JPS6256220B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an aluminum alloy for casting for automobile disc wheels, and more specifically for passenger car disc wheels (hereinafter simply referred to as wheels).
Al is used in an age-hardened state after casting.
Regarding 7%Si-0.3%Mg alloy. Automobile wheels are usually made of Al-7%Si-0.3%
A Mg-based alloy is cast by normal gravity casting, solution treated, and then aged hardened at a temperature of approximately 150°C before use. FIG. 1 typically shows the shape of an aluminum wheel. This Al―7%Si―
0.3%Mg alloy is lightweight and has good castability, and has a beautiful finished surface after machining such as cutting.
In addition, because it is an alloy with excellent mechanical properties, especially elongation impact value, thick alloys are used as mainstream wheel materials in many countries.In Japan, they are specified as AC-4CH of JISH5202-1982 as shown in Table 1. ing.

[Table] In the casting structure of this alloy, iron compounds are crystallized as acicular coarse crystals together with eutectic (α+Si) at the grain boundaries of the primary solid solution α on the aluminum side of the matrix. Due to the solution treatment carried out after casting, the silicon in the eutectic changes from acicular to granular and aggregates, but the iron compound hardly changes its shape and remains as coarse acicular crystals. . Although the aluminum matrix is strengthened by aging treatment after solution treatment, the acicular coarse crystals of the iron compound have a notch effect on the aluminum matrix of the strengthened alloy, which deteriorates the mechanical properties of the alloy as a whole, especially
Reduces elongation and impact value. To date, measures to eliminate the negative effects caused by the crystallization of iron compounds include limiting the iron content in aluminum ingots used during melting, with the intention of reducing the amount of iron compounds themselves present.
Many wheels in Japan depend on using high purity metal with Fe 0.15% or less. Manufacturer is
The standard for adoption is an aluminum base metal with Fe of 0.15% or less, usually around 0.13-0.14%. In view of the increase in the production of automobiles, especially passenger cars, the soaring price of aluminum ingots, and the effective use of aluminum scrap, Al99, which is widely produced as an industrial ingot, is being used instead of high-purity aluminum ingots as mentioned above. Uses aluminum base metal of about 5 to 99.7%, and Fe 0.15%, which is currently in use.
Although there is a demand for the development of a wheel with mechanical properties equal to or better than those made of the following high-purity metals, no wheel has been developed at present. An object of the present invention is to develop a new alloy that can reduce the adverse effects of coarse acicular crystals of iron compounds in the aluminum casting alloy for wheels. Accordingly, an object of the present invention is to provide an aluminum alloy for casting that uses an aluminum base metal of approximately 99.5-99.7% Al and has sufficient strength as an automobile foil material. Another object of the present invention is to provide an aluminum alloy in which the types of additive alloying elements, their effective ingredient ranges, and appropriate heat treatment conditions are clarified to suppress the adverse effects of iron introduced from compound materials such as aluminum ingots. It is to be. The inventors of the present invention used a base metal of about 99.5-99.7% Al in an Al-7%Si-0.3Mg alloy, and by adding new alloying elements, the notch due to the coarse needle-like crystals of iron was removed. A test study was conducted with the expectation that the negative effects of the drug's effects could be eliminated. As samples prepared for test research, Al
Si and Mg, which are the basic components of AC-4CH alloy represented by -7%Si-0.3%Mg, will remain unchanged as a blend composition, and Fe will be specified at 0.13%, which satisfies the current requirement of 0.2% or less. 0.35% above the upper limit of
0.55%, and further improves Mn and Sr.
By adding these two types singly or in combination, and setting the tolerance limits for elements other than the above, such as Cu, Zn, Ni, Ti, Pb, Sn, and Cr, to be the same as the tolerance limit for JIS AC-4CH alloy, We decided to determine the upper limit of Fe that is permissible in automobile wheel materials without deteriorating mechanical properties, and at the same time determine the effective range of Sr and Mn addition amounts. The details of the test will be described later as an example.
To give an overview, 18 types of samples shown by the composition in Table 2 were dissolved,
After casting in a JIS No. 4 heating mold and performing prescribed solution treatment and aging treatment, the tensile strength, yield strength, elongation,
Mechanical properties were determined by measuring impact values. In addition, the microstructures of the ingot samples of each composition and the impact test specimens subjected to solution treatment and aging treatment and the microstructures of the fractured portions of the tensile test specimens were observed using an optical microscope. This is to examine how changes in mechanical properties and microstructure are related to changes in Fe, Mn, and Sr contents. As a result of examining these mechanical properties and microstructures, up to 0.35% of Fe is Sr and or Mn.
Good results can be obtained by adding Fe, but
It was found that tensile strength and yield strength decrease when the content exceeds 0.35% and reaches 0.55% regardless of the addition of the improving elements Mn and/or Sr. Regarding impact value and elongation, it was also observed that when Fe exceeds 0.35%, toughness decreases regardless of the addition of Mn and/or Sr. In addition, Sr is 0.02% higher than when Mn is added alone or when Mn and Sr are combined.
It was found that adding it alone was more effective. From these results, by adding 0.02% Sr alone, even if Fe as an impurity exceeds the conventional upper limit of 0.15% and is present up to 0.35%, strontium (Sr) can be reduced to 0.02% in the alloy. It has been demonstrated that when added, it can be used as a casting alloy for automobile wheels without any practical problems by eliminating the negative effects of the notch effect caused by the formation of coarse acicular grains caused by iron compounds. It is. The results of the test and research on the aluminum casting alloy for automobile wheels according to the present invention will be explained below as an example. Example: (1) Selection and melting of sample Form an iron compound that crystallizes as coarse needle-like crystals
In order to determine how far the upper limit of the Fe content can be allowed by adding Mn and Sr, which are additive elements to eliminate its negative effects, either singly or in combination, the Fe content was increased from the current 0.13. %
and an increase of 0.35% and 0.55%3.
Seed, Mn is 0.3% and 0.5%, Sr is 0.02
%, and the composition of the sample was selected as shown in Table 2. Each sample was prepared to have the compounding composition shown in Table 2. The compounding materials and their melting method are shown below.

