PREPARATION METHOD FOR HIGH-STRENGTH AND HIGH-THERMAL-CONDUCTIVITY MAGNESIUM ALLOY SHEET

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of Chinese Patent Application No. 202411789406.2, filed on Dec. 6, 2024, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure belongs to the technical field of magnesium (Mg) alloys, and specifically relates to a preparation method for a high-strength and high-thermal-conductivity magnesium alloy sheet.

BACKGROUND

With the continuous development of the field of electronic devices, a significant demand for heat dissipation has emerged in areas such as electron devices. Magnesium is the lightest metallic structural material with high thermal conductivity, presenting promising prospects for energy conservation, efficiency improvement, and emission reduction. However, the poor mechanical properties of pure magnesium limit its production and application. To address this, alloying and deformation processing are commonly employed. While widely used commercial magnesium alloys such as AM60 and AZ31 possess good formability and room-temperature mechanical properties, the thermal dissipation capabilities of these alloys are unsatisfactory, with thermal conductivity values of only 61 W/(m·K) and 96.4 W/(m·K), significantly lower than those of pure magnesium. On the other hand, rolling is a common deformation process for magnesium alloys. The rolled magnesium alloy sheets exhibit high mechanical properties, but this process significantly reduces the thermal conductivity, hindering further applications. This trade-off between mechanical properties and thermal conductivity constrains the development and design of high-strength and high-thermal-conductivity magnesium alloys.

SUMMARY

To overcome the deficiencies in the related art and solve the problem of incompatibility between overall mechanical properties and thermal conductivity of existing magnesium alloys, a primary objective of the present disclosure is to develop a preparation method for a low rare-earth content magnesium alloy sheet that exhibits superior mechanical and thermal properties. In the present disclosure, a magnesium alloy material is subjected to vertical-die hot extrusion combined with rolling deformation, followed by aging treatment and heat treatment. This process refines a dynamically recrystallized grain size while enhancing a dispersion degree of second phases, causing the alloy to achieve a thermal conductivity exceeding 130 W/(m·K) while maintaining a strength grade of 450 MPa.

The present disclosure is realized through the following technical solution: a preparation method for a high-strength and high-thermal-conductivity magnesium alloy sheet includes the steps of:

• • (1) annealing a rare-earth magnesium alloy sheet, which is made of magnesium-5zinc-1gadolinium-1yttrium-1manganese (Mg-5Zn-1Gd-1Y-1Mn), at 170° C. for 30 minutes;
  • (2) rolling the annealed rare-earth magnesium alloy sheet with rolls preheated to 120° C. at a roll linear speed of 5 m/min and a 70% reduction rate, to obtain a rare-earth magnesium alloy sheet with a thickness of 1.8 mm;
  • (3) quenching the rare-earth magnesium alloy sheet and cooling the sheet to room temperature; and
  • (4) performing an aging treatment on the quenched rare-earth magnesium alloy sheet at 90° C. for 24 to 48 hours, and air-cooling the sheet to room temperature to obtain the high-strength and high-thermal-conductivity magnesium alloy sheet.

Preferably, the rare-earth magnesium alloy sheet in step (1) is prepared by:

• • S1: preparing a magnesium alloy cast ingot
  • cleaning surfaces of magnesium ingots, zinc granules, Mg—Gd master alloy, Mg—Mn master alloy, and Mg—Y master alloy, followed by heating and melting under a protective atmosphere; and
  • pouring mixed melt into a mold, followed by cooling and demolding to obtain a magnesium alloy cast ingot with low rare-earth content; and
  • S2: performing extrusion forming on the magnesium alloy cast ingot with low rare-earth content
  • S2-1, placing the magnesium alloy cast ingot in a heat treatment furnace for a stepwise solution treatment, with a first stage conducted at a heating temperature of 420 to 480° C. for a holding time of 6 to 8 hours, followed by a second stage performed at a heating temperature of 500 to 520° C. for a holding time of 10 to 12 hours;
  • S2-2, grinding the stepwise-solution-treated magnesium alloy cast ingot until smooth, placing the ground magnesium alloy cast ingot into a mold, and heating the mold to 300° C. and holding for 40 minutes; and performing hot pressing at a loading speed of 10 kN/s, and maintaining a pressure for 100 seconds when the magnesium alloy cast ingot is hot-pressed to a thickness of 60 mm;
  • S2-3, processing the hot-pressed magnesium alloy cast ingot from step S2-2 to standard specimen dimensions and grinding a surface of the cast ingot until smooth; placing the cast ingot into a heat treatment furnace, and heating the cast ingot to 300° C. and holding for 30 minutes; and preheating a punch, a die, an upper pad, and an extrusion mold for subsequent use;
  • S2-4, subjecting the hot-pressed magnesium ingot from step S2-3 to an extrusion process: placing a hot-pressed billet with a 90° flip, and orienting a processing direction-transverse direction (PD-TD) plane after hot pressing as an extrusion direction (ED); performing a three-step extrusion process with an extrusion ratio of 25:1, an initial speed of 5 m/min, and an extrusion force of 2900 kN, and setting an initial extrusion temperature to 300-310° C.; cutting and sampling a sheet after a first extrusion stage; reducing the extrusion temperature by 10-20° C. for each subsequent step, with a corresponding speed increase of 2 m/min, and maintaining the extrusion force at 2700-3000 kN; and setting a second extrusion stage temperature to 280-300° C. at a speed of 7 m/min, and setting a third extrusion stage temperature to 260-280° C. at a corresponding speed of 9 m/min; and
  • S2-5, quenching an extruded slab from step S2-4, cooling the slab to room temperature, chamfering the slab, and grinding a surface of the slab to obtain the magnesium alloy sheet.

