CN113732310B

CN113732310B - Method for preparing complex thin-wall component by adopting laser metal deposition and follow-up rolling

Info

Publication number: CN113732310B
Application number: CN202111018372.3A
Authority: CN
Inventors: 梁江凯; 何祝斌; 杜巍
Original assignee: Dalian University of Technology
Current assignee: Dalian University of Technology
Priority date: 2021-09-01
Filing date: 2021-09-01
Publication date: 2023-03-03
Anticipated expiration: 2041-09-01
Also published as: CN113732310A

Abstract

The invention belongs to the technical field of laser additive manufacturing, and provides a method for preparing a complex thin-wall component by adopting laser metal deposition and follow-up rolling, which comprises the following steps: preparing a material before laser metal deposition forming; layering three-dimensional models of complex thin-wall components; determining laser metal deposition process parameters; determining the follow-up rolling technological parameters of the roller; laser printing the nth layer and finishing follow-up rolling; repeating the third step to the fifth step; and (5) performing aftertreatment on the thin-wall component. The invention can solve the problems that the laser beam can not act on the end surface of the component due to the deformation of the component caused by residual stress when the complex special-shaped thin-wall component is prepared by the existing laser metal deposition technology, so that the continuous printing of the thin-wall component can not be finished, the surface quality of the formed component is poor due to the convex-concave peaks caused by the interlayer lap joint, the reliability is reduced, and the secondary deformation is easily caused when the surface of the component is subjected to mechanical processing or laser polishing treatment.

Description

Method for preparing complex thin-wall component by adopting laser metal deposition and follow-up rolling

Technical Field

The invention belongs to the technical field of laser additive manufacturing, and particularly relates to a method for preparing a complex thin-wall component by adopting laser metal deposition and follow-up rolling.

Background

With the progress of aerospace technology, in order to meet the development requirements of high mach number, high performance and high reliability of new generation aerospace vehicles and engines thereof, the requirements of light-weight high-temperature-resistant complex thin-wall components are continuously increased, and because different use conditions are met, the shapes, the wall thicknesses, the material types, the mechanical properties and the like of the thin-wall components have great differences, and the manufacturing methods are different. In the air intake and exhaust system of the advanced warplane, a complex thin-wall metal component with a complex shape, ultra-thin wall thickness and extremely high precision requirement exists. For example, an air inlet channel in an air inlet system and an air outlet channel in an air outlet system have complicated and changeable section shapes and bending axes in order to meet specific aerodynamic performance and stealth performance, and the components mostly adopt titanium alloy and high-temperature alloy thin-wall plate blanks. Because the traditional rigid die stamping cannot apply effective and reasonable forming load to all parts of the blank, the blank is often manufactured by a method of stamping and forming in blocks and then welding, particularly to a component with negative curvature or a component with a closed section. However, due to the springback of the press forming, the shape accuracy of the blank is difficult to control, and the subsequent tailor welding of a plurality of blanks causes complex deformation, so that the combined action of the two reduces the dimensional accuracy of the part. Meanwhile, the total length of the welding line is large and the reliability of the parts is greatly reduced due to the large number of the parts in blocks. If a large number of curved surface thin-wall rib plates exist on the carrier rocket, the member is usually manufactured by a roll bending or press bending method, and in order to reduce the structural weight, a subsequent material reducing processing technology is usually adopted to cut and remove a local area. However, the problem of deformation of the thin-walled member, excessive local processing, and instability of the rib is likely to occur in the cutting process.

