JP4026563B2 - Aluminum cast alloy piston and method of manufacturing the same - Google Patents

Description

  The present invention relates to an aluminum cast alloy piston and a method for manufacturing the same.

In an internal combustion engine such as an automobile engine, a piston is indispensable as a component constituting the engine, and is conventionally produced using an aluminum cast alloy. Various aluminum casting alloys have been proposed and improved.
For example, in the “aluminum alloy and manufacturing method for an internal combustion engine piston excellent in high-temperature strength” of Patent Document 1, Cu: 3 to 7 wt%, Si: 8 to 13 wt%, Mg: 0.3 to 1 wt% Fe: 0.1 to 1.0 wt%, Ti: 0.01 to 0.3 wt%, P: 0.001 to 0.01 wt%, Ca: 0.0001 to 0.01 wt% and necessary Accordingly, an alloy containing Ni: 0.2 to 2.5% by weight and having P / Ca adjusted to a range of 0.5 to 50 by weight is disclosed.

  The alloy disclosed in Patent Document 1 has the characteristics of maintaining excellent wear resistance and improving high-temperature strength compared to conventional alloys. However, thermal fatigue characteristics are not considered at all, and there is a problem that heat fatigue resistance is low. Further, there is a problem that pores are easily generated and the variation in fatigue characteristics is large.

Further, in the above alloy, the high temperature strength is increased to some extent by increasing the amount of components that enhance heat resistance such as Cu and Ni. However, if the addition amount is further increased, the ductility is lowered, thereby reducing the fatigue strength and thermal fatigue characteristics. Problems arise. Further, when the amount of Cu is high, final solidified portions where the Cu compound crystallizes are scattered in the material, and pores are generated in the portions due to solidification shrinkage.
As described above, the conventional method of increasing the heat-resistant component alone cannot improve the practical fatigue characteristics required for the piston top surface portion including the thermal fatigue characteristics.
JP-A-8-104937

  The present invention has been made in view of such conventional problems, and an object of the present invention is to provide a piston made of an aluminum cast alloy that is more excellent in heat fatigue resistance than the conventional one and a method for manufacturing the same.

  In the first invention, Mg: 0.2 mass% or less, Ti: 0.05 to 0.3 mass%, Si: 10 to 21 mass%, Cu: 2 to 3.5 mass%, Fe: 0.1 to 0.7 mass %, Ni: 1 to 3 mass%, P: 0.001 to 0.02 mass%, and made of an aluminum cast alloy composed of the balance Al and impurities. ).

In the first invention, the means for reducing the amount of Mg described below is used to stabilize the base aluminum structure and increase the ductility, and for the first time, it has been found that the heat fatigue resistance necessary for the piston is improved. .
By reducing the amount of Mg, thermally unstable Mg ₂ Si-based precipitates dispersed in the base aluminum phase can be reduced. This precipitate is coarsened by heating during actual use of the piston, resulting in a structural change. Therefore, the structure stability during heating can be improved by reducing the Mg ₂ Si-based precipitates.

In a thermal fatigue environment, strain concentration occurs around the coarse precipitates, which reduces the ductility of the base aluminum phase and decreases the thermal fatigue resistance. Therefore, it is considered that the reduction of the amount of Mg that forms precipitates suppresses the decrease in ductility of the base aluminum phase in a thermal fatigue environment and improves the thermal fatigue resistance. This improvement in heat-resistant fatigue properties by reducing the amount of Mg (so-called Mg-less) is a completely new concept not found in conventional piston alloys.
The piston can improve the heat fatigue resistance as described above, and the durability of the whole piston can be improved as compared with the conventional case.
The above-described excellent heat fatigue resistance can be obtained even when the piston is in an as-cast state without being heat-treated after casting as will be described later. On the other hand, as described later, various performances can be imparted by applying various heat treatments after casting.

The reasons for limiting the amount of each alloy element are described below.
Mg: 0.2 mass% or less,
Mg was reduced to stabilize the structure and improve ductility. If the Mg content exceeds 0.2%, the ductility of the base aluminum part in a thermal fatigue environment is lowered, and there is a demerit that thermal fatigue cracks are likely to occur. A preferable range is 0.1% or less, and in this case, the above effect is more clearly expressed, and a more preferable range is 0.05% or less. The optimal amount is zero. The smaller the content, the greater the merit, but the higher the cost, so the above limitation is made. Therefore, the Mg content is preferably in the range of 0 to 0.2 mass% (not including 0).

Ti: 0.05 to 0.3 mass%,
When the Ti content is less than 0.05%, the Ti solid solution amount in the base aluminum is low and sufficient solid solution strengthening cannot be obtained. If it is 0.3% or more, the base aluminum phase may become too hard due to Ti solid solution to cause shear fracture, and a coarse Ti compound may be generated to reduce toughness.
Note that when Ti is added by an Al—Ti—B alloy, an Al—Ti—C alloy, or the like, the inclusion of B and C as impurities is allowed. A preferable range of Ti content is 0.15 to 0.3%. If it is 0.15% or more, the structure is homogenized by sufficient crystal grain refinement, so that the fatigue strength is improved.

Cu: 2 to 3.5 mass%,
If the Cu content is less than 2%, the high-temperature proof stress near 350 ° C. is not sufficient, and the fatigue strength is insufficient. If it exceeds 3.5%, the final solidified parts where the Cu compound crystallizes are scattered, and pores are generated in this part due to solidification shrinkage, resulting in a decrease in fatigue strength. A preferable range is 2.5 to 3.25%. Within this range, higher fatigue characteristics can be obtained stably.

Si: 10 to 21 mass%,
If the Si content is less than 10%, hypereutectic solidification cannot be achieved even if P is added, and there is a risk of hypoeutectic solidification. In the case of hypoeutectic solidification, the matrix α-Al phase that releases the gas that causes pores during solidification is dispersed and solidified, and pores are likely to be formed because the final solidified portions are scattered. On the other hand, if it exceeds 21%, a large amount of coarse primary crystal Si is generated, and the ductility and toughness at a low temperature may be remarkably lowered. In addition, machinability may be significantly reduced.
The higher the Si content, the higher the high temperature fatigue strength around 350 ° C. A preferred range is 11-17%. In this range, hypereutectic solidification can be obtained stably, and since the size and amount of primary crystal Si are appropriate, higher fatigue properties and appropriate machinability can be obtained.

