A method and apparatus for controllable preparation of gradient tissues

Abstract

This invention belongs to the field of additive manufacturing and discloses a controllable method and apparatus for preparing gradient structures. The method includes the following steps: S1, performing layer-by-layer additive manufacturing by stacking materials layer by layer; S2, during the layer-by-layer additive manufacturing process, applying an ultrasonic pulse current to the additive component for ultrasonic pulse treatment to complete the manufacturing of the ultrasonic pulse arc additive component; S3, transferring the ultrasonic pulse arc additive component for laser strengthening, applying laser shock strengthening to the surface of the ultrasonic pulse arc additive component. This invention can effectively improve the problem of coarse columnar crystals in arc additive manufacturing. By applying an ultrasonic pulse current, it achieves the effect of refining the grains, transforming coarse columnar crystals into small equiaxed crystals, effectively improving mechanical properties, and performing laser shock strengthening on the additive surface, thereby generating a gradient structure on the surface.

Description

Technical Field

[0001] This invention belongs to the field of additive manufacturing, and more specifically, relates to a method and apparatus for controllable preparation of gradient structures. Background Technology

[0002] Arc additive manufacturing is an advanced digital manufacturing technology based on welding technology. It uses an electric arc or plasma arc as a heat source to melt metal welding wire and, according to the principle of layer-by-layer deposition, manufactures a three-dimensional metal blank that closely approximates the shape and size requirements of the product from a line-surface-volume model. The main forms include gas metal arc welding, tungsten inert gas welding, and plasma arc welding. However, due to the epitaxial growth, the microstructure of additive manufacturing grows into columnar crystals, resulting in uneven structure, anisotropy, large residual stress and tensile stress, and poor mechanical properties.

[0003] Ultrasonic pulsed current refers to the periodically changing arc pressure that stirs the molten pool, breaking dendrites, deepening the crystallization center of the molten pool, and promoting grain refinement. When the pulsed current reaches tens of thousands of Hz or higher, the arc morphology and heat distribution will change significantly. Under the same average current, the electromagnetic contraction effect of the ultrasonic pulsed arc is stronger than that of the continuous DC arc, and the penetration is increased. At the same time, the molten pool is subjected to ultrasonic vibration, which can improve the metallurgical process and increase the fluidity of the molten pool. Compared with ordinary DC of the same power, the arc pressure, the obtained penetration depth, and the welding speed of the ultrasonic pulsed current are all increased by 2 to 3 times.

[0004] Laser shock peening (LSP) is a novel surface strengthening technology that uses laser-induced plasma shock waves to induce plastic deformation in the metal surface, thereby improving the mechanical properties of the material. The principle of LSP involves attaching an absorber layer (aluminum foil or black tape) to the surface of the material to be impacted, followed by a confinement layer (deionized water or transparent glass). A high-power-density (GW / cm²) laser with short pulses (10–30 ns) acts on the absorber layer on the metal surface, causing it to rapidly vaporize and ionize, generating high-temperature, high-pressure plasma. This plasma, constrained by the confinement layer, forms a high-pressure shock wave that propagates into the material. When the peak pressure of the shock wave exceeds the Hugoniot elastic limit (σhel) of the material, plastic deformation occurs on the surface, altering the surface microstructure and residual stress distribution. This results in improved fatigue resistance, wear resistance, and stress corrosion resistance.

[0005] Invention patent CN111088470B discloses a method for preparing a gradient microstructure of high-strength Ti55531 titanium alloy. This method utilizes the resistance heating effect generated by pulsed current to selectively and locally heat titanium alloy bars or plates, ultimately obtaining a gradient microstructure at room temperature. However, this method is only applicable to titanium alloys and is not suitable for additive titanium alloys. Invention patent CN102816981B discloses a method for preparing a zirconium-niobium alloy with a gradient microstructure on its surface. The zirconium-niobium alloy is heat-treated to obtain a basket-like microstructure, followed by mechanical impact deformation with an impact energy of 50–100 J. This energy is too high and unsuitable for use in additive components.

