added to the alloy composition such that any other material is present only as an incidental impurity. The ancillary impurities are present in a total amount of less than 1 wt%, less than 0.5 wt%, or less than 0.1 wt%, or less than 0.05 wt%. In one embodiment, the substantially Pb-free aluminum alloy composition maintains a Bi / Sn ratio (based on weight%; 1.32 is a eutectic ratio of Bi-Sn) of less than 1.32 / 1.

Preferably, the substantially Pb-free aluminum alloy composition of the present invention substantially meets or exceeds the material property requirements of the current free-running material. Specifically, in a preferred embodiment, the substantially Pb-free aluminum alloy composition has a maximum tensile strength of at least 45.0 KSI / 311 MPa, a yield strength of at least 38.0 KSI / 262 MPa, and a minimum elongation percentage of at least 10% 0.0 &gt; T3. &Lt; / RTI &gt;

In general, the phrase " substantially Pb-free &quot; is defined as not intentionally adding Pb to the aluminum alloy composition when the aluminum alloy composition is prepared. Preferably, any Pb In another embodiment, the aluminum alloy composition of the present invention comprises less than 0.01% Pb by weight of Pb in an amount of less than &lt; RTI ID = 0.0 &gt; In another preferred embodiment, the aluminum alloy composition of the present invention contains less than 0.005 wt% Pb. In another preferred embodiment, the aluminum alloy composition of the present invention contains less than 0.003 wt% of Pb.

It is understood that the above-identified ranges for a substantially Pb-free aluminum alloy composition include an upper limit or a lower limit for the selected element, and all numerical ranges and fractions provided within the range can be regarded as an upper limit or a lower limit. For example, it is understood that within the range of Si < 0.40, the upper or lower limit of Si can be selected from 0.30, 0.25, 0.20, 0.15, and 0.10 wt%. In one embodiment, the amount of Si is in the range of less than 0.20% by weight. In another embodiment, the amount of Si is in the range of less than 0.16 wt%. In another embodiment, the amount of Si ranges from 0.10 to 0.16 wt%. For example, it is understood that within the range of Fe < 0.70, the upper or lower limit of Fe can be selected from 0.60, 0.50, 0.40, 0.30, 0.20, and 0.10 wt%. In one embodiment, the amount of Fe is in the range of 0.30 to 0.50 wt%. In another embodiment, the amount of Fe ranges from 0.33 to 0.44 wt%. For example, it is also understood that within the range of Cu 5.0 to 6.0, the upper or lower limit of Cu can be selected from 5.1, 5.2, 5.3, 5.4, 5.5, 5.6, 5.7, 5.8 and 5.9. In one embodiment, the amount of Cu ranges from 5.1 to 5.8 wt%. In another embodiment, the amount of Cu ranges from 5.13 to 5.63 wt%. For example, it is understood that, in the range of Zn < 0.30, the upper or lower limit of Zn can be selected from 0.20, 0.10, 0.05, 0.01, and 0.005 wt%. In one embodiment, the amount of Zn ranges from 0.002 to 0.05. In another embodiment, the amount of Zn ranges from 0.002 to 0.044. For example, it is also understood that within the range of Bi 0.20 to 0.80, the upper or lower limit of Bi can be selected from 0.30, 0.40, 0.50, 0.60, and 0.70. In one embodiment, the amount of Bi is in the range of 0.40 to 0.80. In another embodiment, the amount of Bi is in the range of 0.20 to 0.40. For example, it is also understood that within the range of Sn 0.10 to 0.50, the upper or lower limit of Sn can be selected from 0.20, 0.30, and 0.40. In one embodiment, the amount of Sn is in the range of 0.20 to 0.50. Further, for example, the upper limit or lower limit of the Bi / Sn ratio is 1.30 / 1, 1.25 / 1, 1.20 / 1, 1.15 / 1, 1.10 / 1, 1.05 / 1, 1.00 / 1, and 0.80 / 1. In one embodiment, the Bi / Sn ratio can be 1.32 / 1 to 0.80 / 1. It is further understood that any and all permutations of the identified ranges are included within the scope of the present invention. For example, a substantially Pb-free aluminum alloy composition consists essentially of the following components (wt%): Si <0.15; Fe <0.50; Cu 5.1-5.7; Zn <0.05; Bi 0.40 - 0.80; Sn of 0.20-0.50 (the remainder being aluminum and incidental impurities), and at the same time a Bi / Sn ratio of 1.32 / 1 or less (based on weight%, 1.32 / 1 being the eutectic ratio of Bi-Sn) A Bi / Sn ratio of 0.80 / 1 and minor impurities may be present in a total amount of less than 1 wt%, or less than 0.5 wt%, or less than 0.1 wt%, or less than 0.05 wt%.

