A method for selective rolling and air-blowing forming of 7B04 aluminum alloy ribbed wall plate

Abstract

This invention relates to the field of metal plastic forming technology, specifically disclosing a method for selective rolling and gas expansion forming of 7B04 aluminum alloy ribbed wall panels. The method includes the following steps: preparing two 7B04 aluminum alloy plates, determining the rolling area based on the rib distribution, and performing solution treatment; stacking the plates and spraying a weld inhibitor on the rib area of ​​the contact surface; performing localized multi-pass rolling on the rolling area in a rolling die, with a total reduction of 40%-60%, to achieve pre-forming; transferring the pre-formed double-layer plates to a gas expansion forming die, sealing the die, heating to 400-500℃ and holding at that temperature, and filling the sealed cavity between the plates with high-pressure inert gas, loading at a rate of 0.05-0.2 MPa / min to 3-5 MPa and holding the pressure to achieve plate forming; and performing solution treatment and artificial aging on the formed wall panel to obtain T6 state mechanical properties. This method effectively reduces the overall deformation of subsequent gas expansion forming through selective rolling pre-forming, enabling precise forming of the integral ribbed wall panel while ensuring good dimensional accuracy and comprehensive mechanical properties of the component.

Description

Technical Field

[0001] This invention relates to the field of metal plastic forming technology, and in particular to a method for selective rolling and gas expansion forming of 7B04 aluminum alloy ribbed wall panels. Background Technology

[0002] Ribbed panel structures possess advantages such as high specific strength, high specific stiffness, strong buckling resistance, and significant weight reduction, and are widely used in critical load-bearing components such as fuselage and wing panels of aerospace vehicles, as well as fuel tanks of launch vehicles. Traditional methods for manufacturing rigid panels mainly include mechanical milling, riveting, and welding. However, these methods suffer from problems such as low material utilization, long production cycles, and poor structural integrity. Furthermore, welds and rivet holes can become stress concentration sources, reducing the fatigue life of components and failing to meet the urgent needs of modern aerospace for large, integrated, lightweight, and high-performance components.

[0003] Integral forming technology is a crucial approach to achieving the monolithic manufacturing of ribbed paneling. Superplastic forming / diffusion bonding technology enables the one-time integral forming of complex-shaped ribbed paneling. However, 7B04 aluminum alloy is a high-strength aluminum alloy with poor superplasticity, making it difficult to achieve the monolithic forming of large-sized ribbed paneling using traditional superplastic forming methods. Furthermore, during the forming process of ribbed paneling, problems such as uneven thickness reduction, incomplete rib filling, and poor surface quality easily occur in the rib and skin rib areas, severely affecting the load-bearing capacity and service life of the component.

[0004] Selective rolling with air expansion forming is a novel integral forming process that combines localized rolling pre-forming with subsequent air expansion forming to achieve the integral forming of high-strength aluminum alloy ribbed panels under non-superplastic conditions. This process first uses selective rolling to form pre-formed ribs in localized areas of the sheet metal, then uses air expansion forming to further deform the pre-formed areas and fit them into the mold cavity, ultimately obtaining an integral panel with a complex ribbed structure. However, key issues such as the matching of rolling and air expansion parameters, mold cavity sealing, and air expansion pressure control have not yet been systematically resolved, hindering the engineering application of this technology.

[0005] Therefore, developing a selective rolling gas expansion forming method suitable for 7B04 aluminum alloy stiffened wall panels, and solving key technical problems such as matching rolling and gas expansion process parameters, mold sealing, and precise control of gas expansion pressure, is of great significance for promoting the development of 7B04 aluminum alloy integral wall panel manufacturing technology. Summary of the Invention

[0006] The purpose of this invention is to provide a selective rolling gas expansion forming method for 7B04 aluminum alloy reinforced wall panels, in order to solve the above-mentioned technical problems existing in the prior art.