[0006] Therefore, there is an urgent need for a method and apparatus in additive manufacturing that can refine grains while simultaneously achieving controllable preparation of gradient structures. Summary of the Invention

[0007] In view of this, the present invention proposes a controllable gradient organization preparation method and apparatus, which can obtain controllable gradient organization, and the specific technical solution is as follows:

[0008] This invention provides a gradient organization controllable preparation method, specifically including the following steps:

[0009] S1. Additive manufacturing by stacking materials layer by layer;

[0010] S2. During the layer-by-layer additive manufacturing process, an ultrasonic pulse current is applied to the additive component to perform ultrasonic pulse processing, thereby completing the manufacturing of the ultrasonic pulse arc additive component.

[0011] S3. Transfer the ultrasonic pulse arc additive component to a laser strengthening process, and apply laser shock strengthening to the surface of the ultrasonic pulse arc additive component.

[0012] To address the problems of coarse columnar grains, anisotropy, and large residual tensile stress in arc additive manufacturing, this invention provides a controllable gradient structure preparation method. This method can effectively improve the problem of coarse columnar grains in arc additive manufacturing by applying an ultrasonic pulsed current to refine the grains, transforming coarse columnar grains into smaller equiaxed grains, effectively improving mechanical properties, and performing laser shock peening on the additive surface to generate a gradient structure on the surface.

[0013] Furthermore, in step S2, the pulse current form is divided into several forms, including conventional pulse current, peak composite ultrasonic pulse current, base value composite ultrasonic pulse current, and full-stage composite ultrasonic pulse current. By adding peak composite ultrasonic, base value composite ultrasonic, and full-stage composite ultrasonic to the conventional pulse current, dynamic control of the pulse current is achieved.

[0014] Furthermore, in step S2, the application of ultrasonic pulse current is applicable to different types of metals. Among them, titanium alloys, high-temperature alloys, and carbon steel are suitable for applying ultrasonic current to DC and pulse current, while aluminum alloys and magnesium alloys are suitable for applying ultrasonic current to variable polarity square waves and low-frequency tempered variable polarity square waves.

[0015] Furthermore, in step S3, the laser shock strengthening of the ultrasonic pulse arc additive component is divided into two forms. The first is to perform overall surface laser shock strengthening on the additive component after it is assembled. The second is to perform laser shock strengthening after 3 to 6 layers of additive manufacturing, and then to carry out additive manufacturing on the component after strengthening. This process is repeated to complete the final additive manufacturing and strengthening process.

[0016] Furthermore, in the laser enhancement process of step S3, the surface gradient tissue can be controlled and adjusted by regulating the laser energy, number of impacts, spot diameter, spot overlap rate, and laser travel path.

[0017] Furthermore, the laser energy is 1–20 J, the number of impacts is 1–100, and the spot diameter is 1–6 mm.

[0018] Furthermore, both the electric arc additive manufacturing process and the laser shock peening process are completed in an atmospheric environment at room temperature.

[0019] This invention also provides a gradient fabrication controllable fabrication device, including an arc additive manufacturing mechanism. The arc additive manufacturing mechanism includes an industrial control computer, a data acquisition card, a Hall sensor, a welding power source, a welding torch, a substrate, and a wire feeder. The industrial control computer is connected to the data acquisition card. One end of the data acquisition card is connected to the welding power source through the Hall sensor, and the other end is connected to the welding torch. The welding power source is connected to the substrate, forming a closed loop. The wire feeder is placed on one side of the welding torch. The welding torch is placed on a multi-degree-of-freedom traveler and generates an arc in an argon-protected gas environment. The welding torch is connected to a shielding gas line, a cooling water line, and a welding power source connection line. The arc generated by the welding torch melts the welding wire, and at the same time, the welding torch performs multi-degree-of-freedom movement above the substrate through the multi-degree-of-freedom traveler, thereby performing reciprocating motion to realize layer-by-layer additive manufacturing.