In order to solve the above problems in the manufacture of large-sized complex-shaped thin-walled tubular/plate-shaped members, a laser metal deposition 3D printing technique has been introduced, which prepares a metal member by layer-by-layer deposition by using a laser beam of higher power to generate a molten pool in a deposition region and continuously melting a metal powder material, and which can form a thin-walled metal member having a complex curvature, a large difference in section, and a bending axis. Compared with other 3D printing technologies, the forming component has larger size, higher forming speed and higher powder utilization rate, and is particularly suitable for preparing large-size complex special-shaped thin-wall components in aerospace vehicles. However, the member is easily deformed due to the influence of residual stress during the laser metal deposition process, so that the laser beam cannot be applied to the end face of the member, and continuous printing of the thin-walled member cannot be performed. For the formed thin-wall metal component, mainly from two aspects of precision and performance, hot isostatic pressing densification treatment is usually required after the metal component is prepared by laser metal deposition to eliminate micro-cracks, pores, non-fusion defects and the like on the surface and inside of the formed component. However, in the laser metal deposition technology, metal powder is melted by using a laser beam with high power (1000 w-3000 w), and the height of a single-layer deposition layer is more than 0.1mm, so that convex-concave peaks caused by interlayer overlapping appear on the surface of a printed component, the surface quality is poor, the convex-concave peaks cannot be eliminated even through hot isostatic pressing treatment, and the reliability of the component is greatly reduced. Therefore, it is usually necessary to machine the surface of the formed member after the hot isostatic pressing treatment to remove surface margins such as uneven peaks. However, the thin-walled member has low overall and local rigidity, and is easily affected by various factors such as cutting force of a tool, clamping force of a clamp, self structure, self internal stress and the like in the machining process to deform, so that the dimensional accuracy of the member is reduced.

To reduce the above-mentioned problem of the peaks and valleys caused by the inter-layer lapping during the preparation of the member by laser metal deposition, laser polishing technology has been used to polish the 3D printed and formed metal member, which is based on applying a laser beam to the peak and valley regions on the surface of the formed member to melt the peaks and valleys on the surface, and redistributing the melted liquid metal into the valleys to smooth the original surface. The surface topography after final polishing depends primarily on the parameters of the laser beam applied to the surface of the forming member and the initial topography of the surface of the forming member. Although the method can improve the initial appearance of the component and improve the surface integrity, the laser polishing process is a remelting process, which is equivalent to secondary forming on the surface of a thin-wall component, and the thin-wall component is necessarily deformed under the influence of temperature gradient, so that the dimensional accuracy of the component is reduced.

In order to solve the problems that the laser beam cannot act on the end face of a component due to the deformation of the component caused by residual stress when a complex special-shaped thin-wall component is prepared by the existing laser metal deposition technology, so that the continuous printing of the thin-wall component and the surface of the formed component cannot be finished, the surface quality is poor, the reliability is reduced due to the convex-concave peak caused by the interlayer lap joint, and the secondary deformation is easily caused when the surface of the component is subjected to mechanical processing or laser polishing treatment, a new preparation method of the complex thin-wall component needs to be developed.

Disclosure of Invention

The invention aims to provide a method for preparing a complex thin-wall component by adopting laser metal deposition and follow-up rolling, which can solve the problems that the existing laser metal deposition technology cannot complete continuous printing of a thin-wall component and poor surface quality and low reliability of the formed component surface due to convex-concave peaks caused by interlayer lap joint because of component deformation caused by residual stress when preparing the complex special-shaped thin-wall component, and secondary deformation is easily caused when the component surface is subjected to mechanical processing or laser polishing treatment.

The technical scheme of the invention is as follows:

a method for preparing a complex thin-wall component by adopting laser metal deposition and follow-up rolling comprises the following steps:

step one, preparing a material before laser metal deposition forming: selecting the type of metal powder according to the requirements of the material, the structure and the performance of the complex thin-wall component to be formed, and selecting a metal substrate according to the metal powder selected by the complex thin-wall component; because the residual stress is most concentrated at the joint of the component and the substrate in the printing process, the selected metal substrate and the complex thin-wall component to be formed are ensured to form better metallurgical bonding, and the complex thin-wall component is prevented from being deformed in the forming process due to cracking and residual stress release in the bonding area of the selected metal substrate and the complex thin-wall component to be formed;

step two, layering a three-dimensional model of the complex thin-wall component: establishing a CAD (computer aided design) geometric model of the component according to the three-dimensional shape and size requirements of the complex thin-wall component, extracting an STL (Standard template library) model of the complex thin-wall component, and selecting a layering thickness according to the shape and size of the complex thin-wall component; wherein, for a complex thin-wall component with single curvature, the selected layering thickness is 0.3-0.5 mm; for complex thin-wall components with complex curvature and bending axes, the selected layering thickness is 0.1 mm-0.3 mm; finally, carrying out layering processing on the STL model by using layering slicing software;

