CN113770504B

CN113770504B - Double-body type cavity filling metal/FRP friction stir welding connection method

Info

Publication number: CN113770504B
Application number: CN202111089365.2A
Authority: CN
Inventors: 周利; 刘煜纯; 牛得田; 宋晓国; 王苹; 周宝升; 张涛
Original assignee: National High Speed Train Qingdao Technology Innovation Center
Current assignee: National High Speed Train Qingdao Technology Innovation Center
Priority date: 2021-09-16
Filing date: 2021-09-16
Publication date: 2022-10-14
Anticipated expiration: 2041-09-16
Also published as: CN113770504A

Abstract

The invention provides a double-body type cavity filling metal/FRP friction stir welding connecting method, which solves the technical problems of low bonding strength and small tensile load caused by material loss at an interface because an FRP material is heated to flow and is extruded out of the interface when the metal and the FRP material are connected by friction stir lap welding. The invention can be widely applied to the technical field of welding.

Description

Double-body type cavity filling metal/FRP friction stir welding connection method

Technical Field

The application belongs to the technical field of welding methods, and particularly relates to a double-body type cavity filling metal/FRP friction stir welding connecting method.

Background

With the progress of science and technology and the continuous development of material industry, the concept of light structural design is produced in the fields of vehicle engineering, rail transit, aerospace and the like. FRP (fiber Reinforced Polymer) is a material in which a Polymer material is used as a matrix and carbon fiber or glass fiber is added to the matrix to enhance the mechanical properties of the material, and has excellent properties such as light weight, high specific strength, good corrosion resistance, and good heat resistance. The FRP material partially replaces traditional materials such as aluminum and steel, the advantages of light weight and light weight of the FRP material, high strength and good toughness of metal materials can be fully exerted, the quality of a structural part is further reduced on the premise of ensuring excellent performance, and the design concept of light structure is realized. However, since the FRP substrate is a polymer material, it is relatively difficult to directly connect the FRP and the metal material. The traditional method for connecting FRP and metal material mainly comprises the following steps: gluing and mechanical bonding; the metal/FRP bonding joint is greatly influenced by the temperature and the humidity of the environment, the general bonding treatment time is long, and the adhesive component has certain toxicity and has large influence on the environment. The mechanical connection mainly comprises: riveting and bolting, the bolting often requires holes to be prefabricated on the FRP substrate, and the continuity of the fiber inside the FRP is easily damaged during the drilling process. In addition, stress concentration is easily caused at the rivet and the screw of riveting and bolt connection, and joint breakage is easily caused under high stress conditions. The soldering technique is a bonding method for forming molecular or atomic bonds between materials by applying heat, pressure or a combination of the two. At present, for metal and FRP materials, the connection is carried out by a method of resistance welding, laser welding, friction stir welding and the like by a scholarly. Particularly, in the process of friction stir welding connection, the thermoplastic resin material is softened by heat and has enhanced fluidity, and under the action of certain welding tool pressure, the resin material at the interface position is easily heated to extrude a welding seam, so that on one hand, the material loss is caused, and on the other hand, the softened resin material is difficult to provide enough supporting force for metal/FRP, so that the bonding strength of the friction stir welding metal/FRP interface is influenced.

Chinese invention patent application no: CN201610808910.1: a method for connecting metal material and resin-based composite material by friction stir welding is disclosed, which adopts a shaft shoulder/stirring pin integrated friction stir welding tool, adopts a mode that a metal plate is arranged on the upper part and the resin-based composite material is arranged on the lower part for welding, and a stirring pin is pricked into the metal plate and does not enter an FRP material, thereby primarily realizing the connection of metal and FRP. However, the interface position still has the phenomenon that the resin material is extruded due to the action of heat and pressure, so that the interface connection strength is limited.

Chinese invention patent application no: CN201910225037.7 discloses a friction stir welding method suitable for use between metal and polymer, which adopts a static shaft shoulder and is matched with an auxiliary heat source to improve the joint quality of FSLW, and has certain reference significance. However, the scheme is only suitable for clamping the FRP plate material on the upper side and the metal material on the lower side, an auxiliary heat source device is required to be externally connected, the heating temperature of the shaft shoulder can be obtained only through indirect measurement, real-time and quantitative control through a welding machine is difficult, and certain limitation exists.