Fe: 0.1-0.7 mass%,
The Fe compound is formed as a crystallized product by containing Fe. The high-temperature proof stress is improved by the dispersion strengthening of the crystallized product. If it is less than 0.1%, there are few crystallized substances, and the high-temperature yield strength is not sufficiently improved. If it exceeds 0.7%, a coarse Fe compound is likely to be formed, and the fatigue characteristics may be reduced due to stress concentration. The Fe compound is a generic name for compounds containing Fe.

Ni: 1 to 3 mass%,
Similar to the case of Fe and Mn, Ni crystallizes the compound and contributes to the improvement of the high temperature proof stress by dispersion strengthening. If the Ni content is less than 1%, the crystallization of the Ni compound is small and the improvement in high-temperature proof stress is insufficient. If it exceeds 3%, a coarse Ni compound crystallizes out, and there is a risk that fatigue characteristics will deteriorate due to stress concentration.

P: 0.001 to 0.02 mass%,
By adding P, stable hypereutectic solidification is achieved, and pore generation is prevented. The primary Si is refined to ensure ductility and toughness. If it exceeds 0.02 mass%, the flowability of the hot water deteriorates and casting defects such as poor hot water are likely to occur.

2nd invention is Mg: 0.2 mass% or less, Ti: 0.05-0.3 mass%, Si: 10-21 mass%, Cu: 2-3.5 mass%, Fe: 0.1-0.7 mass %, Ni: 1 to 3 mass%, P: 0.001 to 0.02 mass%, and using an aluminum cast alloy composed of the balance Al and impurities,
A casting process in which the cast aluminum alloy is cast to form a piston;
The manufacturing method of the piston made from an aluminum casting alloy characterized by including the cutting process which cuts the said piston.

According to this manufacturing method, it is possible to easily manufacture the piston having the excellent heat fatigue resistance described above. Here, after the casting process, it can be in an as-cast state without any heat treatment. As will be described later, various heat treatments can be applied after the casting process.
The reasons for limiting the alloy elements of the aluminum cast alloy are the same as described above.

The third invention is Mg: 0.2-2 mass%, Ti: 0.05-0.3 mass%, Si: 10-21 mass%, Cu: 2-3.5 mass%, Fe: 0.1-0. 7 mass%, Ni: 1 to 3 mass%, P: 0.001 to 0.02 mass%, made of an aluminum casting alloy composed of the balance Al and impurities,
A piston made of an aluminum casting alloy having a Vickers hardness before starting use of HV 70 to 100 (claim 15).

The aluminum casting alloy constituting this piston has a Mg amount set higher than that of the alloy on the first side surface, and is 0.2 to 2 mass%. At the same time, in the manufacturing process, for example, annealing is performed, and the Vickers hardness before being used as a piston in an internal combustion engine is adjusted to the range of HV70-100.
Thereby, the said piston can improve heat fatigue resistance, and can improve durability of the whole piston than before.

As described above, the Mg content is increased to 0.2 to 2 mass%. Mg forms Mg ₂ Si-based precipitates, and the yield strength at a low temperature of 200 ° C. or lower can be improved by precipitation strengthening. When the Mg content increases, Mg ₂ Si is generated as a crystallized product during the solidification process. If the Mg content exceeds 2 mass%, the amount of crystallization becomes too large, and it becomes coarser, thereby reducing the toughness. On the other hand, if it is less than 0.2 mass%, the amount of precipitation is small, and the fatigue strength as a material at 200 ° C. is not sufficient.

Further, as described above, the hardness of the piston is such that the Vickers hardness is in the range of HV 70-100. Thereby, even if there is much Mg content, the outstanding heat fatigue resistance is realizable. Furthermore, an additional effect that the dimensional stability is increased can be obtained.
When the Vickers hardness exceeds HV100, the effect of improving the heat fatigue resistance is not obtained so much. Moreover, it is difficult for the hardness of the aluminum cast alloy to be lower than the lower limit value of HV70 within the range of the composition. If the temperature is less than HV80, the fatigue strength in an environment at a temperature of 200 ° C. or less may not be sufficient. Moreover, in the case of HV95 or less, the effect of improving the heat fatigue resistance described above becomes clearer. Therefore, the hardness of the piston is more preferably Vickers hardness HV 80 to 95.
The Vickers hardness described in this specification is based on the value obtained under the conditions of a load of 10 kg and a press-fitting time of 30 seconds. Mean value.

The fourth invention is Mg: 0.2-2 mass%, Ti: 0.05-0.3 mass%, Si: 10-21 mass%, Cu: 2-3.5 mass%, Fe: 0.1-0. 7 mass%, Ni: 1 to 3 mass%, P: 0.001 to 0.02 mass%, and using an aluminum cast alloy composed of the balance Al and impurities,
A casting process in which the cast aluminum alloy is cast to form a piston;
An annealing step of holding the piston at a temperature of 250 to 400 ° C. for 0.5 to 24 hours so that the Vickers hardness of the piston before use is in the range of HV 70 to 100,
And a cutting step of cutting the piston before or after the annealing step (claim 20).

According to this manufacturing method, it is possible to easily manufacture the piston having the excellent heat fatigue resistance described above.
The reasons for limiting the alloy elements of the aluminum cast alloy are the same as described above.

In the first or second invention, the aluminum cast alloy further contains at least one of V: 0.02 to 0.3 mass% or Zr: 0.02 to 0.3 mass%. (Claims 2 and 8).
The addition of V increases the high temperature proof stress and further increases the high temperature fatigue strength. Further, the addition of V can exhibit the additional effect of improving the high temperature fatigue strength without impairing the heat fatigue resistance. If the V content is less than 0.02%, the improvement in high-temperature yield strength is insufficient. If it exceeds 0.3%, uniform dissolution is difficult, and the undissolved V compound may become inclusions and the fatigue strength may be lowered.