step three, determining laser metal deposition process parameters: determining the single-layer lifting amount of the laser head according to the layered thickness determined in the step two, wherein the single-layer lifting amount of the laser head is equal to the layered thickness of the complex thin-wall component; determining other laser metal deposition process parameters including laser power, scanning speed, spot diameter, powder feeding rate, gas composition and pressure flow according to the forming wall thickness and the forming requirement of the complex thin-wall component; determining the laser scanning path of each layer according to the three-dimensional model in the second step in a layering manner;

the preparation of the complex thin-wall component which has active property and is easy to generate oxidation reaction is carried out in the argon environment with the oxygen content lower than 0.05 percent;

for the preparation of the complex thin-wall component with good oxidation resistance, the required atmosphere environment is adjusted according to the used material;

step four, determining the following rolling technological parameters of the roller: determining the distance, the size and the rotating speed of the roller according to the forming wall thickness, the layering thickness and the laser scanning speed of the complex thin-wall component, wherein the distance between the rollers is equal to the forming wall thickness of the complex thin-wall component, and the height of the roller is more than or equal to 3 times of the layering thickness of the complex thin-wall component; the rotating speed of the roller is equal to the laser scanning speed divided by the perimeter of the outer diameter of the roller; in the laser metal deposition process, the roller distance, the roller deflection angle and the substrate deflection angle are adjusted according to the forming requirement of a complex thin-wall component, so that the roller always keeps a linear contact state with a deposition area, and the follow-up rolling of the deposition area is realized by means of bidirectional loading of the roller on the deposition area;

the roller can perform follow-up rolling on the deposition area of the current layer and can perform secondary rolling on the deposition layer of the previous layer;

step five, laser printing the nth layer and finishing follow-up rolling: printing the n-th deposition layer according to the laser metal deposition process parameters determined in the third step, wherein in the printing process, the roller synchronously moves along with the laser head, and the deposition area after laser printing is rolled in a follow-up manner; wherein n is a natural number;

step six, calculating the total number of laser printing layers according to the total forming height of the complex thin-wall component and the layering thickness determined in the step two; wherein, the total printing layer number is equal to the height of the complex thin-wall component divided by the layering thickness; repeating the third step to the fifth step, predicting the deposition condition of the next layer after the laser printing of one layer is finished, and then depositing and forming layer by layer until the printing of the component is finished;

seventhly, post-processing the thin-wall component: after the complex thin-wall component is prepared through laser metal deposition, the complex thin-wall component is subjected to heat treatment under the conditions of high temperature and high pressure, and the end part and the surface of the complex thin-wall component are subjected to machining and cleaning treatment, so that the finally formed complex thin-wall component is obtained.

The beneficial effects of the invention are:

(1) According to the method for preparing the complex thin-wall component by adopting laser metal deposition and follow-up rolling, the roller is adopted to carry out follow-up rolling on the deposition area after the laser metal deposition is finished, and the problems of poor surface quality and low precision caused by convex-concave peaks caused by interlayer lap joint in the process of preparing the thin-wall component by adopting the existing laser metal deposition technology can be solved.

(2) The method for preparing the complex thin-wall component by adopting laser metal deposition and follow-up rolling can prepare the thin-wall component with the complex characteristic region by reasonably matching the roller distance, the deflection angle and the rotation angle of the rotating main shaft connected with the substrate, and can prepare the complex special-shaped thin-wall component with the equal wall thickness and the variable wall thickness with special requirements by adjusting the roller distance and the laser metal deposition process parameters.

(3) According to the method for preparing the complex thin-wall component by adopting the laser metal deposition and the follow-up rolling, the deposition area after the laser metal deposition is finished is rolled in the follow-up manner by adopting the roller, so that the problem that the continuous printing of the thin-wall component cannot be finished because the laser beam cannot act on the end face of the component due to the deformation of the component caused by residual stress is avoided, and in addition, the density and the structural performance uniformity of the formed component can be improved.

(4) According to the method for preparing the complex thin-wall component by adopting laser metal deposition and follow-up rolling, the formed thin-wall component is subjected to hot isostatic pressing only without subsequent machining or laser polishing, and the problem of secondary deformation easily caused by machining or laser polishing is avoided.

Drawings

FIG. 1 is a schematic diagram of the method for manufacturing a complex thin-walled component by laser metal deposition and follow-up rolling according to the present invention.

FIG. 2 is a schematic diagram of a complex thin-walled member to be prepared according to the present invention, (a) a complex curved thin-walled plate-shaped member to be prepared, and (b) a complex variable cross-section thin-walled tubular member to be prepared.