[0007] To achieve the above objectives, the present invention provides the following solution: a method for selective rolling and gas expansion forming of 7B04 aluminum alloy stiffened panel, comprising the following steps: S1. Prepare two 7B04 aluminum alloy plates. Based on the rib distribution of the target ribbed wall panel, determine the rolling area and rib area, and perform solution treatment on the 7B04 aluminum alloy plates. S2. Stack two 7B04 aluminum alloy plates after solution treatment, spray anti-weld agent on the rib area of ​​the contact surface, and place them in the rolling die. Perform local rolling on the rolling area at the preset rolling temperature, using multi-pass rolling, with a total reduction of 40%-60%. S3. Transfer the double-layer sheet material that has been pre-formed by selective rolling to the air-expansion forming mold and seal the mold. Heat the entire mold to 400-500℃ and keep it at that temperature. Inject high-pressure inert gas into the sealed cavity between the double-layer sheet material and apply the gas pressure to 3-5MPa at a rate of 0.05-0.2MPa / min and then hold the pressure to make the sheet material fit into the mold cavity. S4. The formed stiffened wall panel is subjected to solution treatment and artificial aging treatment to obtain T6 state mechanical properties.

[0008] Optionally, in step S1, the width of the rolling zone is designed to be 1.5-2.0 times the final rib width.

[0009] Optionally, in step S1, the rolled area and the unrolled area are transitioned by a rounded arc with a radius of 3-5 mm.

[0010] Optionally, in step S1, the solution treatment process is as follows: after holding at 465-475℃ for 1-2 hours, water quenching is performed, and the water quenching transfer time does not exceed 15 seconds.

[0011] Optionally, in step S2, the rolling area is partially rolled at a rolling temperature of 350-400°C, and the rolling speed is 5-15 mm / s.

[0012] Optionally, in step S2, when multi-pass rolling is used to partially roll the rolling area, the reduction per pass is 20%-40%, and the furnace is kept warm for 10-20 minutes after each pass rolling.

[0013] Optionally, in step S3, the heating rate of the mold is 5-10℃ / min.

[0014] Optionally, in step S4, the solution treatment is carried out at 465-475℃ for 1-1.5h followed by water quenching, and the artificial aging treatment is carried out at 120-140℃ for 24-36h.

[0015] Optionally, after step S4, a surface treatment step is also included: shot peening the formed ribbed wall panel to make its surface roughness Ra≤1.6μm, and then anodizing or chemical oxidation to form an oxide film with a thickness of 5-15μm.

[0016] Optionally, the thickness of the two 7B04 aluminum alloy plates is 1.5-3.0 mm; in step S2, after selective rolling, the thickness of a single layer of plate in the rib area corresponding to the rolling area is 0.8-1.2 mm.

[0017] Furthermore, in step S3, a sealing groove is provided around the cavity of the air-forming mold, and a silicone rubber sealing ring is placed in the groove to achieve sealing through mechanical or hydraulic clamping force.

[0018] Furthermore, in step S3, the pressure holding time is 30-90 minutes.

[0019] Furthermore, in step S3, a limiting block or local constraint structure is provided between the double-layer plates to coordinate the deformation behavior of the rib area and the skin area.

[0020] Furthermore, in step S3, the high-pressure inert gas is argon or nitrogen.

[0021] Furthermore, the upper die of the rolling die is provided with a boss structure corresponding to the shape of the rolling area.

[0022] Furthermore, the clamping force of the air-inflating mold is 300-800kN.

