CN115505723A - Laser shock peening-based aluminum alloy friction stir welding butt joint strengthening and toughening method - Google Patents
Laser shock peening-based aluminum alloy friction stir welding butt joint strengthening and toughening method Download PDFInfo
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- CN115505723A CN115505723A CN202211469641.2A CN202211469641A CN115505723A CN 115505723 A CN115505723 A CN 115505723A CN 202211469641 A CN202211469641 A CN 202211469641A CN 115505723 A CN115505723 A CN 115505723A
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D10/00—Modifying the physical properties by methods other than heat treatment or deformation
- C21D10/005—Modifying the physical properties by methods other than heat treatment or deformation by laser shock processing
Abstract
The invention discloses a strengthening and toughening method of an aluminum alloy friction stir welding butt joint based on laser shock strengthening, and relates to the technical field of post-welding strengthening of aluminum alloy friction stir welding joints, wherein the strengthening and toughening method comprises the following steps: (1) Arranging an absorption layer and a restraint layer on the front surface of the whole softening zone of the friction stir welding butt joint of the aluminum alloy plate and on the symmetrical position of the back surface of the welding line; (2) Carrying out low-energy-density laser shock peening on symmetrical regions of the front and back surfaces of the whole softening region; (3) Then only carrying out high-energy-density laser shock strengthening on the symmetrical regions of the front and back surfaces of the influence region of the advancing side heat machine and the influence region of the retreating side heat machine, and not carrying out laser shock strengthening on the position of the stirring region; and (4) removing the absorption layer and the constraint layer. According to the invention, different laser shock strengthening parameters are adopted to carry out laser shock strengthening on different areas, so that the local performance uniformity of the joint is improved, the yield strength of the joint is greatly improved, and the integral elongation of the joint is improved.
Description
Technical Field
The invention relates to the technical field of post-weld strengthening of aluminum alloy friction stir welding joints, in particular to a strengthening and toughening method of an aluminum alloy friction stir welding butt joint based on laser shock strengthening.
Background
The high-strength aluminum alloy has the advantages of high specific strength, strong corrosion resistance, good fatigue life and the like, and is widely applied to rail transit, ship manufacturing, aerospace and military weapon manufacturing in recent years. The friction stir welding technology is used as one of solid-phase welding, can ensure that an aluminum alloy base metal is not melted when used for welding aluminum alloy, can basically eliminate the problems existing in the fusion welding process, and obtains the high-strength aluminum alloy friction stir welding with mechanical properties which are obviously higher than those of the traditional fusion welding head. However, the mechanical stirring force and the friction heat generated in the friction stir welding process can cause obvious differences of the grain appearance, the grain size and the precipitated phase in each area of a welding joint, a nugget area directly receives the mechanical stirring action of a stirring pin and is subjected to sufficient plastic deformation and welding thermal cycle, the precipitated phase is finally dissolved, and the grain structure is fine isometric crystal; the plastic deformation and welding heat of the heat engine affected zone are far lower than those of the welding nucleus zone, only part of original crystal grains are converted into isometric crystals, but most of the crystal grains are elongated and deformed thick columnar crystal structures, and the precipitated phase of the zone is also seriously dissolved and coarsened after the zone is heated, so that the mechanical property of the heat engine affected zone is the lowest. Obvious nonuniformity appears in microstructure and mechanical properties among all regions of the final joint, early strain concentration appears in a heat engine influence region in an early stage in a stretching process due to lowest hardness and yield strength, so that the joint is broken in the region, the problem of asymmetry of a joint softening region cannot be effectively solved through a conventional welding method and optimization of welding parameters, and the further improvement of the tensile property of the joint is restricted. Therefore, the welding joint with the large difference of the mechanical properties of all areas of the high-strength aluminum alloy friction stir welding joint is solved by adopting a reasonable weld reinforcement method, and the welding joint has important significance for further improving the mechanical properties.
Common strengthening methods of the aluminum alloy friction stir welding joint mainly comprise postweld heat treatment strengthening and weld joint surface strengthening. The residual tensile stress concentration of the shaft shoulder action area of the friction stir welding joint can be released through postweld heat treatment, the tissues of all areas of the joint can be stabilized, and the tissue uniformity is improved. However, it is difficult to perform integral heat treatment on the welded structural member with a large size, and the heat treatment process is generally complicated and has low production efficiency. The weld surface strengthening technology is to process the weld surface through a special process so as to improve various mechanical properties of the weld, common methods in friction stir welding weld strengthening include shot peening, ultrasonic strengthening and laser shock strengthening, and three strengthening methods are all that generate certain plastic deformation on the material surface through the action of external force so as to induce the near surface to generate dislocation activity, and can generally cause the grain refinement of the material surface layer and optimize the residual stress distribution. However, shot peening usually requires micron-level shots, the working environment is poor, a special shot blasting room needs to be arranged, and the production cost is increased; the surface roughness of the joint can be greatly improved in the ultrasonic strengthening process, the surface integrity of a friction stir welding joint is easily damaged, other surface defects are introduced, an ultrasonic transducer and an impact head with larger sizes are required in ultrasonic impact, and the impact strengthening position cannot be accurately controlled.