FIG. 3 is a schematic diagram of the present invention for manufacturing a complex curved surface thin-wall plate-shaped member by laser metal deposition and follow-up rolling.

FIG. 4 is a schematic diagram of the present invention for manufacturing a complex variable cross-section thin-walled tubular member using laser metal deposition and follow-up rolling.

Fig. 5 is a schematic diagram of the complex thin-walled member after printing is completed, wherein (a) is the complex curved surface thin-walled plate-shaped member after forming, and (b) is the complex variable cross-section thin-walled tubular member after forming.

In the figure: the method comprises the following steps of 1 preparing a complex curved surface thin-wall plate-shaped component required to be prepared, 2 preparing a complex variable cross-section thin-wall tubular component required to be prepared, 3 base plates, 4 roller rotating shafts, 5 rollers, 6 laser heads, 7 powder feeders, 8 guide rail rotating shafts, 9 guide rails, 10 powder nozzles, 11 roller turning, 12 rotating main shafts, 13 forming the complex curved surface thin-wall plate-shaped component and 14 forming the complex variable cross-section thin-wall tubular component.

Detailed Description

The following further describes the specific embodiments of the present invention with reference to the drawings and technical solutions.

Example 1: the method for preparing the complex thin-wall component by adopting laser metal deposition and follow-up rolling is described by combining with figures 1, 2, 3, 4 and 5, and is carried out according to the following steps:

step one, preparing materials before laser metal deposition forming. The type of metal powder is selected according to the material, structure and performance requirements of the formed component, the metal substrate is selected according to the type of the material selected by the component, the metal powder needs to be placed in a vacuum drying furnace to remove moisture before being used, and the metal substrate needs to be mechanically ground and cleaned in order to form good metallurgical bonding between the component and the substrate.

And step two, layering the three-dimensional model of the complex thin-wall component. The method comprises the steps of establishing a CAD (computer aided design) geometric model of the component according to the three-dimensional shape and size requirements of the complex thin-wall component, extracting an STL (standard template library) model of the component, selecting a layering thickness according to the shape and size of the component, selecting the layering thickness of the thin-wall metal component with the complex curvature and the bending axis in the range of 0.1-0.3 mm, and finally layering the STL model by using layering slicing software.

And step three, determining laser metal deposition process parameters. Determining the single-layer lifting amount of the laser head according to the determined layered thickness in the step two, wherein the single-layer lifting amount of the laser head is equal to the layered thickness of the component, and determining other laser metal deposition process parameters according to the forming wall thickness and the forming requirement of the component, wherein the process parameters comprise: laser power, scanning speed, spot diameter, powder feeding rate, gas composition, pressure flow and the like, the laser scanning path of each layer is determined according to the three-dimensional model in the step two in a layering mode, and the component is prepared in an argon environment with the oxygen content lower than 0.05%.

And step four, determining the following rolling technological parameters of the roller. The method comprises the steps of determining the distance, the size and the rotating speed of a roller according to the forming wall thickness, the layering thickness and the laser scanning speed of a component, wherein the distance between the rollers is equal to the forming wall thickness of the component, the height of the roller is more than or equal to 3 times of the layering thickness of the component, the roller can perform follow-up rolling on a deposition area of a current layer and can perform re-rolling on a deposition layer of a previous layer, the rotating speed of the roller is equal to the laser scanning speed divided by the outer diameter and the perimeter of the outer diameter of the roller, and in the laser metal deposition process, the distance between the rollers, the deflection angle of the rollers and the deflection angle of a substrate are adjusted according to the forming requirement of the component, so that the rollers are always in line contact with the deposition area, and the follow-up rolling of the deposition area is realized by means of bidirectional loading of the rollers on the deposition area.

And fifthly, laser printing the nth layer and finishing follow-up rolling. And (2) printing the n-th deposition layer according to the laser metal deposition process parameters determined in the third step, (n is 1, 2, 3, 8230; and the like), wherein in the printing process, the roller synchronously moves along with the laser head, and the deposition area after laser printing is rolled in a follow-up manner.

And step six, repeating the step three to the step five. And calculating the total number of laser printing layers according to the total forming height of the component and the layering thickness determined in the second step, wherein the total number of printing layers is equal to the total forming height of the component divided by the layering thickness, repeating the third step to the fifth step, predicting the deposition condition of the next layer after the laser printing of one layer is finished, and then depositing and forming layer by layer until the component is printed.