In addition, manufacturing materials at T8 tempering offers particular advantages in machining applications that are sensitive to machining cracks due to their high material removal rate and thin wall geometry. Thus, aluminum alloys which are not susceptible to free cutting and machining cracks can be produced. The aluminum alloy product was homogenized for improved particle size control to improve recrystallization. In a preferred embodiment, the alloy has a Bi / Sn ratio (wt%) of less than 1.32 / 1. In another preferred embodiment, the alloy has a Bi / Sn ratio (wt%) in the range of 1.32 / 1 to 0.8 / 1. In another preferred embodiment, the alloy has a Bi / Sn ratio (wt%) in the range of 1.20 / 1 to 1/1.

Conversely, certain machining applications that are not susceptible to machining cracks due to more robust part geometry, but that can benefit from much higher material removal rates, can be produced on a T6 temper. Thus, excellent free-cutting aluminum alloy materials can be produced for applications that do not require properties that are not susceptible to mechanical cracks. The aluminum alloy product was homogenized for improved particle size control to improve recrystallization. In a preferred embodiment, the alloy has a Bi / Sn ratio (wt%) of less than 1.32 / 1. In another preferred embodiment, the alloy has a Bi / Sn ratio (wt%) in the range of 1.32 / 1 to 0.8 / 1. In another preferred embodiment, the alloy has a Bi / Sn ratio (wt%) in the range of 1.20 / 1 to 1/1.

It is important to note that the preferred method according to the present application does not involve any natural aging inherent to the processes described herein. Specifically, the present invention does not include any T3 or T4 natural aging of the alloy composition.

Preferred methods of making the alloy compositions of the present invention are similar to those described in U.S. Patent 5,776,269 and U.S. Patent 5,916,385, the disclosures of which are expressly incorporated herein by reference. In one embodiment, the alloy is initially cast as an ingot, the ingot is homogenized for at least one hour at a temperature in the range of about 900 [deg.] To about 1170 [deg.] F but typically less than 24 hours, optionally followed by a fan or air cooling . In one embodiment, the ingot is immersed at about 1020 ° for about 4 hours and then cooled to room temperature. Next, in one embodiment, the ingot is cut into shorter billets, heated to a temperature in the range of about 500 [deg.] To 720 [deg.] F and then extruded to the desired shape. However, those skilled in the art will appreciate that different times and temperatures may be selected and still be within the scope of the present invention.

In one embodiment, the extruded alloy shape is then thermomechanically processed to obtain the desired mechanical and physical properties. For example, to obtain the mechanical and physical properties of a T8 temper, the solution heat treatment may be performed at a temperature in the range of about 930 ° to 1030 ° F., preferably at about 1000 ° F. for about 0.5 to 2 hours , Water quenched to room temperature, cold worked and artificially aged at a temperature in the range of about 250 [deg.] To 400 [deg.] F for about 2 to 12 hours. However, those skilled in the art will appreciate that different times, quenching conditions, and temperatures can be selected and still fall within the scope of the present invention.