[0023] Compared with the prior art, the present invention discloses at least the following beneficial effects: This invention directly improves the overall formability and structural quality of 7B04 aluminum alloy ribbed panels through a composite process combining selective rolling preforming and gas forming. The selective rolling process pre-thinns and tightly adheres the ribbed areas of the sheet metal, effectively reducing the overall deformation required for subsequent gas forming. This allows high-strength aluminum alloy in a non-superplastic state to achieve precise overall forming of complex ribbed structures, avoiding the limitations of traditional superplastic forming on large deformation requirements. The pre-formed ribbed profile formed by local rolling, combined with the uniform loading of high-pressure inert gas during gas forming, promotes full flow and adhesion of the material within the mold cavity, significantly improving the panel's dimensional accuracy and surface forming quality, while reducing defects such as incomplete rib filling and uneven thickness. The subsequent solution treatment and artificial aging further optimize the material's microstructure and properties, enabling the panel to achieve uniform and stable T6 mechanical properties. This method achieves seamless connection between the ribs and skin in a single forming process, with a smooth transition area, effectively reducing stress concentration and improving the fatigue life and structural reliability of the component. The overall process has technical advantages such as short process flow, high material utilization rate, good forming accuracy and strong adaptability, providing a reliable technical approach for the engineering manufacturing of high-strength aluminum alloy integral stiffened wall panels. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is a schematic diagram of the hollow reinforced structure of 7B04 aluminum alloy in this invention; Figure 2 This is a schematic diagram of the isolation zone for the preformed billet in the selective rolling process of this invention. Figure 3 This is a schematic diagram of the selective rolling process using a rolling die in this invention; Figure 4 This is a schematic diagram illustrating the process of heating and bending sheet metal using an air-expansion forming mold in this invention. Figure 5 This is a schematic diagram of the process of filling argon gas and gas expansion forming using a gas expansion forming mold in this invention.

[0026] Figure reference numerals: 1. 7B04 aluminum alloy sheet; 101. Rib area; 102. Rolling area; 2. Double-layer sheet; 3. Rolling die; 4. Air-forming die. Detailed Implementation

[0027] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0029] Reference Figures 1 to 5 As shown in the figure, this embodiment provides a selective rolling gas expansion forming method for 7B04 aluminum alloy stiffened panels. This method aims to solve the technical problems of difficult forming of high-strength aluminum alloy integral stiffened panels and poor adaptability of traditional superplastic forming processes. By combining selective rolling preforming and gas expansion forming, the integral and high-precision manufacturing of 7B04 aluminum alloy stiffened panels in non-superplastic state can be achieved.

[0030] The method includes the following steps: First, design the overall structure of the ribbed panel and the blanks required for pre-forming by selective rolling. The skin area needs to be determined based on the distribution of the ribs on the final panel. Specifically, based on the principle that the final dimensions and volume of the ribbed panel remain unchanged, calculate the thickness reduction and rolling width of the skin area. The thickness of the single-layer sheet in the rolled rib area 101 is typically designed to be 0.8-1.2 mm, while the width of the skin area is typically designed to be 1.5-2.0 times the rib width. In the transition area between the skin area and the unskinned area, a rounded transition design is adopted, with the radius of the rounded corner controlled at 3-5 mm to avoid stress concentration. Next, two 2 mm thick 7B04 aluminum alloy sheets 1 are solution treated at a temperature of 465-475℃, held for 1-2 hours, and then rapidly water-quenched. The water quenching transfer time should not exceed 15 seconds to obtain a microstructure suitable for subsequent plastic forming. Subsequently, the two solution-treated sheets are stacked, coated with a weld inhibitor, and placed in the rolling die 3. The upper die of rolling die 3 is equipped with bosses corresponding to the shape of the skin area, used for localized pressing of specific areas of the sheet metal. The rolling process is carried out at a temperature of 350-400℃, a rolling speed of 5-15mm / s, and multiple passes are used. The reduction per pass is controlled between 20% and 40%, and the total reduction reaches 40%-60%. After each pass, the sheet metal needs to be returned to the furnace for 10-20 minutes to ensure uniform temperature.

[0031] The double-layer sheet 2, after selective rolling pre-forming, is transferred to the air-expansion forming mold 4 for the next forming step. The air-expansion forming mold 4 consists of an upper mold and a lower mold, which form a sealed cavity after closing. After mold closing, high-pressure inert gas, such as argon or nitrogen, is injected into the gap between the double-layer sheet 2 through the air inlet pipe. The gas pressure is slowly increased to 3-5 MPa at a rate of 0.05-0.2 MPa / min and then held, causing the double-layer sheet 2 to undergo plastic deformation under the action of gas pressure, gradually fitting into the cavity of the lower mold, thereby forming the preset rib structure. The air-expansion forming temperature is set in the range of 400-500℃, with a heating rate of 5-10℃ / min. After reaching the set temperature, it needs to be held for 20-40 minutes to ensure that the material is fully softened. The holding time is usually 30-90 minutes, depending on the sheet thickness and structural complexity, to ensure that the sheet completely fits into the cavity and completes the necessary creep deformation.

