CN111254328A

CN111254328A - Method for improving mechanical property of A357 alloy

Info

Publication number: CN111254328A
Application number: CN202010252497.1A
Authority: CN
Inventors: 陈忠伟; 陈子雄; 闫康; 任聪聪
Original assignee: Northwestern Polytechnical University
Current assignee: Northwestern Polytechnical University
Priority date: 2020-04-01
Filing date: 2020-04-01
Publication date: 2020-06-09

Abstract

The invention discloses a method for improving the mechanical property of an A357 alloy, belonging to the technical field of aluminum alloys; scandium (Sc) is added into the aluminum-silicon alloy by smelting, so that the grain refining effect and the modification effect of eutectic Si can be simultaneously realized on the aluminum alloy, the alloy performance is improved, namely, one element is simultaneously used as a grain refiner and a modifier, and the production process is simplified; and the tensile strength and the elongation of the A357 alloy are improved by adjusting the addition of the Sc element, and the mechanical property at normal temperature is greatly improved.

Description

Method for improving mechanical property of A357 alloy

Technical Field

The invention belongs to the technical field of aluminum alloy, and particularly relates to a method for improving the mechanical property of an A357 alloy.

Background

The aluminum-silicon-magnesium casting alloy has good casting process performance and air tightness, has the advantages of small density, high specific strength, small shrinkage rate, small hot cracking tendency and the like, and is widely applied to the aerospace and automobile industries. With the rapid development of science and technology, there is a higher demand for the mechanical properties of cast aluminum alloys, so researchers have attempted to improve the microstructure and mechanical properties of alloys by different treatment methods.

The microstructure of the aluminum-silicon alloy is α -Al primary phase and Al-Si eutectic structure, generally, the mode of improving the mechanical property of the cast aluminum alloy is to add a grain refiner into the alloy, a proper amount of grain refiner is added to obtain fine equiaxial grains, microalloying is the most effective method for improving the comprehensive mechanical property of the aluminum alloy, and the mechanical property of the aluminum alloy can be obviously improved.

In industrial production, the use amounts of a grain refiner and a modifier are required to be determined simultaneously to improve the alloy performance, the steps are complicated, and the matching of the optimum contents of the grain refiner and the modifier is difficult to probe.

Disclosure of Invention

The technical problem to be solved is as follows:

in order to avoid the defects of the prior art, the invention provides a method for improving the mechanical property of the A357 alloy, wherein scandium (Sc) is added into the aluminum-silicon alloy by smelting, so that the scandium (Sc) can simultaneously perform the grain refinement effect and the modification effect of eutectic Si on the aluminum alloy, and the alloy property is improved, namely one element is simultaneously used as a grain refiner and a modifier, so that the production process is simplified.

The technical scheme of the invention is as follows: an a357 alloy, comprising: the A357 alloy comprises the following components in percentage by mass: 6.5-7.5 wt.% of Si, 0.45-0.7 wt.% of Mg, 0-1.0 wt.% of Sc, and the balance of Al.

The further technical scheme of the invention is as follows: the A357 alloy comprises, by mass, 7% of Si, 0.6% of Mg, 0.4-0.6% of Sc and the balance of Al.

A method for improving the mechanical property of an A357 alloy is characterized by comprising the following specific steps:

the method comprises the following steps: taking aluminum with the purity of 99.9 percent and intermediate alloy of three components of Al-2 percent Sc, Al-20 percent Si and Al-10 percent Mg as raw materials to prepare materials; the prepared aluminum alloy comprises the following components in percentage by mass: converting 7% of Si, 0.6% of Mg, 0-1.0% of Sc and the balance of Al to obtain the mass ratio of Al-20% of Si, Al-10% of Mg and Al-2% of Sc;

step two: alloy smelting in a resistance furnace: sequentially adding experimental raw materials into a graphite crucible in the order of melting point from low to high, wherein the order is aluminum, Al-20% Si, Al-10% Mg and Al-2% Sc, heating to 710 ℃ after melting, and ensuring that all the raw materials are fully melted;