In one embodiment, prior to extrusion, the billet is homogenized at a temperature in the range of 950 ° to 1050 ° F and then extruded to near-desired size to obtain T6 properties of the T6511 temper. The rod or bar is then calibrated using any known straightening operation, such as about 1 to 3% stress relaxation stretching. To further improve its physical and mechanical properties, the alloy is heat treated by precipitation artificial age hardening. Generally, this can be achieved at a temperature in the range of about 250 [deg.] To 400 [deg.] F for a period of about 2 to 12 hours. However, those skilled in the art will appreciate that different times, quenching conditions, and temperatures can be selected and still fall within the scope of the present invention.

The following examples illustrate various aspects of the present invention and are not intended to limit the scope of the present invention.

Example 1:

Billets were produced with a 10 inch (254 mm) diameter with the target composition found in Table 1. These billets were extruded and processed into T3, T4, T6 and T8 temperers using the process parameters shown in FIG. 1 to produce a 1.000 inch (25.4 mm) diameter rod. Casting of the billets was performed using conventional direct cooling casting techniques. The 6040 alloy deformation was produced in both press quenching (T6511 tempering) and separate solution heat treating (T651 tempering) processes. Homogenization, extrusion, solution heat treatment, quenching, drawing and artificial aging work were all completed using typical industrial practices. Samples from this material were evaluated for tensile properties and machinability. The results of the tensile properties are shown in Table 2. The mechanical property limits of the 2011-T3 were used as the minimum acceptance criteria. These results show that all materials except the BISN-31- T451 material pass the aluminum-related minimum properties (yield strength 38.0 KSI / 262 MPa; maximum strength 45.0 KSI / 311 MPa; 10% elongation) for 2011-T3 .

Machinability tests were carried out by manufacturing representative parts using several machining operations. This part is conceptually shown in Fig. The material removal rate was kept constant between the materials by keeping the cutting speed and feed rate constant in all machining operations. The chip size is estimated by determining the number of clean dry chips per gram. The results of this evaluation are shown in FIG. 3 and are compared with the current Pb-containing free-cutting material, 2011-T3, as a benchmark comparison. This shows that the alloy / temper combination tested is superior or similar to the conventional one. Also commercially available Pb-free 6040 compositions currently on the market have also been tested in this matrix. This has historically not been performed as well as the 2011-T3, and this test has proved its inferior performance.

To test if the material is not sensitive to cracking in thin walls, severe machining applications, severe machining tests have been developed. This involves drilling the center of a 1.000 " (25.4 mm) rod using a 0.969 " (24.6 mm) diameter twist drill to produce a wall thickness of 0.015 " (0.38 mm), as shown in Fig. RPM and feed rate were kept constant at 1500 RPM and 0.035 " (1.27 mm) / rev feed rate. When this test was completed, the test piece was inspected under the conditions shown in Fig. This test was developed to test the susceptibility of a material to cracking under extreme machining conditions with thin walls, high material removal rates and high torque. This test was repeated at least 12 times for each material tested with acceptable performance in terms of chip size and material properties. The percentage of parts with tear (or crack) and blowout was recorded, and the results are shown in FIG. BISN-31 is designated in this figure as a different temper (T3, T4 and T8) for simplicity. This shows that 2011 (conventional Pb-containing alloys) consistently passed as expected, as did Pb-free 6040 alloy variants (although these alloy variations were not well performed in terms of chip size). The only experimental alloys that passed were BISN-31-T4, but unfortunately this did not pass the tensile character requirements.

Analysis of these results shows that alloy / tempering combinations with low yield strength versus maximum strength are performed excellently in terms of machining crack susceptibility. A more detailed analysis of the BISN-01 to BISN-04 compositions shows that lower Bi + Sn content and lower Bi / Sn ratio are advantageous in view of machining crack susceptibility, given the severity of failure. The Bi / Sn ratio appears to have a stronger effect than the composition-related performance input parameter. This is illustrated in Table 3. On a weight percent basis, the Bi-Sn eutectic composition has a Bi / Sn ratio of 1.32 (shown in FIG. 11).