  Addition of Zr increases the high temperature proof stress and further increases the high temperature fatigue strength. Further, the addition of Zr can exhibit an additional effect of improving the high temperature fatigue strength without impairing the heat fatigue resistance. Furthermore, the addition of Zr makes the crystal grains finer and stable fatigue characteristics can be obtained. If the Zr content is less than 0.02%, the improvement in high-temperature proof stress is insufficient. If it exceeds 0.3%, uniform dissolution is difficult, and undissolved Zr compounds become inclusions, which may reduce fatigue strength.

In the piston of the first or second invention, it is preferable that the aluminum casting alloy further contains Mn: 0.2 to 0.7 mass% (claims 3 and 9).
Mn, like Fe, crystallizes a compound and contributes to the improvement of high-temperature proof stress by dispersion strengthening. Moreover, it has the effect of improving the high temperature proof stress by solid solution in the base aluminum and solid solution strengthening. If it exceeds 0.7%, a coarse Mn compound is likely to be generated, stress concentration occurs, and the fatigue characteristics may be deteriorated. The Mn compound is a general term for compounds containing Mn. Mn is also contained in the Fe compound. For example, since an Al—Si—Fe—Mn compound contains Fe and Mn, it belongs to both the Fe compound and the Mn compound.
Similarly, the Fe content is preferably set to 0.2 mass% or more in order to improve the high-temperature yield strength.

  The piston according to the first aspect of the present invention preferably has a Vickers hardness of HV70 to 100 before the start of use of the piston. Thereby, the heat fatigue resistance can be further improved. When the Vickers hardness exceeds HV100, the effect of improving the heat fatigue resistance is not obtained so much. Moreover, it is difficult for the hardness of the aluminum cast alloy to be lower than the lower limit value of HV70 within the above composition range. When the temperature is less than HV80, the fatigue strength in an environment of a temperature of 200 ° C. or less may not be sufficient. Moreover, in the case of HV95 or less, the effect of improving the heat fatigue resistance described above becomes clearer. Therefore, the hardness of the piston is more preferably Vickers hardness HV 80 to 95. As described above, the heat fatigue resistance is improved when the hardness is lower, as described above, due to an increase in the stability of the structure. Furthermore, the additional effect that dimensional stability increases by reducing hardness is also acquired.

  In the first aspect of the invention, it is preferable that the size of the non-metallic inclusions present in the piston is less than 100 μm. When the size of the inclusion is 100 μm or more, there is a problem that fatigue strength and thermal fatigue life are remarkably lowered. Further, the size of the inclusion is preferably 50 μm or less. Here, the size of the inclusion refers to a representative size of the inclusion having the largest area among the inclusions observed when the metal structure or fracture surface of the piston is observed. As a typical method of obtaining the size, there is a method of taking the square root of the area, for example.

  Examples of the inclusion include oxides mainly composed of Al, sulfides, nitrides, carbides, and silicates.

  Next, in the second aspect of the invention, it is preferable that the piston is formed by the casting process and then allowed to cool to room temperature. That is, it is preferable to use the piston in an as-cast state after cooling the piston without performing a heat treatment. In this case, not only can the manufacturing process be streamlined, but also a new merit that the stability of the structure is enhanced and the thermal fatigue properties are excellent because stable precipitates are formed at high temperatures when allowed to cool after casting. . In addition, it has excellent heat resistance and high temperature fatigue strength.

  In the second invention, after the piston is formed by the casting process, before or after the cutting process, the Vickers hardness before the start of use of the piston is in the range of HV70-100. It is also preferable to perform an annealing step of holding at a temperature of 250 to 400 ° C. for 0.5 to 24 hours (claim 12).

  The annealing process may be performed in any process until the piston is incorporated into the engine and used, and may be performed before or after the cutting process as described above. However, the annealing step is preferably performed before the cutting step. As a result, even when deformation due to thermal effects occurs during annealing, the dimensional accuracy can be improved in the subsequent cutting process.

When the annealing temperature is less than 250 ° C. and when the holding time is less than 0.5 hour, the hardness of the piston cannot be sufficiently softened and it becomes difficult to make the Vickers hardness HV100 or less. . On the other hand, when the annealing temperature exceeds 400 ° C., there is a problem that Cu and Mg are re-dissolved and precipitation hardening occurs during cooling after annealing and holding at room temperature thereafter, resulting in increased hardness. Further, when the holding time exceeds 24 hours, there is a problem that the heat treatment cost is increased. Further, by setting the annealing temperature to 250 ° C. or more, a merit that a dimensional change during use is small is added.
The cooling after the annealing may be either cooling or water cooling.

  In the second aspect of the invention, after the casting step, a solution heating step for holding the piston at a temperature of 450 to 510 ° C. for 1 to 12 hours is performed, and then a quenching step for quenching the piston is performed. After that, it is preferable to perform the annealing step (claim 13).

In this case, the corners of the crystallized material are rounded, stress concentration is reduced, and stable precipitates are generated at high temperatures, so that the coarsening of the precipitates during use and the accompanying decrease in toughness can be suppressed. An effect is obtained.
The quenching step is a heat treatment step of quenching from a high temperature to obtain a supersaturated solid solution, and can be performed by quenching in hot water or cold water, for example.

In addition, when the solution heating temperature is less than 450 ° C., the corners of the crystallized product are not sufficiently rounded, and the solid solution of the precipitate forming elements is insufficient. On the other hand, when the solution heating temperature exceeds 510 ° C., there is a possibility that defects such as pores may occur when a compound containing Cu partially melts and resolidifies.
Further, when the holding time at the solution heating temperature is less than 1 hour, there is a problem similar to that when the solution heating temperature is less than 450 ° C., whereas when it exceeds 12 hours, the heat treatment cost is increased. However, there is a problem that the production efficiency is deteriorated along with the increase in the manufacturing cost.

In the second aspect of the invention, after the quenching step, it is preferable to perform an aging step of holding the piston at a temperature of 180 to 280 ° C. for 1 to 12 hours, and then perform the annealing step. 14). In this case, precipitates are more uniformly dispersed, and stable and excellent heat fatigue resistance can be obtained.
On the other hand, when the aging temperature is less than 180 ° C, there is a problem that the amount of precipitation during aging is not sufficient. On the other hand, when the aging temperature exceeds 280 ° C, precipitation during aging does not occur uniformly. There is a problem that the distribution of the formed precipitates is also inhomogeneous.
In addition, when the holding time at the aging temperature is less than 2 hours, there is a problem that the amount of aging precipitation is not sufficient. However, there is a problem of high cost.
In addition, the aging process described in this specification is a heat treatment process for increasing the hardness, while the annealing process means a heat treatment process for decreasing the hardness.