step three: stirring the melt obtained in the step two by using a stirring tool to uniformly mix the components of the melt, and then adding a degasifier and a slag remover to perform degassing and slag removing treatment so as to reduce the generation of defects in the alloy;

step four: keeping the temperature for 30min and then carrying out alloy casting; selecting a stainless steel casting mold, casting cylindrical ingots with the diameter of 60mm, wherein the Sc element content of the six groups of cast aluminum alloy ingots is different, and the value range of the Sc element is between 0 and 1.0 percent;

step five: observing a sample cut out of the alloy ingot under a scanning electron microscope SEM, analyzing by adopting an electron back scattering diffraction technology EBSD, and counting the refining effect of alloy grains and the modification effect of eutectic Si;

step six: processing the aluminum alloy cast ingot into a plate-shaped sample according to the national standard, performing a room-temperature tensile test on an electronic universal material testing machine, and detecting the mechanical property of the alloy.

The further technical scheme of the invention is as follows: the six groups of aluminum alloy cast ingots respectively contain 0%, 0.2%, 0.4%, 0.6%, 0.8% and 1.0% of Sc element.

Advantageous effects

The invention has the beneficial effects that: according to the invention, the Sc element is added into the aluminum-silicon alloy, and the addition of the Sc element is adjusted, so that the tensile strength and the elongation of the A357 alloy are both improved, and the mechanical property at normal temperature is greatly improved; when the Sc element proportion in the alloy is 0.4-0.6%, the comprehensive mechanical property of the alloy is best, high strength and plasticity are obtained, the tensile strength is improved by about 27.9-28.31% relative to the original alloy, the elongation is improved by 2.32-2.47 times, and a good effect of improving the mechanical property of the alloy is achieved. Compared with the existing production process, the strength is improved, the plasticity is slightly reduced, but the production process is greatly simplified. The following table shows the tensile strength and elongation of the alloy after different contents of Sc element are added into the A357 alloy.

Content of Sc%	Tensile strength MPa	Elongation percentage%
			0	190.06	2.26
0.2	211.56	4.47
			0.4	243.17	5.58
0.6	243.87	5.04
			0.8	256.49	4.42
1.0	249.36	3.88

Drawings

FIG. 1: grain orientation reconfiguration images of A357 alloy with different Sc element contents are formed;

FIG. 2: al (Al)₃Sc particle pictures;

FIG. 3: eutectic Si morphology graphs of alloys with different Sc element contents;

FIG. 4: the original alloy contains iron phase morphology;

FIG. 5: adding an iron-containing phase form in the Sc element alloy;

FIG. 6: newly precipitated phase AlSi₂Sc₂And (4) phase(s).

Detailed Description

The embodiments described below with reference to the drawings are illustrative and intended to be illustrative of the invention and are not to be construed as limiting the invention.

According to the invention, through a series of experiments, the optimal content of Sc added in the A357 alloy is 0.4-0.6%, so that crystal grains in an alloy structure are effectively refined, eutectic Si is well degenerated, and the mechanical property of the alloy is obviously improved.

Different contents of Sc element are added into the A357 alloy, and different effects can be generated. Fig. 1 reflects the grain size of the a357 alloy with various amounts of Sc added, the grains of the original a357 alloy are very coarse and exhibit columnar morphology with in-growth from the cast wall. After 0.2% of Sc element is added, the crystal grains are refined to some extent, but are still relatively coarse columnar crystals. When the content of Sc element reaches 0.4%, the crystal grains are gradually changed into isometric crystals from columnar crystals. The grain size decreased with increasing Sc, with the smallest grain size at Sc levels up to 1.0%, which was about 97% lower than the original a357 alloy, indicating that Sc produces a very effective grain refinement for the a357 alloy. The significant grain refinement that occurs is attributed to the addition of Sc during solidificationFormation of Al by eutectic reaction₃The Sc particles serve as effective heterogeneous nucleation cores to promote the nucleation of α -Al, when the temperature of the metal melt is reduced, α -Al adheres to the formed Al₃Nucleation on Sc particles is large, rather than spontaneous nucleation in the melt, Al₃The Sc particle morphology is shown in FIG. 2. With increasing Sc content, Al₃The Sc particles are increased continuously, so that the grain refining effect is enhanced gradually.