【table】

[Table] AC that has been refined in advance with metallic titanium.
4CH alloy (Sr6.8%-Be0.13%-Mg0.38%-
After melting 4.5 kg of Ti (0.13% remaining Al), adding a Mn master alloy consisting of 16.7% Mn (16.7% remaining Al), and a predetermined amount of iron plate rolled into flakes, the temperature of the molten metal was maintained at 740°C until it melted. Then, after degassing with hexachloroethane (CCl ₆ ) to thoroughly remove the oxides floating on the surface, the strontium (Sr) mother alloy (Si13.12%-
After adding Sr9.15% - Be1.05% residual Al) and holding it at 740℃ for 30 minutes, it was cast into a No. 4 test mold preheated to 150℃.Sr has a low yield in the presence of chlorides. It could get worse, so I was very careful about that. The chemical components of the obtained samples are shown in Table 3.

【table】

[Table] After collecting samples for microscopic structure observation shown in the attached table, these test pieces were subjected to quenching and tempering treatments for 90 tensile tests and 90 impact tests. (2) Heat treatment After solution treatment of the above sample at 535℃ for 9 hours, 15℃
Quenching was carried out in water. Immediately an aging treatment was performed by tempering at 155°C for 5 hours, and a tensile test and an impact test were conducted. The microstructure after heat treatment was determined by cutting and polishing the test piece after the impact test, and the fractured surface was also observed using an optical microscope and partly an electron microscope. (3) Test method For the tensile test, an Instron tensile tester was used. The capacity of the testing machine is 25 tons and the tensile speed is 5
mm/min, and the chart feeding time is 50mm/min.
A load-elongation chart was drawn, and the tensile strength, yield strength, and elongation were determined from this chart. The impact test was conducted using a testing machine for non-ferrous metals. In addition, the impact value was determined from the following formula. E=WR (cosβ−cosα) E: Energy required for fracture W: Weight of hammer R: Distance from center of rotation axis of hammer to center of gravity α: Lifting angle of hammer β: Lifting angle of hammer after specimen fracture The value obtained by dividing E by the initial cross-sectional area (0.8 cm ² ) (Kg·1 cm ² ) was defined as the impact value. (4) Test results and considerations (4)-1 Results of tensile test and impact test Table 4 shows the tensile strength, 0.2% proof stress, elongation, and impact value of each alloy sample after the tensile test and impact test. These results are also illustrated in Figures 2-5.