[0032] Based on the above embodiments, further, during the air-forming process, a limiting block or other local constraint means can be set between the double-layer plates 2 to coordinate the deformation behavior of the rib area 101 and the skin area, prevent the rib area 101 from being excessively thinned due to excessive deformation, and also avoid undesirable excessive deformation of the skin area, thereby ensuring the uniformity of the overall wall panel thickness and the structural integrity.

[0033] Based on the above embodiments, further, after the air-expansion molding is completed, the molded ribbed wall panel is removed from the mold and subjected to heat treatment to strengthen it and obtain the required mechanical properties.

[0034] The heat treatment strengthening process includes: First, solution treatment at 465-475℃, holding for 1-1.5 hours followed by water quenching; then, artificial aging within 24 hours after solution treatment at 120-140℃ for 24-36 hours to obtain the mechanical properties of the T6 state. Finally, the formed stiffened panel undergoes surface treatment, including shot peening to control the surface roughness to Ra≤1.6μm, followed by anodizing or chemical oxidation to form an oxide film with a thickness of 5-15μm to improve its surface corrosion resistance.

[0035] This embodiment utilizes a composite process combining selective rolling preforming and gas forming, effectively reducing the material deformation requirements of subsequent gas forming. This allows high-strength aluminum alloys like 7B04 to achieve integral forming of stiffened panels even under non-superplastic conditions. The upper and lower mold closing and sealing method employed is as follows: Figure 4 and Figure 5As shown, the sealing groove structure ensures the airtightness of the cavity, avoiding the leakage problems of traditional sealing methods. The entire process parameters are highly controllable; by adjusting key parameters such as rolling reduction, air expansion pressure, and forming temperature, it can adapt to the forming requirements of rib structures with different heights, spacings, and shapes, exhibiting good process adaptability. This method can obtain ribbed panels in a single forming process, resulting in high production efficiency, high material utilization, and a smooth transition between the formed ribs and skin, which helps improve the fatigue life and reliability of the components.

[0036] Example 1 In one specific embodiment, two 2mm thick 7B04 aluminum alloy plates 1 are selected to prepare a unidirectional, equally spaced straight-ribbed wall panel. The rib height of the ribbed wall panel is 6mm, the rib width is 10mm, and the rib spacing is 60mm. The specific steps of the selective rolling gas expansion forming method for the 7B04 aluminum alloy plate 1 as the plate material include: S1. Blank design and plate pretreatment.

[0037] First, the blank design is carried out according to the structure and dimensions of the target ribbed panel. Specifically, two 2mm thick 7B04 aluminum alloy sheets are prepared.

[0038] Reference Figure 2 As shown, based on the final rib distribution of the ribbed panel, the area requiring pre-forming by rolling is determined. This area will subsequently form the ribbed region 101 of the ribbed panel. Calculations are performed based on the principle of constant volume to ensure that the thickness of a single layer of sheet material in the ribbed region 101 is controlled to 1.0 mm after rolling, while the unrolled rolled region 102 (i.e., the skin region) retains its original thickness. The width of the skin region is designed to be 1.5 times the final rib width, and a rounded transition with a radius of 4 mm is designed between the skin region and the rolled region 102 to avoid stress concentration during subsequent deformation.

[0039] Subsequently, the two 7B04 aluminum alloy plates were subjected to solution treatment at a temperature of 470℃ for 1.5 hours. After solution treatment, they were quickly quenched in water for 10 seconds to obtain a supersaturated solid solution structure with good plasticity, thus creating conditions for subsequent plastic forming.

[0040] S2, Selective rolling preforming.

[0041] Two 7B04 aluminum alloy plates 1 that have undergone the above solution treatment and water quenching are stacked together, and anti-weld agent is sprayed on their contact surfaces to reduce friction.