Example 2:

The billets were cast to 10 " (254 mm) diameter using the process shown in Figure 1 and the compositions listed in Table 4 and machined into 1 " (25.4 mm) rods. % ROA (area reduction) during withdrawal was evaluated in this study, especially at T3 temper. The homogenization effect was also evaluated by comparing the homogenized cast 1110 with the homogenized cast 1108. A 1 " (25.4 mm) rod was evaluated for mechanical properties, machinability, and machined crack susceptibility using the same technique described in Example 1. [

The mechanical properties are shown in Table 5. This indicates that both the composition and the tempering combination could achieve the minimum 2011-T3 target mechanical properties (yield strength 38 KSI / 262 MPa; maximum strength 45.0 KSI / 311 MPa; 10% elongation). Adding Mg has also been successful in achieving this property in T4 tempering.

The machinability test on chip size was evaluated, and the results are shown in FIG. These results show that the higher Bi + Sn composition (BI39) is performed excellently in terms of machinability, as measured by chip / gram, and is performed as good or better than the existing 2011-T3. The lower Bi + Sn composition (BI26) was generally not as well performed as the conventional 2011-T3, but was similar. It also shows that there is little difference in machinability with respect to the percentage reduction area of the T3 temper, regardless of the Bi + Sn level. The addition of homogenization did not improve machinability, but the examination of the particle structure showed a significant improvement over the surrounding coarse particles (recrystallized particle size at the outer periphery of the rod). Thus, the use of homogenization is not necessary for machinability but may be advantageous in some applications (for example, those that require anodizing) that require improved surface appearance. The T651 temper material was very well performed with small chip size, regardless of alloy composition. The T8 tempering was generally performed better than the T3 counterpart for certain alloys, especially the BI26 composition.

With respect to the machined crack susceptibility test, these results are shown in Fig. 8, in which case wrinkles on the surface (according to Fig. 5) were also considered unacceptable. These results show that while composition BI26 is performed much better than BI39 (confirming that the higher the Bi + Sn is, the more sensitive the material is to machining cracks), the tempering has a much stronger effect. All compositions in this example exhibited Bi / Sn ratios of less than 1.32. The T8 temper, regardless of composition, did not exhibit cracking in this test, but the T6 sample was performed very poorly. The T3 temperers all exhibited some failures and the higher Bi + Sn containing material had a much higher failure rate. The BI26-T3 composition did not exhibit failure with respect to tearing or blow-out according to FIG. 5, and thus Bi + Sn has a significant effect on performance.

Thus, this result demonstrates that by manufacturing the material at the T8 temper, higher Bi + Sn levels can be used and thus superior machinability can also be achieved in terms of chip size.

Example 3:

The billets were cast to 10 " (254 mm) diameter using the process shown in Figure 1 and the compositions listed in Table 6 and machined into 1 " (25.4 mm) and 2 & The rods were evaluated for mechanical properties, machinability, and machined crack susceptibility using the same techniques described in Example 1. [

Mechanical properties are shown in Table 7. This indicates that both the composition and the tempering combination could achieve the minimum 2011-T3 target mechanical properties (yield strength 38 KSI / 262 MPa; maximum strength 45.0 KSI / 311 MPa; 10% elongation).

The machinability test for chip size was evaluated for a 1.000 " (25.4 mm) diameter material, the results of which are shown in Figure 9. The results show that T8 performed better than the Pb-containing 2011 material, The T3 material performed to the Pb-containing 2011 material was not as good as the Pb-containing 2011 material. The test was repeated with a 2.000 " (50.8 mm) diameter to ensure that the material was well machined over a wider range of diameters. It should be noted that the 2.000 "(50.8 mm) diameter result in this test was slightly worse than the Pb-containing 2011 existing material, but better than any of the 1,000" (25.4 mm) diameter test results on a chip-by-gram basis. Thus, it can be concluded that the material performs well in this diameter range.