Chinese patent publication No.: CN113020776A discloses a stirring head for welding metal and thermoplastic polymer and a welding method, wherein a shaft shoulder/stirring needle integrated friction stir welding tool is adopted, a multi-stage flow-resisting circular truncated cone is added on the side edge of the stirring needle, so that the action of polymer flowing upwards to the surface of a welding seam is relieved or even avoided, the surface forming quality of metal and thermoplastic polymer FSLW is improved, an auxiliary heat source is adopted for heating to reduce the residual stress of the welding seam area, and the mechanical property of a metal/FRP joint is improved to a certain extent. However, the length of the stirring pin is fixed relative to the shaft shoulder, the adaptability of the stirring pin to metal plates with different thicknesses is limited, and an external auxiliary heat source is difficult to control in real time and quantitatively.

Therefore, how to avoid the extrusion of the interface resin material in the friction stir welding process of the metal material and the FRP plate, and the formation of a macroscopic mechanical interlocking structure at the interface position to enhance the interface bonding strength, and meanwhile, the preheating process of the metal surface can be controlled in real time and quantitatively, and can be suitable for the welding between the metal and the FRP plate with different thicknesses in a certain range, which becomes the technical bottleneck to be broken through by the invention.

Disclosure of Invention

The invention aims to solve the defects of the background technology, and provides a metal/FRP friction stir welding connection method which adopts a stirring pin and a shaft shoulder split design, has respectively controllable rotating speed, is matched with a friction stir welding main shaft, drives an interface to form a cavity structure through a stirring pin thread, and fills a resin-based material into the cavity by matching with the pressure of a welding tool so as to realize the connection of metal/FRP plates under the condition of different plate thicknesses.

Therefore, the invention provides a double-body type cavity filling metal/FRP friction stir welding connection method, which specifically comprises the following steps:

(1) Cleaning before welding, polishing by using sand paper, cleaning a plate to be welded by using an organic solvent or a cleaning agent, and removing surface oxides and oil stains, wherein the plate to be welded is a metal plate and an FRP plate;

(2) Clamping and fixing the metal plate and the FRP plate by using a clamp to enable butt joint surfaces of the plates to be welded to be close to each other, and adopting a configuration principle that the metal plate is positioned at an upper layer and the FRP plate is positioned at a lower layer;

(3) The method comprises the following steps that a connection mode that a split type friction stir welding tool is matched with a friction stir welding main shaft is adopted, the split type friction stir welding tool is provided with a stirring pin and a shaft shoulder, the stirring pin is matched with an inner clamping ring of a first friction stir welding main shaft, the shaft shoulder is matched and fixed with an outer clamping ring of a second friction stir welding main shaft, and the stirring pin with the inner clamping ring is matched with the shaft shoulder with the outer clamping ring;

(4) Positioning the head of the stirring pin in the step (3) at the initial position of the welding seam of the plate to be welded, simultaneously starting a first stirring friction welding main shaft and a second stirring friction welding main shaft, and adjusting the extending length of the stirring pin relative to the shaft shoulder to ensure that the stirring pin head is just contacted with the upper surface of the metal plate;

(5) Adjusting the rotating speed of the stirring needle and the shaft shoulder, rotating the stirring needle and the shaft shoulder in the same direction and pressing downwards simultaneously, adjusting the rotating speed of the stirring needle to be lower than the rotating speed of the shaft shoulder after the head of the stirring needle is just contacted with the upper surface of the FRP plate, moving the shaft shoulder and the stirring needle along the direction of a welding line for welding simultaneously, adjusting the rotating speed of the stirring needle after the head of the stirring needle reaches the tail end of the welding line, rotating the stirring needle in the opposite direction, and rotating the stirring needle and the shaft shoulder to move upwards simultaneously to obtain a metal/FRP composite component;

(6) And (3) cooling: and (3) placing the metal/FRP composite member in air to cool to room temperature.

Preferably, in step (1), the organic solvent is alcohol or acetone.

Preferably, in the step (3), the inner clamping ring is provided with a first clamping cavity matched with the tail of the stirring pin, the head of the stirring pin is provided with a threaded structure, the outer clamping ring is provided with a second clamping cavity matched with the shaft shoulder, the outer clamping ring is further provided with a containing cavity matched with the stirring pin with the inner clamping ring.