[0020] Furthermore, by adjusting the welding power supply, the conversion of 20kHz to 100kHz ultrasonic pulse current is achieved; the pulse current consists of base current + peak current, the open voltage is 70V, the base current range is 5 to 300A, the peak current range is 5 to 300A, the pulse frequency is 20 to 100KHz, and the duty cycle is 20 to 80%.

[0021] Furthermore, the gradient structure controllable fabrication device also includes a laser shock strengthening mechanism, on which the manufactured ultrasonic pulsed arc additive component is transferred to a laser shock strengthening substrate in the laser shock strengthening mechanism for strengthening.

[0022] Compared to existing technologies, the present invention provides a gradient tissue controllable preparation method and apparatus, which has the following advantages:

[0023] Beneficial effects:

[0024] 1. Without altering the alloy composition, the microstructure transformation can be achieved through ultrasonic pulsed current. The application of ultrasonic current enhances the arc force, causes radial arc contraction, concentrates energy, strengthens arc penetration, increases molten pool temperature and aspect ratio, and reduces elemental segregation. This improves the uniformity of microstructure distribution, refines grains, and significantly enhances mechanical properties.

[0025] 2. Surface laser shock peening is applied to the additive structure to improve the problem of grain refinement failure caused by low heat input on the surface of the ultrasonic pulsed arc additive. The coarse columnar crystals are transformed into fine equiaxed crystals. Laser shock peening is performed on the additive surface to achieve microscale plastic deformation of the surface layer. The additive structure is strengthened, the mechanical properties are improved, and the fatigue life is increased. Furthermore, the residual tensile stress on the surface after additive manufacturing is transformed into residual compressive stress, which further improves the fatigue life of the additive structure.

[0026] 3. Since both the electric arc additive manufacturing process and the laser shock strengthening process are carried out in a room temperature and atmospheric environment, the ultrasonic electric arc additive manufacturing + laser shock strengthening is easy to implement, highly operable, and has low requirements for equipment environment.

[0027] 4. At the same time, the electric arc additive manufacturing process and the laser shock strengthening process can be completed in parallel. That is, the strengthening process can be carried out after 3 to 6 layers of additive manufacturing. The two processes can also be completed independently, that is, the additive construction is completed and then the shock strengthening process is carried out. The equipment has high operational flexibility.

[0028] 5. Surface laser shock peening of additive components can create gradient structures on the surface. Furthermore, by adjusting process parameters such as laser energy, number of shocks, spot diameter, spot overlap rate, and laser travel path, the gradient structure on the surface can be controlled.

[0029] 6. Different current modes applied during additive manufacturing can produce alloy materials that meet different needs. The application of ultrasonic frequencies is suitable for different types of metals. Titanium alloys, high-temperature alloys, and carbon steel are suitable for ultrasonic frequencies applied to DC and pulsed currents. Due to the influence of cathode atomization, aluminum-magnesium alloys are suitable for ultrasonic frequencies applied to variable polarity square waves and low-frequency tempered variable polarity square waves, thereby removing oxide films and further improving the additive microstructure and properties. Attached Figure Description

[0030] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0031] Figure 1 This is a process flow diagram of a gradient organization controllable preparation method according to the present invention;

[0032] Figure 2 Schematic diagram of ultrasonic pulsed arc additive manufacturing;

[0033] Figure 3 This is a schematic diagram of laser shock peening.

[0034] Figure 4 This is a diagram of the original microstructure of a titanium alloy arc additive manufacturing sample in the additive direction.

[0035] Figure 5 Microstructure diagram of titanium alloy in the direction of ultrasonic pulsed arc additive manufacturing;

[0036] Figure 6 The images show the average orientation of the core and the grain boundary angle distribution at different depths of the laser-shock-strengthened titanium alloy sample.

