Patent Application
20200270731
The present invention relates to a substantially Pb-free aluminum alloy composition, and method for making said alloy composition, while achieving the machinability characteristics of their Pb-containing counterparts.
Historically Pb-containing aluminum alloys such as 2011 and 6262 (registered with the Aluminum Association in 1954 and 1960, respectively) have been used for demanding machinability applications. These applications require an alloy 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 jamming the machine tools. Aluminum alloys containing Pb met this need by providing intermetallic phases that acted as chip breakers in the material which enabled faster material removal rates, small machine chips and good machined surfaces. While Pb does provide an effective solution, it is a heavy metal and considered a hazardous material.
In an effort to reduce the adverse health effects and environmental risk these alloys may pose, alternative Pb-free aluminum alloys capable of similar machinability performance are desired. There have been several attempts at developing free machining/Pb free alloys over the years including alloys 2012, 2111, 6020 and 6040. These alloys utilized Bi and/or Sn as a substitute for Pb. While many of these alloys were successful from a machining chip size and machined surface finish perspective, many producers of thin wall, complex parts found they could not achieve the material removal rates that were attained with Pb bearing incumbent alloys because the parts had a tendency to crack. Many of these alloys were thus taken off the market or customers were cautioned to limit material removal rates for some applications. This is problematic, considering many of the applications for the Pb bearing aluminum alloys are sold through distribution channels so the end machining application was unknown to the material producer.
In an effort to avoid potential failures as a result of this crack tendency, the Pb-free alternative alloys that are still available are often restricted in their availability and often have limits placed on the machining parameters that do not achieve the same levels of performance as the Pb-containing alternatives. As a result there is still a market need for a product that meets the machinability characteristics of the Pb-containing alloys, while also meeting the strength requirements. Typically, for example, 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 provides a free machining product that achieves the same or superior machining performance in terms of high material removal rates, machining chip size and machined surface finish as their incumbent Pb-containing predecessors.
The substantially Pb-free aluminum alloy composition of the present invention is not susceptible to cracking in thin wall, complex machining under severe material removal conditions. This is a critical distinction that has not been achieved in other inventions attempting to solve the afore-mentioned technical problem. Materials that are susceptible to such cracking conditions render the machining performance irrelevant either by requiring substantially lower material removal rates or disqualifying the material altogether to ensure the integrity of the final part.
The substantially Pb-free aluminum alloy composition of the present invention substantially meets or exceeds the material property requirements of the current free machining materials. Specifically, in a preferred embodiment, the substantially Pb-free aluminum alloy composition meets the minimum material properties for AA2011-T3 including Ultimate Tensile Strength ≥45.0 KSI/311 MPa, Yield Strength ≥38.0 KSI/262 MPa, and % Elongation minimum ≥10%.
The substantially Pb-free aluminum alloy composition comprises, or consists essentially of, the following components (in weight percent): Si<0.40; Fe<0.70; Cu 5.0-6.0; Zn<0.30; Bi 0.20-0.80; Sn 0.10-0.50 with 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 (in terms of weight percent; 1.32/1 being the eutectic ratio for Bi—Sn). In addition to this, producing the material in a T8 temper provides specific advantages for machining applications that are sensitive to machining cracks because of their high material removal rates and thin wall geometries. Conversely, specific machining applications that are not sensitive to machining cracks because of more robust part geometries, but which would benefit from even higher material removal rates can be produced in a T6 temper.
The features and advantages of the present invention will become apparent from the following detailed description of a preferred embodiment thereof, taken in conjunction with the accompanying drawings, in which:
The substantially Pb-free aluminum alloy composition comprising, or consists essentially of, the following components (in weight percent): Si<0.40; Fe<0.70; Cu 5.0-6.0; Zn<0.30; Bi 0.20-0.80; Sn 0.10-0.50 with the remainder being aluminum and incidental impurities. In a preferred embodiment, Si, Fe, Cu, Zn, Bi, and Sn are the only components intentionally added to the alloy composition such that any other material exist only as incidental impurities. Said incidental impurities are 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 one embodiment, the substantially Pb-free aluminum alloy composition maintains a Bi/Sn ratio of less than 1.32/1 (in terms of weight percent; 1.32 being the eutectic ratio for Bi—Sn).
Preferably, the substantially Pb-free aluminum alloy composition of the present invention substantially meets or exceeds the material property requirements of the current free machining materials. Specifically, in a preferred embodiment, the substantially Pb-free aluminum alloy composition meets the minimum material properties for AA2011-T3 including Ultimate Tensile Strength ≥45.0 KSI/311 MPa, Yield Strength ≥38.0 KSI/262 MPa, and % Elongation minimum ≥10%.