Preferably, the part of the tail part of the stirring pin, which is matched with the inner clamping ring, and the part of the shaft shoulder, which is matched with the outer clamping ring, are provided with trimming planes, which are used for being respectively propped and fixed with screws of the inner clamping ring and the outer clamping ring.

Preferably, in the step (4), the extension length of the stirring pin relative to the shaft shoulder is 1.6-1.8 mm.

Preferably, in the step (5), the rotating speeds of the shaft shoulder and the stirring pin in the pressing process are both 500-1500 rpm, the pressing amount is 0.2-0.8 mm, and the inclination angle of the split friction welding set is 0.5-1.5 degrees.

Preferably, in the step (5), after the head of the stirring pin just contacts the upper surface of the FRP plate, the rotating speed of the stirring pin is adjusted to 600-800 rpm.

Preferably, in the step (5), the rotating speed of the stirring pin moving upwards in the reverse rotation direction is 500-1500 rpm.

Preferably, in the step (5), the welding speed is 1000 to 3000mm/min.

Preferably, in the step (6), the air humidity is 20 to 30%.

The invention has the beneficial effects that:

(1) In the double-body type cavity filling metal/FRP friction stir welding connection method, the design of the welding tool with the stirring pin and the shaft shoulder in a split mode is adopted, the friction stir welding main shaft is matched, the height of the clamping ring in the stirring is adjusted through the first friction stir welding main shaft, and the extending length of the stirring pin relative to the shaft shoulder is further adjusted, so that the friction stir welding tool can be suitable for welding metal/FRP plates with different thicknesses, and the adaptability to different plate thicknesses is improved.

(2) In the double-body cavity filling metal/FRP friction stir welding connection method, the lower surface of the shaft shoulder is concave, so that the loss of aluminum alloy materials is reduced, downward and inward pressure is provided for metal plates in the welding process, and the rotation speed of the shaft shoulder is adjusted through the second friction stir welding main shaft, so that the real-time and quantitative control on interface heat can be realized, and the applicability to welding materials is further improved.

(3) In the double-body type cavity filling metal/FRP friction stir welding connection method, the side surface of the stirring pin is designed with a spiral structure which is matched with the rotation direction of the stirring pin, and the metal material at the interface position can be promoted to flow upwards in the welding process. Therefore, a temporary cavity structure is formed at the metal/FRP interface, the FRP matrix is extruded and filled into the cavity structure under the action of the pressure of the shaft shoulder, so that the extrusion overflow of materials is reduced, a macroscopic mechanical interlocking structure is formed, and the connection strength of the joint is enhanced.

(4) In the double-body type cavity filling metal/FRP friction stir welding connection method, the rotation direction of the stirring pin and the shaft shoulder can be controlled by the main shaft of the friction stir welding, and the rotation direction of the stirring pin promotes the metal material to flow upwards to form a cavity in the pressing stage and the welding stage of the stirring pin; after welding is finished, the stirring pin can reversely rotate to promote the metal material to flow downwards in the stirring pin drawing stage, certain extrusion force can be provided for the interface position, and interface combination of the tail end of the welding seam is promoted.

Drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required to be used in the embodiments or the prior art description will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present application, and for those skilled in the art, other drawings may be obtained according to these drawings without inventive labor.

FIG. 1 is a schematic view of the overall structure of the split friction stir welding tool of the present invention in cooperation with inner and outer clamping rings;

FIG. 2 is an exploded view of the split friction stir welding tool and inner and outer clamp rings of the present invention;

FIG. 3 is a schematic structural view of the stirring pin of the present invention;

FIG. 4 is a schematic view of the welding process at the push down stage of the present invention;

fig. 5 is a schematic view of the welding process at the withdrawal stage of the present invention.

The symbols in the figures illustrate:

1. a metal plate material; 2, FRP plate; 3. an inner clamping ring; 4. an outer clamp ring; 5. a first clamping cavity; 6. a second clamping cavity; 7. an accommodating chamber; 8. a stirring pin; 9. a shaft shoulder; 10. a thread structure; 11. and (5) trimming a plane.

Detailed Description

In order to make the technical problems, technical solutions and advantageous effects to be solved by the present application clearer, the present application is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of and not restrictive on the broad application.