The Sc element has good modification effect on eutectic Si in the A357 alloy, as shown in figure 3, the modification effect is more obvious along with the increase of Sc content, the modification effect is best when the addition amount reaches 1.0%, and the eutectic form is changed from a thick lath-shaped and needle-shaped structure in the original alloy into a highly branched fibrous structure. Through the research result of the invention, the Sc element is added to change the nucleation mode of the eutectic phase and possibly influence the growth mechanism of the eutectic Si. Meanwhile, the addition of Sc element reduces the eutectic reaction temperature, thereby increasing the nucleation supercooling degree and promoting the deterioration of eutectic Si.

After the Sc element is added, the form of the iron-containing phase in the A357 alloy is changed, as shown in figures 4 and 5, the needle-shaped β -AlSiFe phase and the short rod or Chinese character-shaped pi-AlSiMgFe phase in the original alloy are changed into a Chinese character-shaped pi-AlSiMgFeSc phase, which plays a positive role in improving the performance of the alloy.

Generating new precipitated phase AlSi after adding Sc element₂Sc₂As shown in FIG. 6, as the Sc content increases, the precipitated phases increase in size and number. The research result of the invention shows that the small size of AlSi₂Sc₂The phase and the eutectic Si grow in a competition mode, a certain positive effect is achieved on the deterioration of the eutectic, and the eutectic area is strengthened due to the dispersion distribution. Large size AlSi₂Sc₂The phase is essentially still a hard brittle phase which tends to break first when the material is deformed under stress, so that the presence of large-size AlSi in the alloy should be avoided₂Sc₂And (4) phase(s). Excess AlSi₂Sc₂The phases will adversely affect the alloy properties.

Addition of boron to A357 alloyAfter the Sc element with the same content, the tensile strength and the elongation of the A357 alloy are improved, and the mechanical property at normal temperature is greatly improved. The Sc element simultaneously has grain refining and modification effects on the alloy, changes the form of an iron-containing phase, reduces the harm of the iron-containing phase, and has positive effects on the performance improvement of the alloy. However, the large-size AlSi formed after addition of too much Sc element₂Sc₂The phase harms the performance of the alloy and reduces the performance of the alloy, so that the content of Sc in the alloy is prevented from being too high. The results of all the experiments are combined, the optimal Sc content in the A357 alloy is 0.4-0.6%, the tensile strength of the alloy is 243.17-243.87MPa, the elongation is 5.58-5.04%, and the alloy has high strength and plasticity.

Some specific examples are as follows:

example 1:

175g of Al-20% Si alloy, 30g of Al-10% Mg alloy and 295g of pure aluminum are weighed and prepared. The prepared alloy raw materials are smelted in a 12KW resistance furnace, and the used crucible is a graphite crucible with the capacity of about 2L. Experimental raw materials of Al-20% Si alloy, Al-10% Mg alloy and pure aluminum are added into a graphite crucible and heated to 710 ℃. Stirring the melt by using a stirring tool to uniformly mix the components, then adding a degasifier and a deslagging agent, degassing and dredging slag, keeping the temperature for 30min after finishing the steps, and then carrying out alloy casting. And quickly pouring the melt into a stainless steel casting mold, and casting into a cylindrical ingot with the diameter of 60mm, wherein the cylindrical ingot does not contain Sc elements. The alloy ingot is subjected to linear cutting to obtain a required sample, SEM and EBSD experiments are respectively carried out, and the grain refining effect and the modification effect are shown in figures 1 and 3. In the A357 alloy without Sc, the crystal grains are extremely coarse, and the eutectic Si is in a lath shape and a needle shape. The alloy is processed into a plate-shaped sample according to the national standard, and a room-temperature tensile test is carried out on an electronic universal material testing machine produced by INSTRON company in America, so that the tensile strength of the alloy is 190.06MPa, and the elongation is 2.26%.