And seventhly, performing post-treatment on the thin-wall component. After the complex thin-walled component is prepared by laser metal deposition, hot isostatic pressing and solution heat treatment are carried out on the component, and necessary processing and cleaning treatment are carried out on the end part and the surface of the thin-walled component, so that the finally formed complex thin-walled component is obtained.

The beneficial effect of this embodiment is: the method has the advantages that the roller is adopted to roll the deposition area after the laser metal deposition is finished, so that the problems that the laser beam cannot act on the end face of the member due to the deformation of the member caused by residual stress when the thin-wall member is prepared by the existing laser metal deposition technology, the continuous printing of the thin-wall member and the surface of the formed member cannot be finished, the surface quality is poor, the reliability is reduced due to the convex-concave peak caused by interlayer overlapping, and the secondary deformation is easily caused when the surface of the member is subjected to mechanical processing or laser polishing treatment in the follow-up process can be solved; and thin-wall components with complex characteristic regions can be prepared by reasonably matching the roller spacing, the deflection angle and the rotation angle of a rotating main shaft connected with the substrate, and the thin-wall components with special requirements of equal wall thickness and variable wall thickness complex special shapes can be prepared by adjusting the roller spacing and laser metal deposition process parameters.

Example 2: with reference to fig. 2, in the first step, the selected metal powder is GH3536 nickel-base superalloy powder with particle size distribution ranging from 53 um to 106um and prepared by a vacuum atomization process, the metal substrate is 304 stainless steel, the metal powder is placed in a vacuum drying furnace for heat treatment for 3 hours at 120 ℃ before use to remove internal moisture, and other steps are the same as those in example 1.

The beneficial effect of this embodiment is: GH3536 nickel-base superalloys have a high alloy content and are capable of withstanding a wide variety of severe corrosive environments, even where the combination of nickel and chromium is resistant to oxidation reactions, and the presence of molybdenum makes these alloys resistant to pitting and crevice corrosion; in addition, a good metallurgical bonding can be formed between the 304 stainless steel substrate and the formed GH3536 thin-wall component, and the defect of cracking of the two components is avoided.

Example 3: referring to fig. 3, in the third to fifth steps, when a complex curved thin-wall plate-shaped member is prepared, a laser head and a roller are synchronously reciprocated, the deflection angle of the roller is required to be adjusted during the reciprocating movement so that the roller and a deposition area are kept in a linear contact state, and other steps are the same as those in embodiment 1.

The beneficial effect of this embodiment is: when the complex curved surface thin-wall plate-shaped component is formed, the scheme that the laser head and the roller synchronously reciprocate is adopted, the roller rolls the deposition area subjected to laser printing in a follow-up manner, the problem that the component cannot be continuously printed due to the fact that laser beams cannot act on the end face of the component because of deformation caused by residual stress can be solved, and the variable-wall-thickness complex special-shaped thin-wall plate-shaped component with special requirements can be prepared by adjusting the distance between the rollers and laser metal deposition process parameters. In addition, the compactness and the structural property uniformity of the component can be improved.

Example 3: referring to fig. 4, in the third to fifth steps, when the complex variable-section thin-wall tubular member is prepared by the laser metal deposition technology, the laser head and the roller synchronously move in the same direction, the rotation angle of the roller needs to be adjusted by the guide rail rotating shaft in the process of moving in the same direction with the laser head, and other steps are the same as those in embodiment 1.

The beneficial effect of this embodiment is: the forming component with the complicated characteristic region can be prepared by reasonably matching the roller spacing, the deflection angle and the rotation angle of a rotating main shaft connected with the substrate, and the complicated special-shaped thin-wall tubular component with the uniform wall thickness and the variable wall thickness with special requirements can be prepared by adjusting the roller spacing and the laser metal deposition process parameters.

Example 4: and as shown in the figure 5, in the seventh step, the hot isostatic pressing and solution heat treatment are carried out on the component under the conditions of high temperature and high pressure, wherein the hot isostatic pressing process is carried out for 2.5 hours under the conditions of the temperature of 910 ℃ and the pressure of 120MPa, argon is adopted as the inert gas, the solution heat treatment process is carried out for 2 hours under the conditions of the temperature of 1150 ℃, and other steps are the same as those in the example 1.