【table】

[Table] The numerical values for each measurement item in Table 4 are the average values of five actual measurements, and as an example, Table 5 shows the results of each five measurements of tensile strength.

[Table] Figure 2-5 shows the amounts of Fe in the 18 samples No. 1-18 in Table 2, plotted on the X-axis at 0.15%, 0.35%, and 0.55%, and the measured values on the Y-axis. This is a graph with six types of symbols such as 〇, ●, △, ▲..., and each symbol indicates the Mn in the compounded ingredients.
and Sr are the same and Fe is 0.15%, 0.35%, and 0.55
% to represent three types of content. The relationship between each symbol and its representative sample number is shown below for convenient reference of the relationship between the trends shown in these graphs, the ingredients in Table 2, and the analytical values in Table 3. .

[Table] Based on these graphs, we considered the relationship between mechanical properties, Fe content, and added Mn and Sr. Below, tensile strength, yield strength, elongation, and impact value will be examined in this order. (a) Tensile strength The tensile strength shown in Figure 2 is 0.15%Fe, 0.02%Sr.
(Sample No. 10) showed the highest value of 31.20 Kg/mm ² . Furthermore, in this sample, the difference between the maximum and minimum strength values is approximately 2 kg/mm ² . When considering Fe content
There is little change up to 0.35% Fe, but the strength decreases at 0.35% Fe. However, 0.5%Mn, 0.02%Sr
Samples with varying amounts of Fe (No. 12, 15, 18)
0.35%Fe (No. 15) shows a value similar to the others, but 0.15%Fe and 0.55%Fe have lower values than the others. In addition, for those with an increased amount of Fe, it is better to add Mn or Sr alone, and in particular, those with Sr added alone show a low value. However, in JIS (H5202), AC4CH-
Since the tensile strength at T6 is specified to be 25 kg/mm ² or more, it can be recognized that all of these exhibit good values that satisfy the JIS standard in this regard. (b) Yield strength The 0.2% yield strength shown in Figure 3 is 0.35%Fe, 0.02%
Sr and 0.5%Mn (sample No. 15) show the highest values. When considering the amount of Fe, those without Sr are Fe.
Although the tensile strength did not change much even if the amount increased, the samples with added Sr (No. 10, 13; 11, 14;
12, 15) improved with 0.35% Fe and decreased with 0.55% Fe (Nos. 16, 17, 18) as the amount of Fe increased. Due to the above, the yield strength is 0.35%
For Fe, it seems possible to obtain high yield strength by adding Mn and Sr in combination. (c) Elongation The elongation shown in Figure 4 is 0.15%Fe, 0%Mn,
The alloy sample with 0.02% Sr (sample No. 10...symbol ●) has the highest value, showing 13.32%. In this sample, the amount of Fe decreased rapidly as the amount increased, and 0.55% Fe (sample no.
16) shows the lowest value of 6.57%. In other samples, there is little change in Fe amount up to 0.35%, but 0.55%Fe
When it comes to , all samples are decreasing. Next, when considering the amount of Mn, 0%Mn, 0.02%Sr (sample No. 10) is the best for 0.15%Fe, but those with Mn added (symbols △, ▲, □, ■) and those with Sr added Those that do not (symbols 〇, △, □) indicate low values. This trend is the same for 0.35%Fe, but for 0.55%Fe
Added 0.3% Mn and 0.02% Sr (symbol ▲, sample
No. 17) showed a high value, and with this amount of Fe, 0.3%Mn,
The one with 0.02% Sr added is good. (d) Impact value The impact value shown in Figure 5 is 0.15%Fe, 0%
Mn and 0.02% Sr (symbol ●, sample No. 10) show the highest values, and with these Mn and Sr contents, as the Fe amount increases (symbol ●, sample No. 13.16), it rapidly decreases. In addition, when Mn was added, the value was lower than the 0%Mn and 0.02%Sr indicated by the symbol ●, regardless of the amount of Fe, and no improvement effect on Fe was observed even when Mn was added. (e) Consideration First, tensile strength is determined by dispersion strengthening by Fe compounds or solid solution strengthening by solid solution of Fe in the matrix up to 0.35% Fe, where scattered Fe needles crystallize. It seems that there is a slight improvement due to
It is thought that at 0.55% Fe, where needle crystals are greatly developed and crystallized, the notch effect becomes significant and the strength decreases. In addition, as the amount of Fe increases, the elongation and impact values are greatly affected by the notch effect caused by the Fe compound, which seems to reduce the elongation and impact values. Next, the elongation for Mn is 0.55% Fe and 0.3% Mn,
There is a slight improvement in the one with 0.02% Sr added. This seems to be because the shape of the Fe compound changed due to Mn. The addition of Sr had a large effect on elongation and impact value, and was particularly effective and favorable for those with small amounts of Fe and Mn. This is because the eutectic Si is made finer by adding Sr, and from these results, while the addition of Sr alone produces a remarkable improvement effect, adding Sr and Mn in combination does not produce as much as expected. It was shown that the effect of