[0037] In the diagram: 1-Industrial control computer, 2-Data acquisition card, 3-Hall sensor, 4-Welding power supply, 5-Welding torch, 6-Substrate, 7-Wire feeder, 8-Laser shock-strengthened substrate, 9-Laser emitter, 10-Reflector, 11-Focusing lens, 12-Water spray module, 13-Hydraulic cylinder, 14-Constraint layer, 15-Water tank, 16-Clamp. Detailed Implementation

[0038] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0039] In the description of this invention, it should be understood that the terms "upper", "lower", "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. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0040] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0041] First, such as Figure 2 , Figure 3 As shown, the present invention discloses a gradient fabrication controllable preparation device, comprising an electric arc additive manufacturing mechanism and a laser shock stabilization mechanism. The electric arc additive manufacturing mechanism further includes an industrial control computer 1, a data acquisition card 2, a Hall sensor 3, a welding power source 4, a welding torch 5, a substrate 6, and a wire feeder 7. The laser shock stabilization mechanism adopts existing technology and, in addition to the laser shock stabilization substrate 8, also includes a laser emitter 9, a reflector 10, a focusing lens 11, a water spray module 12, a hydraulic cylinder 13, a constraint layer 14, a water tank 15, a fixture 16, and a series of other structures.

[0042] Furthermore, such as Figures 1-3 As shown, the present invention provides a gradient organization controllable preparation method, comprising the following steps:

[0043] Step 1: Connect the industrial computer 1 to the data acquisition card 2, and connect one end of the Hall sensor 3 to the welding power supply 4 and the other end to the welding torch 5. The welding power supply 4 is connected to the substrate 6, forming a closed loop. The wire feeder 7 is placed to the left of the welding torch 5.

[0044] Step 2: After the equipment is turned on, the welding torch 5 generates an electric arc in the argon protective gas environment. The welding torch 5 is placed on the multi-degree-of-freedom traveler. The welding torch 5 is connected to the protective gas line, cooling water line and welding power supply line. The electric arc melts the welding wire. The welding torch 5 can move in multiple degrees of freedom above the substrate 6, thereby performing reciprocating motion to realize layer-by-layer additive manufacturing.

[0045] Step 3: During the additive manufacturing process, the conversion of the ultrasonic pulse current from 20kHz to 100kHz is achieved by adjusting the welding power source 4. During additive manufacturing, the pulse current can take various forms, including conventional pulse current, peak composite ultrasonic pulse current, base value composite ultrasonic pulse current, and full-stage composite ultrasonic pulse current. Peak composite ultrasonic, base value composite ultrasonic, and full-stage composite ultrasonic pulse current can be added to the conventional pulse current to achieve dynamic control of the pulse current. In the layer-by-layer additive manufacturing process, the ultrasonic pulse arc additive component is manufactured. The pulse current consists of a base current (Ib) + a peak current (Ip). The open-circuit voltage is 70V, the base current Ib ranges from 5 to 300A, the peak current Ip ranges from 5 to 300A, the pulse frequency is 20 to 100kHz, and the duty cycle is 20 to 80%.

[0046] Step 4: Transfer the additive component to the laser shock annealing substrate 8 for strengthening. Apply laser shock annealing to the surface of the ultrasonic pulse arc additive component to strengthen the surface tissue. During the strengthening process, set the laser energy, number of impacts, spot diameter, spot overlap rate, and laser path. The laser energy is 1–20 J, the number of impacts is 1–100, the spot diameter is 1–6 mm, and the laser path can have various movements, which can be determined by the set program.

[0047] In step 3 above, the superposition of different current modes in the present invention can refine the microstructure, transform the columnar crystals generated during the additive manufacturing process into equiaxed crystals, thereby improving the mechanical properties.

[0048] In step 4 above, during laser enhancement, the surface gradient tissue can be controlled by adjusting the laser energy, number of impacts, spot diameter, spot overlap rate, and laser path.

[0049] This invention applies laser shock strengthening to the surface of ultrasonic pulsed arc additive components, which improves the phenomenon of insignificant grain refinement on the surface of ultrasonic pulsed arc additive components and prepares a surface gradient structure.