The laser shock strengthening technology adopts the principle that plasma explosion shock waves are generated on the surface of a material through a high-energy-density laser beam so as to induce the material to generate plastic deformation, is a non-contact strengthening method, has good accessibility, is environment-friendly and stable in quality, and has wide application in the field of strengthening titanium alloy, stainless steel, high-temperature alloy and aluminum alloy. At present, the laser shock strengthening method aiming at the welding joint is the same as the metal base material strengthening method, a large-area and full-coverage laser spot path is adopted, the whole welding seam area is strengthened indiscriminately, and finally, the yield strength and the tensile strength of the material can be obviously improved after the laser shock strengthening of the metal base material and the welding joint, but the elongation rate of the material is greatly reduced, and the obvious processing hardening characteristic is realized.
The aluminum alloy friction stir welding joint can be regarded as a heterogeneous composite material because the mechanical properties of all the areas of the aluminum alloy friction stir welding joint are different greatly, the position with lower hardness of the aluminum alloy friction stir welding joint has better corresponding relation with the strain concentration position in the stretching process, and the joint always has strain concentration in the area with the lowest local mechanical property until the joint is broken in the stretching process, so that the mechanical properties of different areas can be regulated and controlled by the technological advantage of accurate local strengthening through laser shock strengthening, the mechanical properties of the weak area of the joint are greatly improved, the uniformity among all the areas is increased, the integral stretching property of the joint is improved, and the reduction of the joint plasticity caused by serious work hardening in the strengthening process is prevented. However, the existing laser shock strengthening technology for the aluminum alloy friction stir welding does not have a reasonable laser shock strengthening region planning method, and does not have a laser shock strengthening method for optimizing the overall mechanical property of the joint by regulating and controlling the local mechanical properties of different regions of the aluminum alloy friction stir welding joint.
Disclosure of Invention
Aiming at the defects of the existing laser shock strengthening method for the friction stir welding butt joint of the aluminum alloy, the invention provides a laser shock strengthening method for reasonably planning a strengthening area according to joint subareas.
In order to achieve the purpose, the invention adopts the following technical scheme:
a laser shock peening-based method for strengthening and toughening an aluminum alloy friction stir welding butt joint comprises the following steps:
(1) Arranging an absorption layer and a restraint layer on the surface and the back of the whole softening region of the friction stir welding butt joint of the aluminum alloy plate;
(2) Carrying out low-energy-density laser shock strengthening on symmetrical regions of the front surface and the back surface of the whole welding seam softening region;
(3) Then only carrying out high-energy density laser shock strengthening on symmetrical regions on the front and back surfaces of an influence region of a side heat machine at the advancing side of the welding seam and an influence region of a side heat machine at the retreating side, and not carrying out laser shock strengthening on the position of a stirring region;
(4) And removing the absorption layer and the restraint layer on the surface and the back of the joint to obtain the toughened aluminum alloy friction stir welding joint.
Further, the actual softening zone position and width, advancing side heat engine affected zone position and width, retreating side heat engine affected zone position and width, and nugget zone position and width are determined from the joint horizontal vickers hardness distribution curve.
Furthermore, because the hardness of the aluminum alloy base material is relatively stable, the average hardness of the base material can be calculated according to data points of which the hardness of the left end and the right end of a horizontal Vickers hardness distribution curve of the joint is close to horizontal distribution, the area with the hardness lower than 95% of the average hardness of the base material is judged as a joint softening area, the width of the softening area and the position opposite to the central line of the welding seam are measured from the curve, the area directly acted by the stirring pin is judged as a joint stirring area, the width of the stirring area is the diameter of the stirring pin, and the stirring area is symmetrically distributed by the center of the welding seam; the widths and relative positions of the forward-side heat-engine affected zone and the backward-side heat-engine affected zone are determined in accordance with the relative relationship between the probe rotation direction and the welding direction.
Further, the aluminum alloy plate may be any one series aluminum alloy, preferably a two-series aluminum alloy and a six-series aluminum alloy.
Further, the parameters of the low energy density laser shock peening in the step (2) are as follows: the single pulse energy is 2 to 5J, the low energy density pulse laser spot size is 1 to 4mm, the square or round spot is overlapped at the spot overlapping rate of 30 to 70 percent, and the reinforcing times are 1 to 2.