  As a method for casting the piston, low-cost gravity casting can be used. However, it can also be manufactured by high pressure casting or die casting.

Next, also in the third and fourth inventions, for the same reason as described above, the aluminum cast alloy further contains V: 0.02 to 0.3 mass%, or Zr: 0.02 to 0.3 mass. % Is preferably contained (claims 16 and 21).
Moreover, also in the said 3rd and 4th side surface, it is preferable to contain Mn: 0.2-0.7mass% from the reason similar to the above. The Fe content is preferably 0.2 mass% or more (claims 17 and 22).

  Also in the third aspect of the invention, for the same reason as described above, it is preferable that the size of the non-metallic inclusions present in the piston is less than 100 μm.

  In the fourth invention, as described above, after the piston is formed by the casting process, before or after the cutting process, the Vickers hardness before the start of use of the piston is in the range of HV70-100. Thus, an annealing process is performed in which the temperature is maintained at 250 to 400 ° C. for 0.5 to 24 hours.

  The annealing process in this case may be performed in any process until the piston is incorporated into the engine and used, and may be performed before or after the cutting process as described above. However, the annealing step is preferably performed before the cutting step in order to improve dimensional accuracy.

When the annealing temperature is less than 250 ° C. and when the holding time is less than 0.5 hour, the hardness of the piston cannot be sufficiently softened and it becomes difficult to make the Vickers hardness HV100 or less. . On the other hand, when the annealing temperature exceeds 400 ° C., there is a problem that Cu and Mg are re-dissolved and precipitation occurs at the time of cooling after annealing and at the normal temperature holding thereafter, and the hardness becomes high. Further, when the holding time exceeds 24 hours, there is a problem that the heat treatment cost is increased. Furthermore, the dimensional stability can be improved by annealing at 250 ° C. or higher.
The cooling after the annealing may be either cooling or water cooling.

  Also in the fourth aspect of the invention, after the casting step, a solution heating step of holding the piston at a temperature of 450 to 510 ° C. for 1 to 12 hours is performed, and then a quenching step of quenching the piston is performed. After that, it is preferable to perform the annealing step (claim 24).

In this case, the corners of the crystallized material are rounded, stress concentration is reduced, and stable precipitates are generated at high temperatures, so that the coarsening of the precipitates during use and the accompanying decrease in toughness can be suppressed. An effect is obtained.
In addition, the said quenching process can be performed by quenching in warm water or cold water, for example.

In addition, when the solution heating temperature is less than 450 ° C., the corners of the crystallized product are not sufficiently rounded, and the solid solution of the precipitate forming elements is insufficient. On the other hand, when the solution heating temperature exceeds 510 ° C., there is a possibility that defects such as pores may occur when a compound containing Cu partially melts and resolidifies.
Further, when the holding time at the solution heating temperature is less than 1 hour, there is a problem similar to that when the solution heating temperature is less than 450 ° C., whereas when it exceeds 12 hours, the heat treatment cost is increased. However, there is a problem that the production efficiency is deteriorated along with the increase in the manufacturing cost.

Also in the fourth aspect, after the quenching step, it is preferable to perform an aging step for holding the piston at a temperature of 180 to 280 ° C. for 1 to 12 hours, and then perform the annealing step. 25). In this case, precipitates are more uniformly dispersed, and stable and excellent heat fatigue resistance can be obtained.
On the other hand, when the aging temperature is less than 180 ° C., there is a problem that the amount of aging precipitation is not sufficient. On the other hand, when the aging temperature exceeds 280 ° C., aging precipitates are generated non-uniformly. There is a problem that the distribution of objects becomes inhomogeneous.
In addition, when the holding time at the aging temperature is less than 2 hours, there is a problem that the amount of aging precipitation is not sufficient. However, there is a problem of high cost.

  Note that low-cost gravity casting can also be used as a piston casting method in the fourth aspect. However, it can also be manufactured by high pressure casting or die casting.

In the first to fourth aspects of the present invention, it is preferable to further perform the following with respect to the component composition of the cast alloy.
<Ca: Addition of 0.0005 to 0.003 mass% (Claims 4, 10, 18, 23)>
For example, when a crystal grain refining element such as Ti, Zr, V or the like is included, the effect of homogenizing the structure can be obtained by further adding a trace amount of Ca to make the crystal grains finer. When the grain refinement element such as Ti is not included, and even if it is contained, the content is not within the scope of the present invention, the effect of grain refinement cannot be obtained even if Ca is added. . Even in the component range of the present invention, if the Ca content is less than 0.0005 mass%, the effect of crystal grain refinement cannot be obtained, and if it exceeds 0.003 mass%, the dendrite structure becomes prominent, This is not preferable because of inhomogeneity. Moreover, since there is a case where pores are likely to be generated when the Ca content is too large, the upper limit of the Ca content is more preferably 0.002% or less.

<Cr: Addition of 0.01 to 0.5 mass%>
The effect of making the crystal grains finer is obtained by adding a small amount of Cr. This effect appears only in the alloy of the present invention that sufficiently contains grain refinement elements such as Ti, as in the case of Ca addition. If the Cr content is less than 0.01%, the effect of crystal grain refinement is small, and if it exceeds 0.5%, a coarse compound containing Cr is generated, and the ductility of the alloy is lowered.

<B: Restricted to less than 0.01 mass%>
As the B content increases, the heat resistance decreases. For this reason, the B content is preferably regulated to less than 0.01 mass%.

<Be: Addition of 0.01 to 0.5 mass%>
Be has the effect of improving the form of the Fe compound and improving the castability. If it is less than 0.01 mass%, the effect of improving the castability is not sufficiently observed, and if it exceeds 0.5 mass%, the improvement of the effect due to the addition of Be is not recognized, which may be costly.