More preferably, the protective atmosphere in step S1-2 is a mixed gas of carbon dioxide (CO₂) and sulfur hexafluoride (SF₆).

Further, a volume ratio of CO₂ to SF₆ is 100:1.

Compared to the related art, the present disclosure has the following beneficial effects.

In the present invention, the challenges of high cost and the difficulty in balancing strength with thermal conductivity in the rare-earth magnesium alloys are addressed by employing low rare-earth micro-alloying and a multi-step deformation process to modulate the quantity and distribution of phases. The alloy sheet, after being annealed at 170° C. for 30 minutes and subjected to single-pass rolling followed by low-temperature aging, achieves a stress strength of 456 MPa, a yield strength of 408 MPa, an elongation of 13%, and a thermal conductivity ranging from 125 to 134 W/(m·K).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of a PD (i.e., perpendicular to a normal direction, ND) and an ED;

FIG. 2 shows microstructure morphology of an Mg-5Zn-1Gd-1Y-1Mn magnesium alloy after solution treatment;

FIG. 3 shows microstructure morphology of the Mg-5Zn-1Gd-1Y-1Mn magnesium alloy after extrusion;

FIG. 4 shows microstructure morphology of the Mg-5Zn-1Gd-1Y-1Mn magnesium alloy after rolling;

FIG. 5 shows microstructure morphology of the Mg-5Zn-1Gd-1Y-1Mn magnesium alloy after aging at 90° C. for 24 hours;

FIG. 6 shows stress curves of an Mg-5Zn-1Gd-1Y magnesium alloy in as-cast, as-extruded, as-rolled, and as-aged states; and

FIG. 7 shows yield strength and thermal conductivity data of the Mg-5Zn-1Gd-1Y magnesium alloy after aging treatment.

DETAILED DESCRIPTION

To make the objectives, technical solutions, and advantages of the present disclosure more apparent, preferred implementations of the present disclosure are described below in detail with reference to embodiments. Based on the embodiments of the present disclosure, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present disclosure.