Example 2:

weighing 175g of Al-20% Si alloy, 30g of Al-10% Mg alloy, 150g of Al-2% Sc alloy and 145g of pure aluminum, and preparing the materials. The prepared alloy raw materials are smelted in a 12KW resistance furnace, and the used crucible is a graphite crucible with the capacity of about 2L. Firstly, Al-20% of Si alloy, Al-10% of Mg alloy and pure aluminum are added into a graphite crucible, after the Al-20% of Si alloy, the Al-10% of Mg alloy and the pure aluminum are approximately melted, Al-2% of Sc alloy is added, and the mixture is heated to 710 ℃. Stirring the melt by using a stirring tool to uniformly mix the components, then adding a degasifier and a deslagging agent, degassing and dredging slag, keeping the temperature for 30min after finishing the steps, and then carrying out alloy casting. And quickly pouring the melt into a stainless steel casting mold, and casting into a cylindrical ingot with the diameter of 60mm, wherein the cylindrical ingot contains 0.6 percent of Sc element. The alloy ingot is subjected to linear cutting to obtain a required sample, SEM and EBSD experiments are respectively carried out, and the grain refining effect and the modification effect are shown in figures 1 and 3. The grain size is effectively refined, the grain size is reduced by 87.0 percent, the eutectic Si is well modified, and the coarse lath shape is converted into a fibrous shape. The alloy is processed into a plate-shaped sample according to the national standard, and a room-temperature tensile test is carried out on an electronic universal material testing machine produced by INSTRON company of America, so that the tensile strength of the obtained alloy is 243.87MPa, the elongation is 5.04%, and the mechanical property is greatly improved compared with that of the original alloy, thereby proving that the mechanical property of the A357 alloy is obviously improved.

Example 3:

weighing 175g of Al-20% Si alloy, 30g of Al-10% Mg alloy, 250g of Al-2% Sc alloy and 45g of pure aluminum, and preparing the materials. The prepared alloy raw materials are smelted in a 12KW resistance furnace, and the used crucible is a graphite crucible with the capacity of about 2L. Firstly, Al-20% of Si alloy, Al-10% of Mg alloy and pure aluminum are added into a graphite crucible, after the Al-20% of Si alloy, the Al-10% of Mg alloy and the pure aluminum are approximately melted, Al-2% of Sc alloy is added, and the mixture is heated to 710 ℃. Stirring the melt by using a stirring tool to uniformly mix the components, then adding a degasifier and a deslagging agent, degassing and dredging slag, keeping the temperature for 30min after finishing the steps, and then carrying out alloy casting. And quickly pouring the melt into a stainless steel casting mold, and casting into a cylindrical ingot with the diameter of 60mm, wherein the Sc element is 1.0 percent. The alloy ingot is subjected to linear cutting to obtain a required sample, SEM and EBSD experiments are respectively carried out, the grain refining effect and the modification effect are shown in figures 1 and 3, the grains are refined to be extremely fine, and the modification effect of the eutectic Si is also good. Processed into plate-shaped samples according to national standard, and subjected to room-temperature tensile test on an electronic universal material testing machine produced by INSTRON company of America to obtain the alloy with the tensile strength of249.36MPa, the elongation is 3.88 percent, because the Sc element content is too high, the newly precipitated phase AlSi₂Sc₂The coarse phase size, the increased number, and the beginning of the alloy decreases its plasticity and strength.

Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention, and that variations, modifications, substitutions and alterations can be made in the above embodiments by those of ordinary skill in the art without departing from the principle and spirit of the present invention.

Claims

1. An a357 alloy, comprising: the A357 alloy comprises the following components in percentage by mass: 6.5-7.5 wt.% of Si, 0.45-0.7 wt.% of Mg, 0-1.0 wt.% of Sc, and the balance of Al.

2. The a357 alloy of claim 1, wherein: the A357 alloy comprises, by mass, 7% of Si, 0.6% of Mg, 0.4-0.6% of Sc and the balance of Al.

3. A method for improving the mechanical properties of the a357 alloy of claim 1, which comprises the following steps:

4. The method of claim 3, further comprising: the six groups of aluminum alloy cast ingots respectively contain 0%, 0.2%, 0.4%, 0.6%, 0.8% and 1.0% of Sc element.