[0050] Furthermore, since both the electric arc additive manufacturing process and the laser shock peening process can be completed at room temperature and in an atmospheric environment, a vacuum environment is not required. The post-additive structure can be directly subjected to laser shock peening, mainly in two forms: the first is to perform overall surface laser shock peening after the additive component is assembled; the second is to perform laser shock peening after 3-6 layers of additive manufacturing, and then place the component on the additive substrate 6 for further additive manufacturing. This process is repeated until the final additive and strengthening process is completed.

[0051] This invention discloses a gradient structure controllable preparation method and apparatus. The application of ultrasonic pulse current is applicable to different types of metals. Among them, titanium alloys, high-temperature alloys, and carbon steel are suitable for applying ultrasonic current to DC and pulse current, while aluminum alloys and magnesium alloys are suitable for applying ultrasonic current to variable polarity square waves and low-frequency tempered variable polarity square waves.

[0052] This invention discloses a controllable method for preparing gradient microstructures, involving arc additive manufacturing technology, ultrasonic pulse technology, and laser shock peening technology. It employs a composite process and apparatus of ultrasonic pulse arc additive manufacturing and laser shock peening to prepare gradient microstructures, achieving controllable preparation of the microstructure gradient. By applying ultrasonic pulses of different current modes to the arc additive manufacturing process, the columnar crystals generated during additive manufacturing are transformed into equiaxed crystals, achieving microstructure refinement. As the melting depth increases, the surface grain refinement weakens. At this point, laser shock peening is used to strengthen the additive surface, causing microscale plastic deformation and thus strengthening, thereby achieving the preparation of gradient microstructures. By combining different parameters for microstructure control, the gradient microstructure exhibits refined and uniform grain distribution, with strengthened surface microstructure and overall improved mechanical properties.

[0053] Example:

[0054] The following is in conjunction with the appendix Figure 1-6 Taking TC17 titanium alloy ultrasonic pulse arc additive manufacturing as an example.

[0055] Step 1: Connect the industrial computer 1 to the data acquisition card 2, and connect one end of the Hall sensor 3 to the welding power supply 4 and the other end to the welding torch 5. The welding power supply 4 is connected to the substrate 6, forming a closed loop. The wire feeder 7 is placed to the left of the welding torch 5, and the wire feeding material is TC17.

[0056] Step 2: After the equipment is turned on, the welding torch 5 generates an electric arc in the argon protective gas environment, and at the same time, the cooling water is turned on for cooling. The current mode is set to full-stage composite ultrasonic pulse current. The electric arc melts the welding wire, and the welding torch 5 can reciprocate above the substrate 6 to stack layer by layer.

[0057] Step 3: During the additive manufacturing process, the base current Ib = 160A, the peak current Ip = 200A, the voltage is 70V, no ultrasonic pulse processing is performed, and the duty cycle is 50%. The wire feed speed is 3m / min, and the welding torch movement speed is 2.4m / min. The microstructure of the sample obtained by additive manufacturing is as follows: Figure 4 As shown.

[0058] Step 4: During the additive manufacturing process, the base current Ib = 160A, the peak current Ip = 200A, the voltage is 70V, and ultrasonic pulse processing is activated with a pulse frequency of 50kHz and a duty cycle of 50%. The wire feed speed is 3m / min, and the welding torch movement speed is 2.4m / min. The microstructure of the sample obtained by ultrasonic additive manufacturing is as follows: Figure 5 As shown.

[0059] Step 5: Transfer the additive component from Step 4 to the fixture table of the laser shock peening mechanism for strengthening treatment. The laser energy is set to 10J, the number of shocks is 5, the spot diameter is 2mm, the spot overlap rate is 20%, and the laser travel path is set to Z-shaped reciprocating motion.