And (2) performing primary strengthening on the whole softening region, so as to generate a certain plastic deformation on the front surface and the back surface of the whole softening region of the joint, generate a symmetrical residual compressive stress layer, generate a certain dislocation structure on the surface close to the softening region of the joint, but not generate a serious work hardening effect to reduce the plasticity of the joint.
Further, in the step (3), the high-energy-density laser shock peening parameters are as follows: the single pulse energy is 10 to 25J, the size of a high-energy density pulse laser light spot is 1 to 2mm, the square or round light spot has the lap joint rate of 50 to 80 percent, and the reinforcement times are 1 to 5.
Further, in the step (3), a single laser beam is used for strengthening the advancing side heat engine affected area and the retreating side heat engine affected area on the front side of the welding seam respectively, or double-beam laser is used for strengthening the advancing side heat engine affected area and the retreating side heat engine affected area on the front side of the welding seam simultaneously, and then the symmetrical area on the back side of the welding seam is strengthened in the same mode, wherein the step is mainly used for generating larger plastic deformation in the heat engine affected area, inducing coarse grains in the area to generate grain refinement and improving local dislocation density, generating the effects of fine grain strengthening and dislocation strengthening, and compensating the serious softening of the area, so that the mechanical properties among the advancing side heat engine affected area, the retreating side heat engine affected area and the stirring area are more uniform.
Further, the absorption layer is an aluminum foil, a black adhesive tape or black paint with the thickness of 10 to 100 mu m; the restraint layer is a flowing water layer with the thickness of 1-3mm or K9 glass.
Further, the joint horizontal vickers hardness distribution curve drawing method comprises the following steps:
1) Cutting an aluminum alloy base metal simulation piece with a proper size according to the size of a tool clamp of a friction stir welding workbench, and carrying out butt welding on the simulation piece by adopting welding equipment and welding parameters used in actual production;
2) Taking out metallographic samples from the simulation piece, grinding and polishing the cross section of a welding line to a mirror surface, testing the horizontal Vickers microhardness distribution of the joint, testing the hardness in the horizontal direction at a depth of 0.8mm from the upper surface of the welding line during testing, wherein the hardness testing force is 50 to 300g, the distance between the testing points is 0.1 to 1mm, the testing length is equal to the length of the metallographic samples, each metallographic sample is subjected to horizontal microhardness testing by adopting the same testing parameters, calculating the average value of all metallographic sample hardness testing results corresponding to each point, taking the central line of the welding line as the origin of coordinates, and drawing the horizontal Vickers hardness distribution curve of the joint by corresponding the coordinate of each average value to the hardness value.
Further, if the base material size in the preparation process of the simulation piece is too small, the heat dissipation condition and the clamping constraint state are greatly different from the actual production, and the length of the simulation piece in the step 1) is more than 30cm, and the width of the simulation piece is more than 20cm.
Further, the metallographic sample in the step 2) is cut by adopting a wire cut electrical discharge machining method, the sampling position is at least 5cm away from the starting end and the ending end of the welding line, the center of the welding line must be positioned at the center of the metallographic sample, the length of the metallographic sample exceeds the diameter of a shaft shoulder selected in the welding process by 10-40mm, the friction stir welding process is stable, but the possible slight difference of the welding line outline in different areas of the same welding line can not be avoided, so that 3-5 metallographic samples are taken, and the sampling interval is more than 50mm; and polishing the metallographic specimen to a mirror surface by using polishing paste of 3.5 to 0.5 mu m.
The strengthening principle of the aluminum alloy friction stir welding joint provided by the invention is as follows: the initial structure of the stirring area is fine isometric crystal, the mechanical properties such as local hardness and yield strength are high, the crystal grains in the heat engine influence area are large and the dissolution coarsening of the strengthening precipitated phase is serious, and the mechanical property is the worst. Firstly, carrying out laser shock strengthening on symmetrical regions on the front and back surfaces of the whole softening region by low-energy-density laser, generating a residual compressive stress field on the surface of the joint, and generating a certain amount of dislocation structures on the near surface of each region of the joint to generate dislocation strengthening to a certain degree; by adopting high-power-density laser to carry out laser shock strengthening on symmetrical regions of the front and back surfaces of the heat machine influence regions on the advancing side and the retreating side, obvious grain refinement is purposefully generated in the two regions, original coarse grains are broken, and dislocation with higher density is generated in the two regions, so that more obvious fine grain strengthening and dislocation strengthening are realized, serious softening of the two regions is compensated, and the yield strength of the two regions is improved. The hardness differences among the stirring zone, the advancing side heat machine influence zone and the retreating side heat machine influence zone can be effectively reduced aiming at the reinforcement of different zones, so that the local mechanical properties among the zones are more uniform, the strain can be more uniformly distributed among the zones in the stretching process, and the cooperative improvement of the overall yield strength and the elongation of the joint is finally realized.