<Optimal content of Ti, Zr, V>
When both Ti, Zr and V are contained, Ti: 0.15 to 0.3 mass%, Zr: 0.05 to 0.12 mass%, V: 0.03 to 0.12 mass% are preferable. . As a result, the crystal grains are sufficiently refined and an optimal homogeneous structure is obtained. When both the Zr content and the V content exceed 0.12 mass%, a compound with Ti is likely to be formed at the time of dissolution, the crystal grains are insufficiently refined, and the dendrite is aligned and the microstructure is not good. May become homogeneous.

<Preferable content of P>
When the P content is small, a dendrite remains in a hypoeutectic structure, and the microstructure may be inhomogeneous. Therefore, the P content is preferably 0.005 mass% or more.

(First embodiment)
In this example, as shown in Tables 1 to 4 as aluminum casting alloys for pistons, as examples of the present invention, 19 kinds of aluminum casting alloys (less than 0.2 mass% Mg content) made of Mg-less material ( Table 1) and three types of cast aluminum alloys (Table 2) made of Mg-containing material (Mg content of 0.2 mass% or more) were prepared. Furthermore, as a comparative example, six types of Mg-containing materials (Table 3) and three types of Mg-less material cast aluminum (Table 4) were prepared. Then, various manufacturing methods were applied to each to prepare test pieces, and the thermal fatigue test was performed.

  First, various aluminum alloys having chemical compositions shown in Tables 1 to 4 were melted. The molten metal temperature was 740 to 760 ° C., and after deoxidation treatment by adding flux, vacuum degassing treatment was performed for 20 minutes in vacuum. Thereafter, the molten metal was cast into a JIS No. 4 test piece collecting boat having a surface coated with BN. The pouring temperature is 700 ° C. ± 20 ° C. The boat type used was heated in advance with a burner to remove water sufficiently and then cooled to room temperature.

Next, as shown in Tables 5 to 8, the following heat treatment was performed on the obtained casting material as necessary.
<T6 treatment>:
After heating at 495 ° C for 3 hours, solution treatment was performed by quenching in warm water at 50 ° C, followed by aging treatment at 210 ° C for 3 hours.
<T5 treatment>:
After casting into a mold, it is allowed to cool to room temperature, and then an aging treatment is performed at 220 ° C for 6 hours.
<F treatment>:
After casting into a mold, it is only allowed to cool to room temperature.
<Water-cooled T5 treatment>:
Immediately after casting into a mold, it was quenched from a high temperature state of 400 ° C or higher into hot water of 50 ° C and then subjected to aging treatment at 220 ° C for 6 hours.
<T6 + S treatment>:
After the T6 treatment, an annealing treatment of 350 ° C x 2 hours was performed.
<T5 + S treatment>:
After the T5 treatment, an annealing treatment of 350 ° C x 2 hours was performed.
<T6 + S4 process>:
After the T6 treatment, an annealing treatment of 400 ° C x 2 hours was performed.
<TS processing>:
After heating at 495 ° C for 3 hours, solution treatment is performed by quenching in warm water at 50 ° C, followed by annealing at 350 ° C for 2 hours.

Next, thermal fatigue test pieces and hardness measurement samples were collected from the cast material subjected to the heat treatment by machining.
The parallel part of the thermal fatigue test piece was φ4 mm × length 6 mm, and was processed with the position 14 mm high from the boat bottom as the center of the axis of the test piece.

  The thermal fatigue test was carried out by attaching a test aluminum alloy to a restraining holder made of a low thermal expansion alloy and repeating heating and cooling (for example, (1) Japanese Patent Application Laid-Open No. 7-20031, (2) Japanese Patent Application No. 2001-222081. (3) Proceedings of CAMP2002 on High-tempareature Fatigue Eds .: Thermal fatigue test method shown in G. Biallas et al., Pp.171-178).

  Specifically, as shown in FIGS. 1 to 3, a rod-shaped thermal fatigue test piece 1 having an evaluation portion (parallel portion) 10 having a smaller cross-sectional area than both ends at the center is prepared. The thermal fatigue test piece 1 is made of a low expansion material having a smaller thermal expansion coefficient than the thermal fatigue test piece 1 and having no temperature singularity where the thermal expansion coefficient changes rapidly. Two holders 2 having a plurality of V-shaped blades 22 provided on the fixed end portions 21 on both sides in contact with each other are prepared.

  Then, as shown in FIG. 3, when the thermal fatigue test piece 1 is constrained to be sandwiched by the holder 2 from both sides, the blade 22 of the holder 2 is press-fitted into both end portions 11 of the thermal fatigue test piece 1. To do. At the same time, in order to prevent loosening of the connection between the both end portions 11 of the thermal fatigue test piece 1 and the fixed end portion 21 of the holder 2, the thermal fatigue test piece is provided by the coupling means 3 via the elastic member 31. 1 both ends 11 and the fixed end 21 of the holder 2 are restrained.

Further, a strain gauge 59 for measuring thermal strain is disposed in the evaluation portion 10 of the thermal fatigue test piece 1 as required (only when strain is measured).
And while heating and cooling cycles are repeatedly given to the thermal fatigue test piece 1 and the holder 2 as a whole in this restrained state, both end portions 11 of the thermal fatigue test piece 1 and the fixed end portion 21 of the holder 2 are applied. Further tightening of the coupling means for suppressing a decrease in the coupling force is performed as necessary.

  And when the thermal fatigue test piece 1 breaks when the thermal strain caused by the thermal expansion difference between the thermal fatigue test piece 1 and the holder 2 is locally concentrated on the evaluation portion 10 of the thermal fatigue test piece 1 The thermal fatigue life is obtained from the number of heating / cooling cycles. Further, the total strain range is obtained by the strain gauge.

In this example, the test temperature range was 50 to 350 ° C., and the repetition rate was 4 min / cycle.
The material of the holder is Incoloy903, and the shape of the test piece and the holder is as shown in FIGS. 1 and 2 with the dimensions L0 = 48 mm, L1 = 46 mm, L2 = 32 mm, L3 = 22 mm, t = 6 mm. .