[0042] Reference Figure 3As shown, the stacked plates are placed in a dedicated rolling die 3. This rolling die 3 consists of two 10mm diameter rolling mills, with the upper die designed with a boss structure corresponding to the shape of the skinned area determined in step S1. The laminated plates are locally rolled at a rolling temperature of 380℃ and a rolling speed of 10mm / s. The rolling process uses two passes: the first pass has a reduction of 0.8mm, the second pass has a reduction of 1.2mm, and the total reduction is 2mm. After each pass, the plates are held in the furnace for 15 minutes. After this selective rolling process, at the position corresponding to the future ribs, the two plates are locally thinned and tightly bonded, forming a pre-formed blank with a preset thickness difference, i.e., the double-layer plate 2. The thinned area forms the prototype of the rib area 101, while the unskinned area is the rolled area 102.

[0043] S3, air-inflated molding.

[0044] The double-layer sheet 2, after selective rolling preforming, is transferred to the air-expansion forming mold 4. (Refer to...) Figure 4 and Figure 5 The air-inflating forming mold 4 consists of an upper mold and a lower mold. Its cavity is precisely machined according to the shape of the final ribbed wall plate. The mold material can be medium silicon molybdenum ductile iron. Silicon rubber sealing rings are placed in the sealing groove around the mold cavity. The clamping force is 500kN.

[0045] Place the double-layer sheet 2 on the lower mold, aligning its edges with the mold edges, and ensuring that the gap between the double-layer sheet 2 is connected to the air inlet pipe on the mold. Then close the upper mold, using mechanical or hydraulic clamping force to press and seal the upper and lower molds. After mold closing, heat the entire mold to the air expansion forming temperature, specifically 475℃, at a heating rate of 8℃ / min. After reaching the set temperature, hold it at that temperature for 30 minutes to allow the double-layer sheet 2 to fully soften.

[0046] After the heat preservation is completed, argon gas is injected into the sealed cavity between the double-layer plates 2 through the air inlet pipe. The gas pressure is slowly increased to 4 MPa at a rate of 0.1 MPa / min and then held for 60 minutes. Under the action of gas pressure, the double-layer plates 2 undergo plastic deformation and gradually fit into the lower mold cavity. The rib area 101 is further expanded to form the preset rib structure, while the rolling area 102 forms the skin of the wall panel.

[0047] Building upon the above embodiments, further in the air-forming step S3, to more precisely control the deformation coordination between the rib region 101 (rib) and the rolling region 102 (skin), and to avoid excessive thinning of the rib region 101 or wrinkling of the skin region, limiting blocks or other forms of local constraints can be pre-set at specific locations between the double-layer plates 2. This measure can guide pressure distribution and material flow, thereby optimizing the overall thickness uniformity and structural integrity of the wall panel.

[0048] S4, Heat treatment strengthening.

[0049] After air forming is completed, the formed ribbed panel is removed from the air forming mold 4 and immediately subjected to heat treatment to restore and improve its mechanical properties. The heat treatment strengthening process includes: first, solution treatment at 470℃ for 1.2 hours, followed by water quenching; then, artificial aging treatment within 12 hours after solution treatment at 135℃ for 30 hours. Through this heat treatment, 7B04 aluminum alloy can obtain excellent mechanical properties in the T6 condition.

[0050] S5. Surface treatment.

[0051] Finally, the heat-treated stiffened panel undergoes surface treatment to improve its surface quality and corrosion resistance. First, the surface of the component is shot-peened to achieve a surface roughness Ra of 1.2 μm. Subsequently, anodizing or chemical oxidation is performed to form a dense oxide film with a thickness of 10 μm on the component surface, significantly improving its resistance to environmental corrosion.

[0052] This embodiment utilizes selective rolling pre-forming to pre-thin the ribbed region 101, significantly reducing the overall deformation required for subsequent gas expansion forming. This allows high-strength aluminum alloys like 7B04 to achieve integral forming of complex structures even under non-superplastic conditions. The use of upper and lower molds for closing and sealing with sealing rings effectively solves the cavity sealing problem, ensuring the stability of gas expansion pressure and the uniformity of forming quality. The entire process is clear, and the parameters are controllable. By adjusting the rolling reduction, gas expansion pressure, and forming temperature, it can adapt to the production needs of ribbed panels of different specifications.