The gain effect of the invention is as follows:
1. the method overcomes the limitation that the conventional method greatly reduces the extensibility of the aluminum alloy friction stir welding joint after laser shock strengthening, regulates and controls the local mechanical property of the joint by designing a unique shock strengthening area, promotes the strain of the joint in the stretching process to be more uniformly distributed in the whole joint, and realizes the cooperative improvement of the strength and the elongation of the joint.
2. According to the invention, the width and the relative position of each region of the joint are accurately determined by adopting the hardness distribution of the joint, and the grain size and the dislocation density of different regions of the joint can be more accurately regulated and controlled by combining the advantage of laser shock strengthening controllability, so that more uniform local mechanical properties are obtained.
3. The invention adopts the simulation piece to determine the hardness distribution of the joint, only needs conventional laser shock strengthening equipment, does not need to develop extra equipment according to the friction stir welding component, and has strong operability and low cost.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the embodiments or the prior art descriptions will be briefly described below, it is obvious that the drawings in the following description are only embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the provided drawings without creative efforts.
FIG. 1 is a schematic diagram of a metallographic specimen sampling device.
FIG. 2 is a schematic diagram of a cross-section microhardness test position of a metallographic sample.
FIG. 3 is a hardness profile plotted for example 1 of the present invention.
FIG. 4 is a schematic diagram of a laser shock peening process according to the present invention.
Fig. 5 is a schematic diagram of a laser spot movement path when different impact areas are strengthened by laser shock peening in the present invention.
FIG. 6 is a graph showing a tensile curve (a) before and after strengthening of an aluminum alloy friction stir welded joint obtained in example 1 of the present invention and a statistical comparison of joint properties (b).
FIG. 7 is a drawing curve (a) before and after reinforcing an aluminum alloy friction stir welded joint obtained by comparative example 1 of the present invention and a statistical comparison chart (b) of joint properties;
FIG. 8 is a drawing curve (a) before and after reinforcing an aluminum alloy friction stir welded joint obtained by comparative example 2 of the present invention and a statistical comparison chart (b) of joint properties;
FIG. 9 is a drawing curve (a) before and after the aluminum alloy friction stir welded joint obtained in comparative example 3 according to the present invention was strengthened and a statistical comparison of joint properties (b).
In the figure: 1-a simulation piece; 2-metallographic specimen sampling position; 3, friction stir welding the welding seam of the simulation piece; 4-metallographic specimen; 5-horizontal microhardness test position; 6-actual workpiece friction stir welding seam to be strengthened; 7-the centre line of the weld; 8-Joint softening zone range; 9-forward side heat engine affected zone range; 10-stirring zone range; 11-retreating side heat engine affected zone; 12-a constraining layer; 13-an absorbent layer; 14-low energy density laser shock peening region; 15-high energy density laser shock peening region; 16-low fluence pulsed laser spot; 17-laser spot movement path; 18-high energy density pulsed laser spot.
Detailed Description
The technical solutions in the embodiments of the present invention will be described clearly and completely below, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be obtained by a person skilled in the art without making any creative effort based on the embodiments in the present invention, belong to the protection scope of the present invention.
According to the invention, firstly, simulation samples are made of the same material and welding parameters to test the horizontal hardness distribution of the section of the welded joint, the total width of a softening area of the joint, the width of an influence area of a heat engine and the width of a welding core area are determined, then different laser shock strengthening parameters are adopted to carry out laser shock strengthening on different areas, the local performance uniformity of the joint is improved, the deformation potential of the welding core area is exerted, and thus the yield strength of the joint is greatly improved and the integral elongation of the joint is ensured to be increased.