Generally, the phrase “substantially Pb-free” is defined as having no intentional additions of Pb to the aluminum alloy composition as it is being produced. Preferably, any Pb that may be contained in the aluminum alloy composition is the result of tramp contamination. In a preferred embodiment, the aluminum alloy composition of the present invention contains <0.05 wt. % Pb. In another embodiment, the aluminum alloy composition of the present invention contains <0.01 wt. % Pb. In another preferred embodiment, the aluminum alloy composition of the present invention contains <0.005 wt. % Pb. In another preferred embodiment, the aluminum alloy composition of the present invention contains ≤0.003 wt. % Pb.
It is understood that the ranges identified above for the substantially Pb-free aluminum alloy composition include the upper or lower limits for the element selected and every numerical range and fraction provided within the range may be considered an upper or lower limit. For example, it is understood that within the range of Si<0.40, the upper or lower limit for Si may be selected from 0.30, 0.25, 0.20, 0.15, and 0.10 wt. %. In one embodiment, the amount of Si ranges from <0.20 wt. %. In another embodiment, the amount of Si ranges from <0.16 wt. %. In another embodiment, the amount of Si ranges from 0.10-0.16 wt. %. For example, it is also understood that within the range of Fe<0.70, the upper or lower limit for Fe may be selected from 0.60, 0.50, 0.40, 0.30, 0.20, and 0.10 wt. %. In one embodiment, the amount of Fe ranges from 0.30-0.50 wt. %. In another embodiment, the amount of Fe ranges from 0.33-0.44 wt. %. For example, it is also understood that within the range of Cu 5.0-6.0, the upper or lower limit for Cu may 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 5.1-5.8 wt. %. In another embodiment, the amount of Cu ranges from 5.13-5.63 wt. %. For example, it is also understood that with the range of Zn<0.30, the upper or lower limit for Zn may be selected from 0.20, 0.10, 0.05, 0.01, and 0.005 wt. % In one embodiment, the amount of Zn ranges from 0.002-0.05. In another embodiment, the amount of Zn ranges from 0.002-0.044. For example, it is also understood that within the range of Bi 0.20-0.80, the upper or lower limit for Bi may be selected from 0.30, 0.40, 0.50, 0.60, and 0.70. In one embodiment, the amount of Bi ranges from 0.40-0.80. In another embodiment, the amount of Bi ranges from 0.20-0.40. For example, it is also understood that within the range of Sn 0.10-0.50, the upper or lower limit for Sn may be selected from 0.20, 0.30, and 0.40. In one embodiment, the amount of Sn ranges from 0.20-0.50. Additionally, for example, it is also understood that within the range of Bi/Sn ratio of less than 1.32/1, the upper or lower limit for Bi/Sn ratio may be selected from 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 ration may be between 1.32/1-0.80/1. It is further understood that any and all permutations of the ranges identified above are included within the scope of the present invention. For example, the substantially Pb-free aluminum alloy composition may consist essentially of the following components (in weight percent): Si<0.15; Fe<0.50; Cu 5.1-5.7; Zn<0.05; Bi 0.40-0.80; Sn 0.20-0.50 with the remainder being aluminum and incidental impurities, while maintaining a Bi/Sn ratio of less than 1.32/1 (in terms of weight percent; 1.32/1 being the eutectic ratio for Bi—Sn) or a Bi/Sn ratio from 1.32/1 to 0.80/1, having incidental impurities 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 to this, producing the material in a T8 temper provides specific advantages for machining applications that are sensitive to machining cracks because of their high material removal rates and thin wall geometries. As such, a free machining, machining crack insensitive aluminum alloy may be produced. The aluminum alloy product has been homogenized to improve the recrystallization for improved grain size control. In a preferred embodiment, the alloy has a Bi/Sn ratio (in weight percent) of less than 1.32/1. In yet another preferred embodiment, the alloy has a Bi/Sn ratio (in weight percent) ranging from 1.32/1 to 0.8/1. In yet another preferred embodiment, the alloy has a Bi/Sn ratio (in weight percent) ranging from 1.20/1 to 1/1.
Conversely, specific machining applications that are not sensitive to machining cracks because of more robust part geometries, but which would benefit from even higher material removal rates can be produced in a T6 temper. As such, a superior free machining aluminum alloy material for applications that do not require machine crack insensitive properties may be produced. The aluminum alloy product has been homogenized to improve the recrystallization for improved grain size control. In a preferred embodiment, the alloy has a Bi/Sn ratio (in weight percent) is less than 1.32/1. In yet another preferred embodiment, the alloy has a Bi/Sn ratio (in weight percent) ranging from 1.32/1 to 0.8/1. In yet another preferred embodiment, the alloy has a Bi/Sn ratio (in weight percent) ranging from 1.20/1 to 1/1.
It is important to note that the preferred process in accordance with the present application does not include any naturally aging beyond that which is inherent in the described processes disclosed herein. Specifically, the present invention does not include any T3 or T4 naturally aging of the alloy composition.