First, in order to verify the validity of the thermal fatigue test and clarify the test conditions, a thermal fatigue test is performed using a test piece made of JIS-AC8A alloy, and the total strain range at the initial stage of the test is measured with a high-temperature strain gauge. Measured. As a result, the total strain range at the beginning of the test was about 0.65%. In addition, it was confirmed that the fracture occurred at the central parallel part of the test piece and the thermal fatigue characteristics could be evaluated.
In the thermal fatigue test using the specimens of the above-mentioned alloys, in order to prevent loosening of the bolts and nuts that fasten the holder and the specimen, the bolts and The nuts were tightened.

  The thermal fatigue life (Nf) was judged from visual observation of cracks and a change in temperature difference between the upper and lower sides of the specimen (because the temperature distribution changes when it breaks). In this test, when a macro crack that can be visually confirmed is generated, it rapidly progresses and breaks. Therefore, the generation life and the fracture life of the macro crack are considered to be almost the same, and are unified with the thermal fatigue life.

  Tables 5 to 8 show the performance evaluation results of the alloys of the comparative examples and the examples. In these tables, the presence or absence of Mg indicates that the Mg content is 0.2 mass% or less (actually less than 0.01 mass%) as no Mg (Mg-less). HV represents Vickers hardness. Nf indicates the cycle number of the heating / cooling cycle when the test piece is broken in the thermal fatigue test.

The characteristics of the match money are shown again as follows.
Comparative Examples 1 to 6 are Mg-containing alloys, and heat treatments are T6 treatment in Comparative Examples 1 and 2, T5 treatment in Comparative Examples 3 to 5, and water-cooled T5 treatment in Comparative Example 6.
Comparative Examples 7 to 9 are Mg-less materials, and heat treatments are Comparative Examples 7 and 8 for F treatment and Comparative Example 9 for T5 + S treatment.

Examples 1 to 19 and Examples 23 and 24 are Mg-less alloys, and heat treatments are T6 treatment in Examples 1 to 3, T6 + S treatment in Examples 4 to 11, TS treatment in Example 12, and Example 13 T6 + S4 treatment, Examples 14 and 15 are T5 treatment, Example 16 is water-cooled T5 treatment, Examples 17 and 18 are F treatment, Example 19 is water-cooled T5 + S treatment, and Examples 23 and 24 are T6 treatment. In Examples 23 and 24, the Mn content was substantially 0 (less than 0.01 mass%).
Examples 20 to 22 and Examples 25 and 26 are Mg-containing alloys, and heat treatments are T6 + S treatment in Examples 20 and 21, Water-cooled T5 + S treatment in Example 22, and T6 + S treatment in Examples 25 and 26. In Examples 25 and 26, the Mn content was substantially 0 (less than 0.01 mass%).

As can be seen from Table 5, the Mg-less alloy having an Mg content of less than 0.01 mass% has a longer thermal fatigue life than the alloy of the comparative example.
As can be seen from Table 7, since the Mg amount of the alloys of Comparative Examples 1 to 6 is 0.25 mass% or more, at least the Mg amount is 0.2 mass% or less in order to obtain an excellent thermal fatigue life. It turns out that is effective.

As shown in Table 5, the alloys of Examples 4 to 13 subjected to annealing among the Mg-less alloys have a particularly long thermal fatigue life.
Example 12 is an alloy in which the aging treatment of T6 is omitted and annealing is performed, and similarly shows an excellent thermal fatigue life.
In Examples 14 to 19, the heat treatment is T5 treatment, water-cooled T5 treatment, F-treatment or water-cooled T5 + S, but the heat fatigue life is longer than that of the comparative example in any heat treatment. In particular, Examples 17 and 18 subjected to the F treatment show excellent thermal fatigue life.
Also, Examples 23 and 24, which are Mg-less alloys and are Mn-less (Mn content is substantially 0), showed excellent characteristics equivalent to those obtained by the same heat treatment.

As shown in Table 6, Examples 20 to 22 are annealed materials of Mg-containing alloys, and also show excellent thermal fatigue life as compared with Comparative Examples 1 to 6. This shows that excellent thermal fatigue life can be obtained even with Mg-containing materials if the base aluminum phase is softened to some extent by annealing. The degree of softening of the base aluminum phase by annealing can be almost evaluated by the hardness.
Further, Examples 25 and 26, which are Mg-containing alloys and are Mn-less (Mn content is substantially 0), exhibited excellent characteristics equivalent to those obtained by the same heat treatment.

Further, from the results of Tables 4 to 8, it is clearly recognized that the thermal fatigue life is improved in the specimens whose Vickers hardness is adjusted to HV100 or less by annealing. In particular, the effect is remarkable at HV95 or lower.
In addition, since the hardness of those subjected to T5 treatment and T6 treatment (Comparative Examples 1 to 6) without annealing is HV101 or higher, the thermal fatigue life is as long as the annealing conditions are such that the hardness is HV70-100. It is judged that the improvement effect is obtained.
From the above results, it is the alloy of Examples 1 to 19 belonging to the first aspect of the present invention that has been subjected to heat treatment to reduce the amount of Mg and anneal or cool after casting, and the Mg-containing material belonging to the third aspect. However, it was found that the one using an aluminum cast alloy whose Vickers hardness was adjusted in the range of HV 70 to 100 was excellent in the heat fatigue characteristics required for the piston.

  Tables 4 to 8 show the sizes of non-metallic inclusions recognized at the fracture starting point of the fracture surface of each test piece. This inclusion is, for example, an oxide mainly composed of Al, such as alumina. The size of the inclusion was obtained from the square root of the area.

  As can be seen from Tables 4 to 8, when the size of the nonmetallic inclusion is 100 μm or more, the thermal fatigue life is short, and the excellent heat fatigue resistance inherent in the alloy cannot be fully exhibited. In addition, it is appropriate to consider inclusions that are the starting point of fracture as the largest inclusions in the specimen.

Next, an example of a piston manufactured using the aluminum casting alloy of Example 1 will be described.
As shown in FIG. 4, the piston 5 of this example has a substantially cylindrical main body portion 50, a top surface portion 530 disposed so as to close one end of the main body portion 50, and the main body portion 50 penetrating in the radial direction. It has the pin boss | hub part 52 which provided the pin hole 520 provided so that it might do. Each pin hole 520 is configured to insert a piston pin for fixing a connecting rod (not shown).