The invention provides a double-body type cavity filling metal/FRP friction stir welding connection method, which comprises the following specific steps:

(1) Cleaning before welding, polishing by using 600-1500-mesh abrasive paper, cleaning and wiping the metal plate 1 by using acetone, cleaning and wiping the FRP plate 2 by using alcohol, and respectively removing oxides and oil stains on the surfaces of the metal plate 1 and the FRP plate 2, wherein the metal material is aluminum alloy, magnesium alloy, titanium alloy, low-carbon steel or stainless steel; the fiber material in the FRP plate 2 is carbon fiber or glass fiber.

(2) The metal plate 1 and the FRP plate 2 are clamped and fixed by using the clamp, the clamping form adopts a lap joint form, and even if butt joint surfaces of the plates to be welded are close to each other, the configuration principle that the metal plate is positioned on the upper layer and the FRP plate is positioned on the lower layer is adopted.

(3) Adopt split type friction stir welding utensil cooperation friction stir welding main shaft's connected mode, as shown in fig. 1, split type friction stir welding utensil is equipped with stirring pin 8 and shaft shoulder 9, for the friction stir welding main shaft that for the convenience stirring pin 8 and shaft shoulder 9 dispose rather than mutually respectively is connected, and stirring pin 8 total length is 60mm ~ 80mm, and shaft shoulder 9 total length is 45mm ~ 70mm. As shown in fig. 3, the head of the stirring pin 8 is tapered and provided with a screw structure 10. The lower surface of the shaft shoulder 9 is concave, so that the loss of aluminum alloy materials is reduced, and downward and inward pressure is provided for the metal plate 1 in the welding process. The friction stir welding main shaft is equipped with two, is first friction stir welding main shaft and second friction stir welding main shaft respectively, and first friction stir welding main shaft is equipped with interior grip ring 3, and interior grip ring 3 and stirring needle 8 cooperation are connected, and the second friction stir welding main shaft is equipped with outer grip ring 4, and outer grip ring 4 and shaft shoulder 9 cooperation are connected.

Specifically, as shown in fig. 2, the inner clamping ring 3 is provided with a first clamping cavity 5, and the first clamping cavity 5 is of a rectangular parallelepiped structure and is used for being matched and connected with the tail part of the stirring pin 8 and fastened through a screw; the outer clamping ring 4 is provided with a second clamping cavity 6, and the second clamping cavity 6 is of a concave structure and is used for being matched and connected with the shaft shoulder 9 and fastened through screws. Wherein, the matching part of the tail part of the stirring pin 8 and the first clamping cavity 5 and the matching part of the shaft shoulder 9 and the second clamping cavity 6 are provided with tangent plane planes 11 which are respectively used for the screw top fixation of the inner clamping ring 3 and the outer clamping ring 4.

Further, outer clamping ring 4 still is equipped with and holds chamber 7, holds chamber 7 and is the shape of falling the dogbone for hold the stirring needle 8 that has interior clamping ring 3, with stirring needle 8 and shaft shoulder 9 cooperation connection.

(4) And (3) simultaneously starting welding to enable the first friction stir welding main shaft and the second friction stir welding main shaft to rotate, adjusting the shaft shoulder 9 and the stirring pin 8 to enable the shaft shoulder and the stirring pin 8 to freely move up and down in a limiting range, and then positioning the head of the stirring pin 8 in the step (3) at the initial position of the welding seam of the plate to be welded. Through the height of the inner clamping ring 3, the extension length of the stirring pin 8 relative to the shaft shoulder 9 is adjusted, so that the split type friction stir welding tool can be suitable for welding between metal/FRP plates with different thicknesses, and the adaptability to different plate thicknesses is improved. Specifically, the extension length of the stirring pin 8 relative to the shaft shoulder 9 is 1.6-1.8 mm. Then, the height between the stirring pins 8 and the metal plate 1 is adjusted so that the head portions of the stirring pins 8 just contact the upper surface of the metal plate 1.