Preferred processes for making the alloy composition of the present invention are similar to the processes described in U.S. Pat. Nos. 5,776,269 and 5,916,385, the contents of which are expressly incorporated herein by reference. In one embodiment, the alloy is initially cast into ingots and the ingots homogenized at a temperature ranging from about 900° to 1170° F. for at least 1 hour but generally not more than 24 hours, optionally followed either by fan or air cooling. In one embodiment, the ingot is soaked at about 1020° F. for about 4 hours and then cooled to room temperature. Next, in one embodiment, the ingots are cut into shorter billets, heated to a temperature ranging from about 500° to 720° F. and then extruded into a desired shape. However, it should be understood that one of ordinary skill in the art may select different times and temperatures and still remain within the scope of the present invention.
In one embodiment, the extruded alloy shapes are then thermomechanically treated to obtain the desired mechanical and physical properties. For example, to obtain the mechanical and physical properties of a T8 temper, solution heat treatment is conducted at a temperature ranging from about 930° to 1030° F., preferably at about 1000° F., for a time period ranging from about 0.5 to 2 hours, water quenched to room temperature, cold worked, and artificial aged at a temperature ranging from about 250° to 400° F. for about 2 to 12 hours. However, it should be understood that one of ordinary skill in the art may select different times, quenching conditions, and temperatures and still remain within the scope of the present invention.
In one embodiment, to obtain the properties of a T6 of T6511 temper, prior to extrusion, the billets are homogenized at a temperature ranging from about 950° to 1050° F. and then extruded to a near desired size. The rod or bar is then straightened using any known straightening operation such as stress relieved stretching of about 1 to 3%. To further improve its physical and mechanical properties, the alloy is heat treated by precipitation artificial age hardening. Generally, this may be accomplished at a temperature ranging from about 250° to 400° F. for a time period from about 2 to 12 hours. However, it should be understood that one of ordinary skill in the art may select different times, quenching conditions, and temperatures and still remain within the scope of the present invention.
The following examples illustrate various aspects of the invention and are not intended to limit the scope of the invention.
Billets were produced in 10 inch (254 mm) diameter with the target compositions found in Table 1. These billets were extruded and processed into T3, T4, T6 and T8 tempers using the process parameters shown in
Machinability testing was conducted by producing a representative part that utilizes several machining operations. This part is depicted conceptually in
In order to test that the materials were not susceptible to cracking in thin wall, severe machining applications, a severe machining test was developed. This involves drilling out the center of the 1.000″ (25.4 mm) rod using 0.969″ (24.6 mm) diameter twist drill, resulting in a 0.015″ (0.38 mm) wall thickness, as shown in
Analysis of these results indicates that alloy/temper combinations with lower yield to ultimate strength ratios perform better from a machining crack susceptibility perspective. Closer analysis of BISN-01 through BISN-04 compositions indicates that lower Bi+Sn content and lower Bi/Sn ratios are beneficial from a machining crack susceptibility perspective when taking into account the severity of the failures. The Bi/Sn ratio appears to be the stronger influence relative to the composition related performance input variables. This is illustrated in Table 3. Note that the Bi—Sn eutectic composition from a weight percent basis is at a ratio of 1.32 Bi/Sn (as shown in
Billets were cast in 10″ (254 mm) diameter and processed into 1″ (25.4 mm) rod using the process depicted in
The mechanical properties are shown in Table 5. This shows that all of the composition and temper combinations were capable of achieving the minimum 2011-T3 target mechanical properties (Yield Strength 38 KSI/262 MPa; Ultimate Strength 45.0 KSI/311 MPa; 10% Elongation). The addition of Mg was successful in achieving these properties as well in the T4 temper.
The machinability test, relative to chip size was evaluated with the results depicted in
In terms of the machining crack susceptibility test, these results are shown in
These results therefore demonstrate that by producing the material in a T8 temper, higher Bi+Sn levels can be utilized, thus achieving the superior machinability from a chip size perspective as well.
Billets were cast in 10″ (254 mm) diameter and processed into 1″ (25.4 mm) and 2″ (50.8 mm) T3 and T8 rod using the process depicted in
The mechanical properties are shown in Table 7. This shows that all of the composition and temper combinations were capable of achieving the minimum 2011-T3 target mechanical properties (Yield Strength 38 KSI/262 MPa; Ultimate Strength 45.0 KSI/311 MPa; 10% Elongation).
The machinability test relative to chip size was evaluated with the results depicted in
Machining crack susceptibility testing was also performed on the 1.000″ (25.4 mm) diameter material considering wrinkles, tears and blow-outs (per
These results confirm that for applications with severe material removal rates and part geometries with thin walls that are susceptible to tearing, processing the material in a T8 temper and maintaining Bi/Sn ratios less than 1.32 virtually eliminates this failure mechanism.
Although the present invention has been disclosed in terms of a preferred embodiment, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention as defined by the following claims:
This application is a divisional application of U.S. Ser. No. 15/640,722 filed Jul. 3, 2017, the entirety of which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 15640722 | Jul 2017 | US |
| Child | 15930768 | US |