The method for strengthening and toughening the aluminum alloy friction stir welding joint based on laser shock strengthening provided by the invention is described in detail by combining the drawings 1-5:
(1) According to the size of a tool clamp of a friction stir welding workbench, cutting a sample piece welding base metal with the length of more than 30cm and the width of more than 20cm from an aluminum alloy welding base metal (2219 aluminum alloy) used in actual production, and carrying out butt welding on a simulation piece by adopting welding equipment and welding parameters used in actual production to obtain a simulation piece 1;
(2) Measuring a metallographic sample sampling position 2 from a region of a simulation part weld joint 3 on a welded simulation part 1, cutting a metallographic sample 4 at the metallographic sample sampling position 2 by adopting a wire-cut electrical discharge machining method, wherein the position of the metallographic sample sampling position 2 is at least 5cm away from a weld joint starting end and a weld joint finishing end, a weld joint central line 6 must be positioned at the center of the metallographic sample 4, the length of the metallographic sample 4 is 10 to 40mm greater than that of a shaft shoulder selected in a welding process in order to ensure that each region of a joint is completely contained, the metallographic sample is 3 to 5, the sampling interval is greater than 50mm, polishing the cross section of the weld joint to a mirror surface by using polishing paste of 3.5 to 0.5 mu m, determining a horizontal microhardness testing position 5 at a depth of 0.8mm away from the upper surface of the metallographic sample 4, the hardness testing force is 50 to 300g, the interval is 0.1 to 1mm, the testing length is equal to the metallographic sample length, and each sample adopts the same testing parameters to carry out horizontal microhardness testing;
(3) Calculating the average value of all metallographic specimen hardness test results corresponding to each point, taking a welding seam central line 7 as a coordinate origin, and drawing a hardness distribution curve by corresponding the coordinate corresponding to each point average value with the hardness value, wherein the metallographic specimen microhardness test width is larger than the welding joint width and comprises a part of base metal area, the base metal hardness distribution is stable, so that the average hardness of the base metal can be calculated by selecting the hardness values of data points close to horizontal distribution on the left side and the right side of the hardness distribution curve, then judging the area with the hardness lower than 95% of the base metal average hardness as a joint softening area, measuring the width and the relative position with the welding seam central line 7 from the figure, determining the relative position and the width of a stirring area according to the diameter of a stirring pin, and determining the width and the relative position of an advancing side heat machine influence area and a retreating side heat machine influence area by combining the relative relationship between the rotation direction and the welding direction of the stirring pin;
(4) Determining the softening area range 8 of the whole joint on the surface of the actual workpiece friction stir welding line 6 to be strengthened according to the hardness value measured in the step (3), further determining a stirring area 10, an advancing side heat engine influence area 9 and a retreating side heat engine influence area position 11 in the softening area, and arranging an aluminum foil, a black tape or black paint with the thickness of 10-100 mu m as an absorption layer 13, and a flowing water layer with the thickness of 1-3 mm or K9 glass as a restraint layer 12 on the surface of the whole softening area range 8 and the symmetrical position of the back of the welding line;
(5) Firstly, carrying out low-energy-density laser shock peening on the whole softening region range 8, wherein the parameters are as follows: the single pulse energy is 2 to 5J, the size of a low-energy-density pulse light spot is 1 to 4mm, a square or round light spot can be used, the overlapping ratio of the light spot is 30 to 70 percent, the strengthening times are 1 to 2 times, and in order to ensure the symmetrical distribution of residual stress, the laser shock strengthening is also needed to be carried out on a symmetrical area of the back surface of a welding seam, so that a certain residual compression stress layer is generated, a certain dislocation structure is generated on the near surface of a whole joint softening area, but the serious processing hardening is not generated, so that the plasticity of the joint is reduced;
(6) Then only high-energy-density laser shock strengthening is carried out on the forward-side heat engine affected zone 9 and the backward-side heat engine affected zone 11, laser shock strengthening is not carried out on the stirring zone 8, a single laser beam can be used for strengthening the forward-side heat engine affected zone and the backward-side heat engine affected zone of the welding seam respectively, double-beam laser can also be used for strengthening the two zones simultaneously, and then the symmetrical zone of the back surface of the welding seam is strengthened by adopting the same process, the step is mainly to generate larger plastic deformation in the heat engine affected zone, induce the coarse grains in the zone to generate grain refinement and improve the local dislocation density, make up the severe softening of the zone, and enable the mechanical properties among the forward-side heat engine affected zone, the backward-side heat engine affected zone and the stirring zone to be more uniform, and the specific parameters are as follows: the single-pulse energy is 10-25J, the size of a high-energy-density pulse laser spot is 1-2mm, a square or round spot can be used, the overlap joint rate of the spot is 50% -80%, the strengthening times are 1-5 times, and in order to ensure that the distribution of residual stress is symmetrical, laser shock strengthening is carried out on a symmetrical area of the back of the welding seam.
(7) And finally removing the absorption layer 13 and the restraint layer 12 to obtain the high-quality aluminum alloy friction stir welding joint with the strength and the elongation rate synergistically improved.