[0060] Step 6: Characterize the surface tissue from Step 5. Observe and characterize the tissue at distances of 0 μm, 100 μm, 400 μm, 700 μm, and 1000 μm from the surface, respectively. The results are as follows: Figure 6 As shown. By observing the average orientation of the kernel ( Figure 6 (a)-(e)) revealed that the dislocation density was highest at the surface, gradually decreasing with increasing distance from the surface, indicating a significant gradient structure at the surface. Statistical analysis of the grain boundary angle distribution diagrams ( Figure 6 (f)-(j) revealed that, at the same scale, the number of grain boundaries was highest on the surface, and decreased with increasing distance from the surface, indicating the formation of a gradient structure on the surface after laser shock peening. Furthermore, the increase in the number of surface grain boundaries and the increase in dislocation density both demonstrate that the surface of the additive component was strengthened by laser shock peening. Based on these changes in microstructure and internal structure, the mechanical properties of the component were significantly and comprehensively improved.

[0061] Therefore, after processing with a composite process and device of ultrasonic pulsed arc additive manufacturing and laser shock peening, the coarse columnar crystals of TC17 titanium alloy additive manufacturing are transformed into small equiaxed crystals. While the surface layer initially failed to achieve significant grain refinement, laser shock peening refined the surface grains and produced a distinct gradient structure. This gradient structure can be controlled by adjusting the process parameters.

[0062] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the apparatus disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple; relevant parts can be referred to the method section.

[0063] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A gradient-organized controllable preparation method, characterized in that, Specifically, the following steps are included: S1. Additive manufacturing by stacking materials layer by layer; S2. During the layer-by-layer additive manufacturing process, an ultrasonic pulse current is applied to the additive component to perform ultrasonic pulse processing, thereby completing the manufacturing of the ultrasonic pulse arc additive component. S3. Transfer the ultrasonic pulse arc additive component to a laser strengthening process, and apply laser shock strengthening to the surface of the ultrasonic pulse arc additive component; This gradient fabrication controllable preparation method refines the microstructure by applying ultrasonic pulses of different current modes on the basis of arc additive manufacturing, transforming columnar crystals generated during the additive manufacturing process into equiaxed crystals. As the melting depth increases, the surface grain refinement weakens. At this point, laser is used to impact strengthen the additive surface, causing microscale plastic deformation on the surface to achieve strengthening, thereby realizing the preparation of gradient structure. In step S2, the pulse current can be divided into several forms, including conventional pulse current, peak composite ultrasonic pulse current, base value composite ultrasonic pulse current, and full-stage composite ultrasonic pulse current. By adding peak composite ultrasonic, base value composite ultrasonic, and full-stage composite ultrasonic to the conventional pulse current, dynamic control of the pulse current can be achieved.

2. The gradient organization controllable preparation method according to claim 1, characterized in that, In step S2, the application of ultrasonic pulse current is applicable to different types of metals. Among them, titanium alloys, high-temperature alloys, and carbon steel are suitable for applying ultrasonic current to DC and pulse current, while aluminum alloys and magnesium alloys are suitable for applying ultrasonic current to variable polarity square waves and low-frequency tempered variable polarity square waves.

3. The gradient organization controllable preparation method according to claim 1, characterized in that, In step S3, laser shock strengthening of the ultrasonic pulse arc additive component is carried out in two forms. The first is to perform overall surface laser shock strengthening on the additive component after it is assembled. The second is to perform laser shock strengthening after 3 to 6 layers of additive manufacturing, and then to carry out additive manufacturing on the component after strengthening. This process is repeated to complete the final additive manufacturing and strengthening process.

4. A gradient organization controllable preparation method according to claim 1 or 3, characterized in that, In the laser enhancement process of step S3, the surface gradient tissue can be controlled and adjusted by regulating the laser energy, number of impacts, spot diameter, spot overlap rate and laser travel path.

5. The gradient organization controllable preparation method according to claim 4, characterized in that, The laser energy is 1~20J, the number of impacts is 1~100, and the spot diameter is 1~6mm.

6. The gradient organization controllable preparation method according to claim 1, characterized in that, Both the electric arc additive manufacturing process and the laser shock peening process are completed at room temperature and in an atmospheric environment.

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