The beneficial effect of this embodiment is: and carrying out hot isostatic pressing and solution heat treatment on the formed thin-walled component, wherein the hot isostatic pressing process can eliminate micro-cracks, air holes, non-fusion defects and the like existing in the formed component, and the solution heat treatment process can improve the structure of the component, improve the solid solution degree of alloy elements and enhance the strength.

Claims

1. A method for preparing a complex thin-wall component by adopting laser metal deposition and follow-up rolling is characterized by comprising the following steps:

step one, preparing a material before laser metal deposition forming: the method comprises the following steps of selecting the type of metal powder according to the material, structure and performance requirements of a complex thin-wall component to be formed, selecting a metal substrate according to the metal powder selected by the complex thin-wall component, ensuring that good metallurgical bonding is formed between the selected metal substrate and the complex thin-wall component to be formed, avoiding cracking and residual stress release in a bonding area between the selected metal substrate and the complex thin-wall component to be formed, and avoiding deformation of the complex thin-wall component in the forming process;

step two, layering a three-dimensional model of the complex thin-wall component: establishing a CAD geometric model of the complex thin-wall component according to the three-dimensional shape and size requirements of the complex thin-wall component, extracting an STL model of the complex thin-wall component, and then selecting a layering thickness according to the shape and size of the complex thin-wall component; wherein, for a complex thin-wall component with single curvature, the selected layering thickness is 0.3 to 0.5mm; for a complex thin-wall component with complex curvature and a bending axis, the selected layering thickness is 0.1mm-0.3mm; finally, carrying out layering processing on the STL model;

step three, determining laser metal deposition process parameters: determining the single-layer lifting amount of the laser head according to the layered thickness determined in the step two, wherein the single-layer lifting amount of the laser head is equal to the layered thickness of the complex thin-wall component; determining other laser metal deposition process parameters including laser power, scanning speed, spot diameter, powder feeding rate, gas composition and pressure flow according to the forming wall thickness and the forming requirement of the complex thin-wall component; determining the laser scanning path of each layer according to the three-dimensional model in the second step in a layering manner; the preparation of the complex thin-wall component is carried out in an argon environment with oxygen content lower than 0.05 percent;

step four, determining the following rolling technological parameters of the roller: determining the distance, the size and the rotating speed of the roller according to the forming wall thickness, the layering thickness and the laser scanning speed of the complex thin-wall component, wherein the distance between the rollers is equal to the forming wall thickness of the complex thin-wall component, and the height of the roller is more than or equal to 3 times of the layering thickness of the complex thin-wall component; the rotating speed of the roller is equal to the laser scanning speed divided by the circumference of the outer diameter of the roller; in the laser metal deposition process, the roller distance, the roller deflection angle and the substrate deflection angle are adjusted according to the forming requirement of a complex thin-wall component, so that the roller always keeps a linear contact state with a deposition area, and the follow-up rolling of the deposition area is realized by means of bidirectional loading of the roller on the deposition area;

step five, depositing the nth layer by laser metal and finishing follow-up rolling: depositing the nth deposition layer according to the laser metal deposition process parameters determined in the third step, wherein in the laser metal deposition process, the roller synchronously moves along with the laser head, and performs follow-up rolling on the deposition area subjected to laser metal deposition;

step six, calculating the total layer number of the laser metal deposition according to the total forming height of the complex thin-wall component and the layering thickness determined in the step two; the total laser metal deposition layer number is equal to the total assembly height of the complex thin-wall component divided by the layering thickness; repeating the third step to the fifth step, predicting the deposition condition of the next layer after the laser metal deposition is finished for one layer, and then performing deposition forming layer by layer until the component is finished by the laser metal deposition;

seventhly, post-processing the thin-wall component: after the complex thin-walled component is prepared by laser metal deposition, hot isostatic pressing and solution heat treatment are carried out on the complex thin-walled component, and the end part and the surface of the complex thin-walled component are processed and cleaned, so that the finally formed complex thin-walled component is obtained.

2. The method for manufacturing complex thin-walled component by laser metal deposition and follow-up rolling according to claim 1,

in the fourth step of the method, the first step,

the roller rolls the deposition layer of the previous layer again while rolling the deposition area of the current layer in a follow-up manner.