Example 1
The joint to be strengthened in this example was a 2219 aluminum alloy friction stir welded butt joint 7mm thick with a stir head diameter of 10mm, a shoulder diameter of 25mm, a welding rotation speed of 500rpm, and a welding speed of 170mm/min. The laser shock strengthening is carried out by adopting the strengthening method provided by the invention. Firstly, according to the size of a tool clamp of an actual friction stir welding workbench, cutting two sample piece welding parent metals with the length of 50cm and the width of 30cm from 2219 aluminum alloy welding parent metal used in actual production, and carrying out butt welding on a simulation piece by adopting welding parameters such as the diameter of a stirring head of 10mm, the diameter of a shaft shoulder of 25mm, the matching of welding rotation speed of 500rpm, welding speed of 170mm/min and the like to obtain the simulation piece 1. Measuring 3 metallographic sample sampling positions 2 in an area 10cm away from a starting end and an ending end of a welding line of a simulation part 1, wherein the distance between the metallographic sample sampling positions is 60mm, cutting a metallographic sample 4 at the position of the metallographic sample sampling position 2 by adopting a wire cut electrical discharge machining method, the length of the metallographic sample 4 is 60mm, polishing the cross section of the welding line to a mirror surface by adopting 3.5 mu m polishing paste, determining a horizontal microhardness testing position 5 at a depth of 0.8mm away from the upper surface of the metallographic sample 4, selecting a hardness testing force value of 200g, wherein the distance between the testing points is 1mm, the testing length is equal to that of the metallographic sample, and performing horizontal microhardness testing on each metallographic sample by adopting the same testing parameters. Calculating the average value of all metallographic samples corresponding to each point in the hardness test result, taking the central line 7 of the weld as the origin of coordinates, and drawing a hardness distribution curve by corresponding the coordinate corresponding to each point to the hardness value, as shown in FIG. 3, because the microhardness test width of the metallographic sample is greater than the width of a welded joint, a part of base material regions are included, and it can be seen from the hardness distribution curve chart 3 that the hardness distribution of the base material regions on the left side and the right side is close to a horizontal line, the average hardness of the base material can be calculated by taking the average value of the data points on the two sides, and the region with the hardness lower than 95% of the average hardness of the base material is a joint softening region, namely the region with the hardness lower than 140HV, and the total width measured from FIG. 3 is 37.5mm, the width on the advancing side is 18.5mm, the width on the retreating side is 19mm, and the diameter of the stirring pin is 10mm, therefore, the central coordinate point of the stirring pin is determined to be 0mm,0mm to 5mm and 0 to 5mm and the heat engine region on the retreating side is determined by combining the relative relationship between the rotation direction of the stirring pin and the welding direction. Determining the softening area range 8 of the whole joint on the surface of the actual workpiece friction stir welding seam 6 required to be strengthened according to the hardness value measured in the step (3), further determining the stirring area 10, the advancing side heat engine influence area 9 and the retreating side heat engine influence area position 11 in the softening area, and arranging 50 mu m thick aluminum foil as an absorbing layer 13 and a 1mm thick flowing water layer constraint layer 12 on the surface of the whole softening area range 8 and the symmetrical position of the welding seam back surface. Firstly, carrying out low-energy-density laser shock peening on the whole softening region range 8, wherein the parameters are as follows: the single pulse energy is 2J, the size of a low-energy-density pulse light spot is 2mm, a circular light spot is used, the overlapping rate of the light spot is 50%, the strengthening times are 1 time, and in order to ensure symmetrical distribution of residual stress, a symmetrical area on the back of the welding seam needs to be subjected to laser shock strengthening. Then only carrying out high-energy-density laser shock strengthening on the advancing side heat engine affected zone 9 and the retreating side heat engine affected zone 11, not carrying out laser shock strengthening on the stirring zone 8, respectively strengthening the advancing side heat engine affected zone and the retreating side heat engine affected zone by using a single laser beam, and strengthening a weld back symmetrical zone by using the same process, wherein the step is mainly to generate larger plastic deformation in the heat engine affected zone, induce coarse grains in the zone to generate grain refinement and improve local dislocation density, make up for severe softening of the zone, and enable the mechanical properties among the advancing side heat engine affected zone, the retreating side heat engine affected zone and the stirring zone to be more uniform, and the specific parameters are as follows: the single pulse energy is 15J, the size of a high-energy density pulse laser spot is 2mm, a circular spot is used, the overlap ratio of the spot is 70%, the strengthening times are 3 times, and in order to ensure symmetrical distribution of residual stress, laser shock strengthening needs to be carried out on a symmetrical area of the back surface of the welding seam. And finally removing the absorption layer 13 and the restraint layer 12 to obtain the high-quality aluminum alloy friction stir welding joint with the strength and the elongation rate synergistically improved.
Fig. 6 is a drawing curve and drawing property statistics of the friction stir welding joint obtained by the reinforcement method provided by the present invention, it can be seen that the strength is greatly improved, especially the yield strength is improved by about 56%, the elongation is improved by 6% instead of being reduced, and the strengthening and toughening of the joint performance are realized.