We decided not to add Mn as an improvement element.
However, from the test results, it was found that presence of about 0.3% does not have such an effect as to deteriorate the characteristics, and may even show good results. Therefore, when adding Sr as in the present invention, it is necessary to avoid impurities. Mn as
It was found that there is no need to limit the content to 0.1% or less as in the JIS AC-4CH alloy, but it is sufficient to limit the content to 0.3% or less. (4)-2 Microstructure observation Results of microstructural observation using an optical microscope of cast alloy samples and alloy samples that were solution treated at 535℃ for 9 hours, water quenched in 15℃ water, and aged at 155℃ for 5 hours. are shown in Figures 6-10 and 11-14, respectively. First of all, cast materials that do not contain Sr are No. 6-8.
As shown in the figure, even when Fe is as low as 0.15%, eutectic Si
The crystals are irregular and vary in size, and the crystal grains become smaller when Fe is 0.35% (Figure 7) and 0.55% (Figure 8), but eutectic Si is precipitated in needle shapes. I understand that. In addition, those with Sr added are
As shown in Figure 10, it can be seen that the eutectic Si becomes finer due to Sr. Next, as shown in Figure 11, in the structure of the heat-treated alloy sample without the addition of Sr, the eutectic Si is finer than in the cast structure, and even in the case of a relatively large Si, the tip is rounded. It can be seen that crystallization is occurring. In addition, the one with Sr added is the 12th,
As shown in Figure 13, the size of the eutectic Si crystal grains does not change much, but it becomes more spherical.
It is finer than the one without Sr added. Then Fe
Observing the compound, 0.15% Fe has the 12th
As shown in the figure, it can be seen that needle-like crystals of Fe are crystallized, albeit in a small amount. Also, the amount of Fe is 0.35%
It can be seen that as Fe increases to 0.55% Fe, the needle-like crystals of Fe also increase and are greatly developed, as shown in FIGS. 13 and 14. Next, Mn is added, but the structure is Mn
No change in the shape of the Fe compound was observed. It can be seen that addition of Sr improves mechanical properties just by observing the structure. It is also considered that it is better to reduce the amount of Fe as much as possible. (4)-3 Component range of each alloying element and reason for limitation When developing the alloy of the present invention, as already mentioned in the first part, JIS H5202 (1982)
The purpose was to improve the properties of aluminum casting alloys for automobile wheels specified in AC-4CH and expand the upper limit of Fe content in the alloy, so Si and Mg
The above standard remains as is, and Si is 6.5-7.5
%, Mg was 0.20-0.40%. Sr has a remarkable effect as an improving element in the present invention
The upper limit of 0.02% was set for Al―Si― in Figure 15.
From the Sr ternary phase diagram, it is clear that E _T1 (Al-
13.1%Si-0.03%Sr) and E _T2 (Al-1.1%Si-2.4
It was found that E _T1 , which has two ternary eutectic points (%Sr), has the most refinement effect. Therefore, it was decided that the upper limit of the Sr content should be 0.02% or less based on the presence of the E _T1 eutectic point mentioned above and the experimental results. The reason for setting the Fe content to 0.35% or less was based on the test results mentioned above.The addition of Sr significantly improved the mechanical properties with 0.13% Fe, and even with 0.35% Fe, the current AC-4CH
This is because it was found that although properties equal to or superior to those of Fe can be obtained, if Fe exceeds 0.35%, mechanical properties deteriorate and it is also unfavorable from the viewpoint of microstructure. Effects and industrial applicability of the alloy of the present invention As is clear from the test results as examples,
The addition of a small amount of Sr, about 0.02%, improves the mechanical properties in general, and as a result, it is possible to improve the mechanical properties of products, even for bare metals that conventionally require Fe of less than 0.15% or 0.13% or 0.14%. Fe in the cast product
Since the amount will be allowed up to 0.35%, even the widely produced Al99.5-99.7% base metal will be able to be used as automobile wheel material. In addition to expanding the range of use of ingots, reducing costs, and increasing the utilization of scrap, mechanical properties, especially ductility and toughness, are significantly improved when the Fe content is 0.20% or less, as in the past. Of course, it can be used as an automobile wheel.
It can be widely applied to products that require castability, strength, and toughness.