The machined crack susceptibility test was also performed on materials with a diameter of 1.000 "(25.4 mm), with wrinkles, tears and blow-out (according to Figure 5) considered unsuccessful.

These results show that for applications with severe material removal rates and thin-walled part geometry sensitive to phosphorus, the material is processed at the T8 temper and the failure mechanism is virtually eliminated by keeping the Bi / Sn ratio below 1.32 .

Although the present invention has been disclosed with respect to preferred embodiments, it will be understood that many further modifications and variations can be made without departing from the scope of the invention as defined by the following claims.

Claims (15)

As a substantially Pb-free aluminum alloy composition,
The weight percent of the aluminum alloy composition being Pb0-0.10; Si 0-0.40; Fe 0-0.70; Cu 5.0-6.0; Zn 0 - 0.30; Bi 0.20-0.80; Sn 0.10-0.50; Except for incidental impurities, the remainder includes aluminum,
The alloy composition has a Bi / 1 Sn weight ratio of less than 1.32 / 1,
The alloy composition prepared using only T8 or T6 temper to provide alloy compositions has an Ultimate Tensile Strength of at least 45.0 KSI / 311 MPa, a yield strength of at least 38.0 KSI / 262 MPa, and a yield strength of at least 10% And a minimum elongation percentage (%). The composition of claim 1, wherein the aluminum alloy composition has less than 0.05 wt% Pb. The composition of claim 1, wherein the aluminum alloy composition comprises 0.10 to 0.16% Si by weight. The composition of claim 1, wherein the aluminum alloy composition comprises 0.30 to 0.50 wt% Fe. The composition of claim 1, wherein the aluminum alloy composition comprises 5.1 to 5.8 wt% Cu. The composition of claim 1, wherein the aluminum alloy composition comprises 0.002 to 0.05 wt% Zn. The composition of claim 1, wherein the aluminum alloy composition comprises from 0.20 to 0.40 wt% Bi. The composition of claim 1, wherein the aluminum alloy composition comprises 0.20 to 0.50 wt% Sn. The composition of claim 1 wherein the aluminum alloy composition comprises and optionally consists of the following components:% (weight / weight) of the aluminum alloy composition:
Si 0-0.16; Fe 0-0.50; Cu 5.1-5.8; Zn 0 - 0.05; Bi 0.20 - 0.40; And Sn 0.20 - 0.50. The composition of claim 1, wherein the aluminum alloy composition has a Bi / Sn weight ratio in the range of 1.32 / 1 to 0.8 / 1. 2. The composition of claim 1, wherein the minor impurity is present in a total amount of less than 0.5% by weight. The composition of claim 1, wherein the preparation comprises only a T8 temper. The composition of claim 1, wherein the aluminum alloy composition is free of T3 or T4 tempering. As a method for forming an aluminum alloy,
a. Casting an alloy billet of the aluminum alloy composition of claim 1,
b. Homogenizing the optionally cast billet;
c. Extruding the cast billet to form an extrudate having a profile shape;
d. Heating the extrudate to an immersion temperature of 900 to 1060 ° F (482 to 571 ° C) to solution heat treatment, and quenching the immersion temperature to room temperature;
e. Cold-working the extrudate after step d) with a minimum 5% reduction in cross-sectional area through drawing, stretching or rolling; And
f. The extrudates of step e) are artificially aged in T8 or T6 tempering to a peak hardness to obtain a maximum tensile strength of 45.0 KSI / 311 MPa or higher, a yield strength of 38.0 KSI / 262 MPa or higher, and a minimum elongation (% &Lt; / RTI &gt; 15. The method of claim 14,
The step of homogenizing the cast billet takes place over a period of at least 1 hour at a temperature in the range of 900 to 1050 F;
Wherein the step of heating the extrudate to a temperature of 900 to 1060 占 ((482 to 571 占 폚) and performing a solution heat treatment occurs for 0.5 to 2 hours.

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