(5) The rotating speeds of the stirring pin 8 and the shaft shoulder 9 are both adjusted to be 500-1500 rpm, and under the drive of the first friction stir welding main shaft and the second friction stir welding main shaft, as shown in figure 4, the stirring pin 8 and the shaft shoulder 9 rotate in the same direction and are pressed downwards, and the pressing quantity is 0.2-0.8 mm. And after the head of the stirring pin 8 just contacts the upper surface of the FRP plate 2, adjusting the rotating speed of the stirring pin 8 to be lower than that of the shaft shoulder 9. This is because the additional heat generation at the shoulder 9 portion is mainly used to bring the metal plate material 1 into a thermoplastic state and to bring the FRP matrix into a plastic flow state at the interface position by heat conduction. The pin 8 is mainly responsible for stirring the material at the interface location and forming a temporary cavity, and is not responsible for mainly generating heat, so the rotational speed of the pin 8 is adjusted to 600-800 rpm. At the moment, the shaft shoulder 9 and the stirring pin 8 are simultaneously welded in a moving mode along the direction of a welding line, the welding speed is 1000-3000 mm/min, the head of the stirring pin 8 is provided with the spiral structure 10 which is matched with the rotation direction of the stirring pin 8 and can promote metal materials at the interface position to flow upwards, so that a temporary cavity structure is formed at a metal/FRP interface, the FRP matrix is extruded and filled into the cavity structure under the pressure action of the shaft shoulder 9, the extrusion overflow of the materials is reduced, a macroscopic mechanical interlocking structure is formed, and the connection strength of the joint is enhanced. After the welding seam reaches the tail end, the rotating speed of the stirring pin 8 is adjusted to be 500-1500 rpm, as shown in fig. 5, the stirring pin 8 rotates reversely, moves upwards together with the shaft shoulder 9, and in the process of reverse rotation, the metal material is promoted to flow downwards, certain extrusion force can be provided for the interface position, the interface combination of the tail end of the welding seam is promoted, and finally the metal/FRP composite member is obtained.

Furthermore, in order to reduce the resistance of the material at the front end of the head of the stirring pin 8 in the welding process and reduce the flash, the inclination angle of the split type friction welding set is 0.5-1.5 degrees.

(6) And (3) cooling: and (3) cooling the welded metal/FRP composite member in the air, controlling the humidity of the air to be 20% -30% so as to prevent a part of the FRP resin matrix from absorbing moisture at high temperature, and cooling to solidify the interface resin material to complete the whole welding process.

Example 2

A double-body type cavity filling metal/FRP friction stir welding connection method comprises the following specific steps of:

(1) Preparing an aluminum alloy plate with the thickness of 2mm and a carbon fiber reinforced PA66 plate with the thickness of 3mm, polishing by using 600-mesh, 1000-mesh and 1500-mesh abrasive paper, cleaning and wiping the aluminum alloy plate by using acetone, cleaning and wiping the carbon fiber reinforced PA66 plate by using alcohol, and respectively removing oxides and oil stains on the surfaces of the aluminum alloy plate and the carbon fiber reinforced PA66 plate.

(2) And (3) clamping and fixing the aluminum alloy plate and the carbon fiber reinforced PA66 plate under the matching of the pressing plate 3.

(3) Selecting a stirring pin 8 with the length of 60-80 mm and a first friction stir welding main shaft configured with the stirring pin 8, selecting a shaft shoulder 9 with the length of 45-70 mm and a second friction stir welding main shaft configured with the shaft shoulder 9, mounting the stirring pin 8 and an inner clamping ring 3 of the first friction stir welding main shaft in a matched manner, mounting the shaft shoulder 9 and an outer clamping ring 4 of the second friction stir welding main shaft in a matched manner, and respectively jacking and fixing by using screws.

(4) Then, the welding machine is started simultaneously to enable the first friction stir welding main shaft and the second friction stir welding main shaft to start rotating, the shaft shoulder 9 and the stirring pin 8 are adjusted to enable the shaft shoulder 9 and the stirring pin 8 to freely move up and down in a limiting range, and the extending length of the stirring pin 8 relative to the shaft shoulder 9 is adjusted to be 1.6-1.8 mm. Then, the height between the stirring pin 8 and the aluminum alloy plate is adjusted so that the head of the stirring pin 8 just contacts with the upper surface of the aluminum alloy plate.