[0053] In the description of this invention, it should be understood that the terms "longitudinal", "lateral", "up", "down", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this invention, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention.

[0054] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for selective rolling and gas expansion forming of 7B04 aluminum alloy reinforced wall panels, characterized in that, Includes the following steps: S1. Prepare two 7B04 aluminum alloy plates. Based on the rib distribution of the target ribbed wall panel, determine the rolling area and rib area, and perform solution treatment on the 7B04 aluminum alloy plates. S2. Stack two 7B04 aluminum alloy plates after solution treatment, spray anti-weld agent on the rib area of ​​the contact surface, and place them in the rolling die. Perform local rolling on the rolling area at the preset rolling temperature, using multi-pass rolling, with a total reduction of 40%-60%. S3. Transfer the double-layer sheet material that has been pre-formed by selective rolling to the air-expansion forming mold and seal the mold. Heat the entire mold to 400-500℃ and keep it at that temperature. Inject high-pressure inert gas into the sealed cavity between the double-layer sheet material and apply the gas pressure to 3-5MPa at a rate of 0.05-0.2MPa / min and then hold the pressure to make the sheet material fit into the mold cavity. S4. The formed stiffened wall panel is subjected to solution treatment and artificial aging treatment to obtain T6 state mechanical properties.

2. The method for selective rolling and gas expansion forming of 7B04 aluminum alloy reinforced wall panels according to claim 1, characterized in that, In step S1, the width of the rolling zone is designed to be 1.5-2.0 times the final rib width.

3. The method for selective rolling and gas expansion forming of 7B04 aluminum alloy reinforced wall panels according to claim 1, characterized in that, In step S1, the rolled area and the unrolled area are transitioned by a rounded arc with a radius of 3-5 mm.

4. The method for selective rolling and gas expansion forming of 7B04 aluminum alloy reinforced wall panels according to claim 1, characterized in that, In step S1, the solution treatment process is as follows: after holding at 465-475℃ for 1-2 hours, water quenching is performed, and the water quenching transfer time does not exceed 15 seconds.

5. The method for selective rolling and gas expansion forming of 7B04 aluminum alloy reinforced wall panels according to claim 1, characterized in that, In step S2, the rolling area is partially rolled at a rolling temperature of 350-400℃, and the rolling speed is 5-15 mm / s.

6. The method for selective rolling and gas expansion forming of 7B04 aluminum alloy reinforced wall panels according to claim 1, characterized in that, In step S2, when multi-pass rolling is used to locally roll the rolling area, the reduction per pass is 20%-40%, and the furnace is kept warm for 10-20 minutes after each pass rolling.

7. The method for selective rolling and gas expansion forming of 7B04 aluminum alloy reinforced wall panels according to claim 1, characterized in that, In step S3, the heating rate of the mold is 5-10℃ / min.

8. The method for selective rolling and gas expansion forming of 7B04 aluminum alloy reinforced wall panels according to claim 1, characterized in that, In step S4, the solution treatment is carried out at 465-475℃ for 1-1.5h followed by water quenching, and the artificial aging treatment is carried out at 120-140℃ for 24-36h.

9. The method for selective rolling and gas expansion forming of 7B04 aluminum alloy reinforced wall panels according to claim 1, characterized in that, After step S4, a surface treatment step is also included: the formed stiffened wall panel is shot peened to make its surface roughness Ra≤1.6μm, and then anodized or chemically oxidized to form an oxide film with a thickness of 5-15μm.

10. The method for selective rolling and gas expansion forming of 7B04 aluminum alloy reinforced wall panels according to claim 1, characterized in that, The thickness of the two 7B04 aluminum alloy plates is 1.5-3.0 mm; in step S2, after selective rolling, the thickness of a single layer of plate in the rib area corresponding to the rolling area is 0.8-1.2 mm.

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