[Brief explanation of the drawing]

Figure 1a is a front view of an automobile disc wheel;
FIG. 1b is also a cross-sectional side view of the disc foil, but for ease of understanding, the spokes have been excluded and the scale is different from FIG. 1a. Second
Figure 3 shows the relationship between tensile strength and Fe content for each sample, Figure 3 shows the relationship between 0.2% proof stress and Fe content, Figure 4 shows the relationship between elongation and Fe content, and Figure 5 shows the impact value. It is a graph showing the relationship between the amount of Fe. Figure 6~
FIG. 10 is an optical microscopic structure showing the microstructure of a typical sample of a cast material, and FIGS. 11 to 14 are optical microscopic photographs showing the microstructure of a sample subjected to solution treatment and aging treatment after casting. Figure 15 shows Al
-Al side phase diagram of Si-Sr ternary alloy.

Claims (1)

[Scope of Claims] 1. 6.5-7.5% Si and 0.20-0.4% Si by weight
Mg, 0.005-0.02% Sr, 0.35% or less Fe
, 0.20% or less Ti, 0.30% or less Mn, 0.20
% or less Cu, 0.10% or less Zn, 0.05% or less Ni, 0.05% or less Pb, 0.05% or less Sn,
Consisting of 0.05% or less Cr and the balance Al, it has good castability and can be used after solution treatment and aging treatment.
By adding Sr, Si in the eutectic consisting of α phase and Si
An aluminum-based cast alloy characterized by having a metal structure in which Fe is refined and granulated, and crystallization of needle-shaped Fe crystals that reduce the strength of the matrix due to a notch effect is suppressed. 2. The alloy according to claim 1 has good castability and good tensile strength, yield strength, elongation, and impact value after solution treatment and aging treatment, and is suitable for casting automobile disc wheels. Aluminum based casting alloy suitable for use.