In manufacturing this piston 5, it can carry out similarly to the case where the test piece of the said Examples 1-20 is manufactured.
The obtained piston 5 can particularly improve the heat fatigue resistance of the top surface portion 530, and the effect of improving the heat fatigue resistance of the lip portion 53 is particularly great. Further, the high temperature fatigue strength of the entire top surface portion 530 is high. Due to these effects, the durability of the entire piston can be improved as compared with the prior art. Moreover, since this alloy is excellent in heat resistance, an effect can be expected even if it is used for improving the heat resistance of the ring groove portion 54.

(Second embodiment)
In this example, as shown in Table 9, Ca was added as a component of the cast alloy, and the lower limit of the addition amount was verified.
As shown in Table 9, in this example, two types of test materials (Examples A1 and A2) were prepared as examples of the present invention, and two types of test materials (Comparative Examples A3 and A4) as comparative examples were prepared. ) Was prepared. All the specimens were cast by the same casting method as in the first embodiment, and then allowed to cool to room temperature.
Macrostructure photographs of the obtained specimens are shown in FIGS. FIG. 5 is for Example A1, FIG. 6 is for Example A2, FIG. 7 is for Comparative Example A3, and FIG. 8 is for Comparative Example A4.

As can be seen from FIGS. 5 to 8, the alloys of Examples A1 and A2 having a Ca content of 0.0005 mass% or more are compared with the alloys of Comparative Examples A3 and A4 having a Ca content of less than 0.0005 mass%. The crystal grains are fine and the structure is more homogeneous.
In Table 9, ◯ means that the structure is fine and homogeneous, and Δ means that the structure is slightly coarse and slightly heterogeneous.

(Third embodiment)
In this example, as shown in Table 10, Ca was added as a component of the cast alloy, and the upper limit of the amount added was verified.
As shown in Table 10, in this example, one type of test material (Example B1) was prepared as an example of the present invention, and two types of test materials (Comparative Examples B2 and B3) as comparative examples were prepared. Got ready. All the specimens were cast by the same casting method as in the first embodiment, and then allowed to cool to room temperature.

Microstructure photographs of the obtained test materials are shown in FIGS. 9 is for Example B1, FIG. 10 is for Comparative Example B2, and FIG. 11 is for Comparative Example B3.
As is known from FIGS. 9 to 11, Example B1 having a Ca content of 0.003 mass% or less has almost no dendrite alignment and a homogeneous microstructure, but the Ca content is 0.003 mass%. In the alloys of comparative examples B2 and B3, the dendrite alignment is clear and the microstructure is inhomogeneous.
In Table 10, “O” means that the structure is homogeneous, and “X” means that the structure is heterogeneous.

(Fourth embodiment)
In this example, as shown in Table 11, the effect of adding Cr as a component of the cast alloy was verified.
As shown in Table 11, in this example, two types of test materials (Examples C1 and C2) were prepared as examples of the present invention, and two types of test materials (Comparative Examples C3 and C4) as comparative examples were prepared. ) Was prepared. All the specimens were cast by the same casting method as in the first embodiment, and then allowed to cool to room temperature.
Macrostructure photographs of the obtained specimens are shown in FIGS. 12 is for Example C1, FIG. 13 is for Example C2, FIG. 14 is for Comparative Example C3, and FIG. 15 is for Comparative Example C4.
As is known from FIG. 12 to FIG. 15, Examples C1 and C2 containing Cr have crystal grains smaller than those of Comparative Examples C3 and C4 having a Cr amount of less than 0.01 mass% and substantially not containing Cr. Fine and macro structure is more homogeneous.
In Table 11, ◯ means that the structure is sufficiently fine and homogeneous, and Δ means that the structure is slightly coarse and slightly heterogeneous.

(5th Example)
In this example, as shown in Table 12, the effect when B was contained as a component of the cast alloy was verified.
As shown in Table 12, in this example, one type of test material (Example D1) was prepared as an example of the present invention, and two types of test materials (Comparative Examples D2 and D3) as comparative examples were prepared. Got ready. All the test materials were cast by the same casting method as in the first example, and then allowed to cool to room temperature, and then subjected to aging treatment at 220 ° C. for 6 hours (T5 treatment). The sample was held at 350 ° C. for 100 hours and then cooled to room temperature.
In this example, the Vickers hardness of the obtained specimen was measured. The results are shown in Table 12.
As is known from the table, the hardness of Example D1 having a B content of less than 0.01 mass% and containing substantially no B is higher than that of Comparative Examples D2 and D3 having a B content of 0.01 mass% or more. , High hardness after holding at high temperature and excellent heat resistance.

The (a) top view and (b) front view which show the shape and dimension of a thermal fatigue test piece in 1st Example. The shape and dimension of a holder in 1st Example are shown, (a) Front view, (b) Side view, (c) Expansion explanatory drawing of a blade part. The (a) top view and (b) front view which show the restraint state of the thermal fatigue test piece and holder in 1st Example. The partial notch perspective view of a piston in 1st Example. The drawing substitute photograph which shows the macro structure of Example A1 in 2nd Example. The drawing substitute photograph which shows the macro structure of Example A2 in 2nd Example. The drawing substitute photograph which shows the macro structure of comparative example A3 in 2nd Example. The drawing substitute photograph which shows the macro structure of comparative example A4 in 2nd Example. The drawing substitute photograph which shows the microstructure of Example B1 in 3rd Example. The drawing substitute photograph which shows the microstructure of comparative example B2 in 3rd Example. The drawing substitute photograph which shows the microstructure of comparative example B3 in 3rd Example. The drawing substitute photograph which shows the macro structure of Example C1 in 4th Example. Drawing substitute photograph which shows macro structure of Example C2 in 4th Example. The drawing substitute photograph which shows the macro structure of the comparative example C3 in 4th Example. The drawing substitute photograph which shows the macro structure of the comparative example C4 in 4th Example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Thermal fatigue test piece 10 Evaluation part 11 Both ends 115 Insertion hole 2 Holder 21 Fixed end 215 Insertion hole 22 Blade 3 Coupling means 301 Bolt 302 Nut 31 Elastic member (disc spring)
5 Piston 52 Pin boss part 520 Pin hole 53 Lip part 530 Top face part 54 Ring groove part