Comparative example 1
The joint to be reinforced in this comparative example was laser shock reinforced by a conventional large area reinforcement method as in example 1. As can be seen from example 1, the width of the softened region was 37.5mm, and in this example, the reinforcing region was wider than the softened region, covering the entire surface of the heat-engine-affected zone, the stirring zone and a part of the base material zone, and the width of the final reinforcing region was 50mm. Firstly, adopting small energy density laser shock peening, wherein the parameters are as follows: the single pulse energy is 2J, the size of a low-energy-density pulse light spot is 2mm, a circular light spot is used, the overlapping rate of the light spot is 50%, the strengthening times are 1 time, and then the laser shock strengthening is carried out by adopting the symmetrical area of the back surface of the welding seam with the same parameters. And then, performing large-area reinforcement on the reinforced area by using high energy density, wherein the specific parameters are as follows: the single pulse energy is 15J, the size of a high-energy-density pulse laser spot is 2mm, a circular spot is used, the overlap ratio of the spot is 70%, the strengthening times are 3 times, and then the laser shock strengthening is carried out on a symmetrical area of the back of the welding seam by adopting the same parameters.
Fig. 7 is a drawing curve and drawing performance statistics of the friction stir welding joint obtained by the common large-area laser shock peening method in the present comparative example, it can be found that although the strength is significantly improved, the elongation is reduced by about 25%, a severe work hardening effect occurs, and the strengthening and toughening cannot be achieved. The partition strengthening method provided by the invention is very important for strengthening and toughening the performance of the joint.
Comparative example 2
The joint to be reinforced in this comparative example was the same as in example 1, and the range of the thermo-mechanical affected zones of the softening zone, the stirring zone, the advancing side and the receding side was determined by the zoning method provided in example 1 for zoning reinforcement, but the laser power density was not changed during reinforcement. Firstly, carrying out high-energy density laser shock peening on the whole softening region range, wherein the specific parameters are as follows: the single pulse energy is 15J, the size of a high-energy-density pulse laser spot is 2mm, a circular spot is used, the overlap ratio of the spot is 70%, the front and back symmetrical areas are respectively strengthened for 1 time, then the high-energy-density laser shock strengthening is continuously used for strengthening the positions of an advancing side heat machine influence area and a retreating side heat machine influence area, the laser shock strengthening is not carried out on the position of a stirring area, and the specific parameters are as follows: the single pulse energy is 15J, the size of a high-energy density pulse laser spot is 2mm, a circular spot is used, the overlapping rate of the spot is 70%, and the symmetrical areas on the front side and the back side are strengthened for 3 times.
Fig. 8 is a drawing curve and drawing performance statistics of the friction stir welding joint obtained by performing zone strengthening only by high-energy-density laser shock in this comparative example, it can be found that although the strength is significantly improved, the elongation is reduced by about 13%, a significant work hardening effect occurs, and strengthening and toughening cannot be achieved. The laser energy density selection method provided by the invention is very important for strengthening and toughening the performance of the joint.
Comparative example 3
The joint to be reinforced in this comparative example was the same as in example 1, and the range of the heat mechanical influence region of the softening region, the stirring region, the advancing side and the receding side was determined by the zoning method provided in example 1 for zoning reinforcement, but the laser power density was not changed during reinforcement. Firstly, carrying out low-energy-density laser shock peening on the whole softening region range, wherein the specific parameters are as follows: the single pulse energy is 4J, the size of a low-energy-density pulse light spot is 2mm, a circular light spot is used, the light spot overlapping rate is 50%, the strengthening times of front and back symmetrical areas are 1 time respectively, then the low-energy-density laser shock strengthening is continuously used for strengthening the positions of an advancing side heat machine influence area and a retreating side heat machine influence area, the laser shock strengthening is not carried out on the position of a stirring area, and the specific parameters are as follows: single pulse energy is 4J, the size of a low-energy-density pulse light spot is 2mm, a circular light spot is used, the overlapping rate of the light spot is 50 percent, the strengthening times of the front and back symmetrical areas are 3 times respectively,
fig. 9 is a drawing curve and drawing performance statistics of the friction stir welding joint obtained by performing zone strengthening only by low-energy-density laser shock in the comparative example, and it can be found that although the strength improvement effect is significantly weakened, the elongation is still reduced by 4%, and strengthening and toughening cannot be achieved. It is again demonstrated that the laser energy density selection method provided in the present invention is very important to strengthen the joint performance.
In the present specification, the embodiments are described in a progressive manner, each embodiment focuses on differences from other embodiments, and the same and similar parts among the embodiments are referred to each other. The device disclosed in the embodiment corresponds to the method disclosed in the embodiment, so that the description is simple, and the relevant points can be referred to the description of the method part.
The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended 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 (10)
1. A laser shock peening-based aluminum alloy friction stir welding butt joint strengthening and toughening method is characterized by comprising the following steps:
(1) Arranging an absorption layer and a restraint layer on the front surface and the back surface of the whole softening zone of the friction stir welding butt joint of the aluminum alloy plate;
(2) Carrying out low-energy-density laser shock peening on symmetrical regions of the front and back surfaces of the whole softening region;
(3) Then only carrying out high-energy-density laser shock strengthening on the symmetrical regions of the front and back surfaces of the influence region of the advancing side heat machine and the influence region of the retreating side heat machine, and not carrying out laser shock strengthening on the position of the stirring region;
(4) And removing the absorption layers and the restraint layers on the front surface and the back surface of the joint to obtain the toughened aluminum alloy friction stir welding joint.