(5) Under the drive of the first friction stir welding main shaft and the second friction stir welding main shaft, the stirring pin 8 and the shaft shoulder 9 rotate in the same direction and press down simultaneously, the rotating speed of the shaft shoulder and the stirring pin 8 in the pressing down process is adjusted to be 800-1200 rpm, and the pressing down amount is 0.2-0.8 mm. After the head of the stirring pin 8 just contacts the upper surface of the carbon fiber reinforced PA66 plate, keeping the rotating speed of the shaft shoulder 9 unchanged, adjusting the rotating speed of the stirring pin 8 to 600-800 rpm, simultaneously moving and welding the shaft shoulder 9 and the stirring pin 8 along the direction of a welding seam, wherein the welding speed is 1000-3000 mm/min, the inclination angle of the split type friction welding tool is 0.5-1 degrees, and the horizontal moving distance of the welding tool in the welding process is 200-2000 mm. And after the welding line reaches the tail end of the welding line, adjusting the rotating speed of the stirring pin 8 to be 800-1200 rpm, and rotating the stirring pin 8 reversely and simultaneously moving the stirring pin and the shaft shoulder 9 upwards to obtain the metal/FRP composite component.

(6) And (3) placing the welded metal/FRP composite plate in 20-30% air humidity, and cooling to room temperature.

The above description is only exemplary of the present application and should not be taken as limiting the present application, as any modification, equivalent replacement, or improvement made within the spirit and principle of the present application should be included in the protection scope of the present application.

Claims

1. A double-body type cavity filling metal/FRP friction stir welding connection method is characterized by comprising the following steps:

(3) The split friction stir welding tool is connected with a friction stir welding spindle in a matched mode, the split friction stir welding tool is provided with a stirring needle and a shaft shoulder, the head of the stirring needle is provided with a threaded structure, the lower surface of the shaft shoulder is concave, the stirring needle is matched with an inner clamping ring of a first friction stir welding spindle, the inner clamping ring is provided with a first clamping cavity and matched with the tail of the stirring needle, the shaft shoulder and an outer clamping ring of a second friction stir welding spindle are matched and fixed, the stirring needle with the inner clamping ring is matched with the shaft shoulder with outer clamping, the outer clamping ring is provided with a second clamping cavity, the second clamping cavity is matched with the shaft shoulder, the outer clamping ring is further provided with a containing cavity, the containing cavity is matched with the stirring needle with the inner clamping ring, and the part, matched with the shaft shoulder and the outer clamping ring, of the tail of the stirring needle, is provided with a trimming plane and is used for being respectively abutted and fixed with screws of the inner clamping ring and the outer clamping ring;

(4) Positioning the head of the stirring pin in the step (3) at the initial position of the welding line of the plate to be welded, simultaneously starting a welding machine to enable a first stirring friction welding main shaft and a second stirring friction welding main shaft to rotate, and adjusting the extending length of the stirring pin relative to a shaft shoulder to enable the stirring pin head to be just contacted with the upper surface of the metal plate;

2. The two-body type cavity-filled metal/FRP friction stir welding connection method according to claim 1, wherein in the step (1), the organic solvent is alcohol or acetone.

3. The friction stir welding method of metal/FRP filled cavities of two bodies as recited in claim 1, wherein in step (4), the extension length of the stirring pin relative to the shoulder is 1.6 to 1.8mm.

4. The friction stir welding method of two-body type cavity filled metal/FRP according to claim 1, wherein in step (5), the rotating speed of the shaft shoulder and the stirring pin in the pressing down process is 500-1500 rpm, the pressing down amount is 0.2-0.8 mm, and the inclination angle of the split type friction welding tool is 0.5-1.5 °.

5. The friction stir welding connection method of metal/FRP filled with double-body cavities according to claim 1, wherein in the step (5), the rotating speed of the stirring pin is adjusted to 600-800 rpm just after the head of the stirring pin contacts the upper surface of the FRP plate.

6. The friction stir welding method of metal/FRP filled cavities of two bodies as recited in claim 1, wherein in the step (5), the rotating speed of the pin moving up in the reverse direction is 500 to 1500rpm.

7. The friction stir welding method of metal/FRP filled cavities of two bodies according to claim 1, wherein in step (5), the welding speed is 1000 to 3000mm/min.

8. The two-body type cavity-filling metal/FRP friction stir welding connection method according to claim 1, wherein in the step (6), the air humidity is 20 to 30%.