Claims (25)

  Mg: 0.2 mass% or less, Ti: 0.05 to 0.3 mass%, Si: 10 to 21 mass%, Cu: 2 to 3.5 mass%, Fe: 0.1 to 0.7 mass%, Ni: 1 to 1 A piston made of an aluminum cast alloy, comprising 3 mass%, P: 0.001 to 0.02 mass%, and made of an aluminum cast alloy comprising the balance Al and impurities.   2. The aluminum according to claim 1, wherein the aluminum cast alloy further contains at least one of V: 0.02 to 0.3 mass% or Zr: 0.02 to 0.3 mass%. Cast alloy piston.   The aluminum cast alloy piston according to claim 1, wherein the aluminum cast alloy further contains Mn: 0.2 to 0.7 mass%.   The aluminum cast alloy piston according to claim 1, wherein the aluminum cast alloy further contains Ca: 0.0005 to 0.003 mass%.   2. The aluminum cast alloy piston according to claim 1, wherein the piston has a Vickers hardness of HV 70 to 100 before the start of use of the piston.   2. The aluminum casting alloy piston according to claim 1, wherein the size of the nonmetallic inclusions existing in the piston is less than 100 [mu] m. Mg: 0.2 mass% or less, Ti: 0.05 to 0.3 mass%, Si: 10 to 21 mass%, Cu: 2 to 3.5 mass%, Fe: 0.1 to 0.7 mass%, Ni: 1 to 1 3 mass%, P: 0.001 to 0.02 mass% is contained, and an aluminum casting alloy composed of the balance Al and impurities is used.
A casting step of casting the aluminum cast alloy to form a piston;
A method for producing an aluminum cast alloy piston, comprising: a cutting step of cutting the piston.   8. The aluminum according to claim 7, wherein the aluminum cast alloy further contains at least one of V: 0.02 to 0.3 mass% or Zr: 0.02 to 0.3 mass%. A method for producing a cast alloy piston.   8. The method for producing an aluminum cast alloy piston according to claim 7, wherein the aluminum cast alloy further contains Mn: 0.2 to 0.7 mass%.   8. The method for producing an aluminum cast alloy piston according to claim 7, wherein the aluminum cast alloy further contains Ca: 0.0005 to 0.003 mass%.   8. The method for producing an aluminum cast alloy piston according to claim 7, wherein the piston is formed by the casting step and then allowed to cool to room temperature.   In Claim 7, After forming the said piston by the said casting process, before or after the said cutting process, temperature 250-400 degreeC so that the Vickers hardness before the start of use of this piston may become the range of HV70-100. A method for producing a piston made of an aluminum cast alloy, characterized in that an annealing step is performed for 0.5 to 24 hours.   In Claim 12, after the said casting process, the solution heating process which hold | maintains the said piston at the temperature of 450-510 degreeC for 1 to 12 hours is performed, Then, the quenching process which quenches the said piston is given, Then, the above-mentioned A method for producing an aluminum cast alloy piston, characterized by performing an annealing step.   The aluminum casting alloy piston according to claim 13, wherein after the quenching step, the piston is subjected to an aging step of holding the piston at a temperature of 180 to 280 ° C for 1 to 12 hours, and then the annealing step is performed. Production method. Mg: 0.2-2 mass%, Ti: 0.05-0.3 mass%, Si: 10-21 mass%, Cu: 2-3.5 mass%, Fe: 0.1-0.7 mass%, Ni: 1 ~ 3mass%, P: 0.001 ~ 0.02mass% containing, consisting of an aluminum cast alloy consisting of the balance Al and impurities,
A piston made of an aluminum casting alloy, having a Vickers hardness before starting use of HV 70-100.   16. The aluminum according to claim 15, wherein the aluminum cast alloy further contains at least one of V: 0.02 to 0.3 mass% or Zr: 0.02 to 0.3 mass%. Cast alloy piston.   The aluminum cast alloy piston according to claim 15, wherein the aluminum cast alloy further contains Mn: 0.2 to 0.7 mass%.   16. The aluminum cast alloy piston according to claim 15, wherein the aluminum cast alloy further contains Ca: 0.0005 to 0.003 mass%.   The aluminum casting alloy piston according to claim 15, wherein the size of the nonmetallic inclusions existing in the piston is less than 100 µm. Mg: 0.2-2 mass%, Ti: 0.05-0.3 mass%, Si: 10-21 mass%, Cu: 2-3.5 mass%, Fe: 0.1-0.7 mass%, Ni: 1 ~ 3mass%, P: 0.001 ~ 0.02mass% containing, using an aluminum cast alloy consisting of the balance Al and impurities,
A casting step of casting the aluminum cast alloy to form a piston;
An annealing step of holding the temperature at 250 to 400 ° C. for 0.5 to 24 hours so that the Vickers hardness before the start of use of the piston is in the range of HV 70 to 100,
And a cutting step of cutting the piston before or after the annealing step.   The aluminum cast alloy according to claim 20, wherein the aluminum cast alloy further contains at least one of V: 0.02 to 0.3 mass% or Zr: 0.02 to 0.3 mass%. A method for producing a cast alloy piston.   21. The method for producing an aluminum cast alloy piston according to claim 20, wherein the aluminum cast alloy further contains Mn: 0.2 to 0.7 mass%.   21. The method for producing an aluminum cast alloy piston according to claim 20, wherein the aluminum cast alloy further contains Ca: 0.0005 to 0.003 mass%.   In Claim 20, after the said casting process, the solution heating process which hold | maintains the said piston at the temperature of 450-510 degreeC for 1 to 12 hours is performed, Then, the quenching process which quenches the said piston is given, Then, the above-mentioned A method for producing an aluminum cast alloy piston, characterized by performing an annealing step.   25. The aluminum cast alloy piston according to claim 24, wherein after the quenching step, the piston is subjected to an aging step of holding the piston at a temperature of 180 to 280 ° C. for 1 to 12 hours, and then the annealing step is performed. Production method.

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