2. The method for strengthening and toughening an aluminum alloy friction stir welding butt joint based on laser shock peening as recited in claim 1, wherein the actual softening zone position and width, advancing side heat engine affected zone position and width, retreating side heat engine affected zone position and width, and nugget zone position and width are determined from the joint horizontal Vickers hardness distribution curve.
3. The method for strengthening and toughening an aluminum alloy friction stir welding butt joint based on laser shock peening as recited in claim 2, wherein the average hardness of the base metal is calculated according to a horizontal vickers hardness distribution curve of the joint, a region with hardness lower than 95% of the average hardness of the base metal is determined as a joint softening region, the width of the softening region and the position of the softening region relative to the center line of the weld are measured from the curve, a region directly acted by the stirring pin is determined as a joint stirring region, the width of the stirring region is the diameter of the stirring pin, and the stirring region is symmetrically distributed around the center of the weld; the widths and relative positions of the forward-side heat-engine affected zone and the backward-side heat-engine affected zone are determined in accordance with the relative relationship between the probe rotation direction and the welding direction.
4. The laser shock peening-based aluminum alloy friction stir welding butt joint strengthening and toughening method according to claim 1, wherein the low energy density laser shock peening parameters in the step (2) are as follows: the single pulse energy is 2 to 5J, the low energy density pulse laser spot size is 1 to 4mm, the square or round spot is overlapped at the spot overlapping rate of 30 to 70 percent, and the reinforcing times are 1 to 2.
5. The laser shock peening-based aluminum alloy friction stir welding butt joint strengthening and toughening method according to claim 1, wherein in the step (3), the high-energy-density laser shock peening parameters are as follows: the single pulse energy is 10-25J, the size of a high-energy density pulse laser spot is 1-2mm, the square or round spot is overlapped at a ratio of 50-80%, and the reinforcement times are 1-5.
6. The method for strengthening the butt joint of the aluminum alloy by friction stir welding based on the laser shock peening as claimed in claim 1, wherein step (3) uses a single laser beam to strengthen the forward-side heat engine affected zone and the backward-side heat engine affected zone respectively, or uses a double-beam laser to strengthen the forward-side heat engine affected zone and the backward-side heat engine affected zone simultaneously.
7. The laser shock peening-based aluminum alloy friction stir welding butt joint strengthening and toughening method according to claim 1, wherein the absorption layer is an aluminum foil, a black tape or a black paint with the thickness of 10-100 μm; the restraint layer is a flowing water layer with the thickness of 1-3mm or K9 glass.
8. The laser shock peening-based method for strengthening and toughening the aluminum alloy friction stir welding butt joints according to claim 2, wherein the method for drawing the horizontal Vickers hardness distribution curve of the joints comprises the following steps:
1) Cutting an aluminum alloy base metal simulation piece with a proper size according to the size of a tool clamp of a friction stir welding workbench, and carrying out butt welding on the simulation piece by adopting welding equipment and welding parameters used in actual production;
2) Taking out metallographic samples from the simulation piece, grinding and polishing the cross section of a welding line to a mirror surface, testing the horizontal Vickers microhardness distribution of the joint, testing the hardness in the horizontal direction at a depth of 0.8mm from the upper surface of the welding line during testing, wherein the hardness testing force is 50 to 300g, the distance between the testing points is 0.1 to 1mm, the testing length is equal to the length of the metallographic samples, each metallographic sample is subjected to horizontal microhardness testing by adopting the same testing parameters, calculating the average value of all metallographic sample hardness testing results corresponding to each point, taking the central line of the welding line as the origin of coordinates, and drawing the horizontal Vickers hardness distribution curve of the joint by corresponding the coordinate of each average value to the hardness value.
9. The laser shock peening-based method for strengthening and toughening an aluminum alloy friction stir welding butt joints according to claim 8, wherein in the step 1), the length of the simulation piece is greater than 30cm, and the width of the simulation piece is greater than 20cm.
10. The laser shock peening-based aluminum alloy friction stir welding butt joint strengthening and toughening method according to claim 8, characterized in that the metallographic sample in step 2) is cut by a wire cut electrical discharge machining method, the sampling position is at least 5cm away from the starting end and the ending end of the weld, the center of the weld must be located at the center of the metallographic sample, the length of the metallographic sample exceeds the diameter of a shaft shoulder selected in the welding process by 10-40mm, 3-5 metallographic samples are taken, and the sampling interval is more than 50mm; and polishing the metallographic specimen to a mirror surface by adopting polishing paste of 3.5 to 0.5 mu m.
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