Embodiment 1

• • In S1, a low rare-earth content Mg-5Zn-1Gd-1Y-1Mn magnesium alloy cast ingot was prepared, including the following steps:
  • In S1-1, pre-prepared pure Mg was immersed in an acidic solution to remove surface oxides. The treated pure Mg, pure Zn, Mg-30Gd (wt. %) master alloy, Mg-30Mn (wt. %) master alloy, and Mg-30Y (wt. %) master alloy were ground to a bright finish using a grinding wheel and angle grinder, thereby eliminating the influence of oxide layers and impurities on material preparation. After grinding, these raw materials were wrapped in aluminum foil and stored in a sealed condition.
  • In S1-2, a stainless steel crucible was preheated in a pit furnace. After preheating to 500° C., the crucible was removed and coated with talc and zinc oxide coatings to prevent direct contact between Mg melt and a crucible wall, thereby avoiding impurity introduction. The crucible was returned to the pit furnace for heating. When the temperature reached 720° C., the pure Mg was added while introducing a mixed gas of CO₂ and SF₆ (with a volume ratio of 100:1) as a protective atmosphere during melting.
  • In S1-3, after the pure Mg was completely melted, the melt was held for 15 minutes. Zinc granules were added and stirred for 1 minute. Meanwhile, Mg-30Gd and Mg-30Y master alloys were wrapped in aluminum foil and preheated near the furnace. The crucible temperature was subsequently raised to 760° C., and the preheated Mg-30Gd, Mg-30Y, and Mg-30Mn master alloys were added, followed by rapid stirring for 3 minutes and holding for 10 minutes. The crucible temperature was lowered to 720° C., and the molten alloy was poured into a mold preheated to 250° C. The alloy was naturally air-cooled to room temperature to obtain the alloy cast ingot.
  • In S2, the magnesium alloy cast ingot was subjected to extrusion forming, including the following steps:
  • In S2-1, the magnesium alloy cast ingot was placed in a heat treatment furnace for a stepwise solution treatment, with a first stage conducted at a heating temperature of 430° C. for a holding time of 6 hours, followed by a second stage performed at a heating temperature of 500° C. for a holding time of 10 hours.
  • In S2-2, the homogenized magnesium alloy cast ingot from step S2-1 was ground until its surface was smooth, placed into a mold, and preheated along with the mold in the furnace to 300° C. and held for 40 minutes. Hot pressing was performed at a loading speed of 10 kN/s, and the pressure was maintained for 100 seconds after this specimen was compressed to a thickness of 60 mm.
  • In S2-3, the hot-pressed magnesium alloy cast ingot from step S2-2 was processed to standard specimen dimensions, and the surface was ground until smooth. The cast ingot was placed into a heat treatment furnace and heated to 300° C., followed by holding at this temperature for 30 minutes. Meanwhile, a punch, a die, an upper pad, and an extrusion mold were preheated for subsequent use.
  • In S2-4, the hot-pressed magnesium ingot from step S2-3 was subjected to an extrusion process. After a hot-pressed billet was positioned with a 90° flip, orienting a PD-TD plane of this hot-pressed material as an ED. A three-step extrusion process was performed with an extrusion ratio of 25:1, an initial speed of 5 m/min, an extrusion force of 2900 kN, and an initial extrusion temperature of 300° C. After a first extrusion stage, a sheet was cut and sampled. For each subsequent step, the extrusion temperature was reduced by 20° C. with a corresponding speed increase of 2 m/min, while the extrusion force was maintained at 2900 kN. A second extrusion stage temperature was set to 280° C. at a speed of 7 m/min, and a third extrusion stage temperature was set to 260° C. at a corresponding speed of 9 m/min.
  • In S2-5, an extruded slab from step S2-4 was quenched in 60° C. warm water. After the slab cooled to room temperature, it was removed, chamfered, and its surface was ground smooth for su1bsequent use.

In S3, the magnesium alloy was processed by single-pass rolling and heat treatment, including the following steps:

• • In S3-1, the slab obtained from step S2 was annealed in a heat treatment furnace at 170° C. for 30 minutes.
  • In S3-2, the slab obtained from step S3-1 was placed into a BKDΦ350×400 experimental rolling mill and rapidly fed into rolls for rolling. During rolling, the rolls were preheated to 120° C., a rolling linear speed was set to 5 m/min, and a reduction rate was 70%, resulting in a sheet with a thickness of 1.8 mm.
  • In S3-3, the sheet obtained from step S3-2 was quenched in 60° C. warm water and removed after cooling to room temperature.
  • In S3-4, the magnesium alloy sheet rolled in step S4-2 was subjected to an aging treatment at 90° C. for 24 hours, followed by air-cooling to room temperature, thereby obtaining a high-strength and high-thermal-conductivity magnesium alloy sheet.

By comparing a microstructure of the extruded magnesium alloy with that of the rolled magnesium alloy, it can be observed that most of second phases at grain boundaries after solid solution have dissolved into a matrix. After extrusion deformation, significant dynamic recrystallization occurs around the second phases. The interaction between soft deformation zones and hard recrystallized regions substantially enhances a plastic deformation capability of the sheet. When annealed at 170° C. for 30 minutes followed by single-pass rolling, the magnesium alloy sheet achieves a stress strength of 385 MPa, a yield strength of 337 MPa, and an elongation ranging from 13% to 17%.

After low-temperature aging, the alloy achieves a stress strength up to 456 MPa, a yield strength of 408 MPa, an elongation of 13%, and a thermal conductivity of 135 W/(m·K).

Embodiment 2

Compared with Embodiment 1, the aging time in step S3 is 36 hours, while all other conditions remain the same as in Embodiment 1. After low-temperature aging, the alloy achieves a stress strength up to 416 MPa, a yield strength of 352 MPa, an elongation of 12%, and a thermal conductivity of 125 W/(m·K).

Embodiment 3

Compared with Embodiment 1, the aging time in step S3 is 48 hours, while all other conditions remain the same as in Embodiment 1. After low-temperature aging, the alloy achieves a stress strength up to 422 MPa, a yield strength of 338 MPa, an elongation of 14%, and a thermal conductivity of 127 W/(m·K).

The embodiments described above represent only a portion of the embodiments of the present disclosure, and not all possible embodiments. The detailed description of the embodiments of the present disclosure is not intended to limit the scope of the present disclosure claimed but merely represents selected embodiments of the present disclosure. Based on the embodiments of the present disclosure, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present disclosure.