| Rotary friction welding,being a solid-state welding method,has been widely applied in civil engineering that shows the high quality of the joint,high efficiency and energy saving of the process.However,rotary friction welding has not been well applied in aerospace industry owing to the lack of rules for process design.Essentially,the rules for process design is supported by the basic process theory such as model,criterion and mechanism.Therefore,this paper focuses on the manufacturing process of the rotary friction welding,in which different red-hardness austenitic alloys SUS304,GH2132,GH4169 were employed as typical materials and experiments were carried out as the main research methods.The work started from the essence of interface evolution characterized by corona-bond to systematically reveal the joint formation mechanisms of three stages as initiation,evolution and heat-pattern formation.As a result,theoretical models such as location and width of corona-bond at initiation,spreading criterion of corona-bond,evolution model of corona-bond and formation mechanism of heatpattern were constructed.Thereafter,the grain structures inside the corona-bond were further characterized to reveal the formation mechanism of friction welding.The main research contents and achievements are as follows:Firstly,the friction coefficient models were constructed for the first stage in friction welding process based upon the rotary friction welding experiments on samples of pipestructure.Based on the results,the heat liberation models of the first stage for rod-structure samples(Φ25mm)were established and the temperature fields were characterized,which were verified by the experimental results of the initiation location and morphology(width)of coronabond.Based on the results,models that describe the location and width of corona-bond at initiation with rotation speed and red-hardness were constructed.The corona-bond initiates at the high temperature zone.As a result,the corona-bond of low red-harness metal SUS304 initiates at 2/3R,which varies little with rotation speed.Whereas,the width increases from 0.4R to 0.7R as the rotation speed from 500 rpm to 2000 rpm.As the improvement of red-hardness(GH2132,GH4169),the corona-bond initiates at 0.33R～R at a low rotation speed,width of which decreases with the increase of rotation speed(GH2132: 0.75R(300rpm)to 0.45R(2000rpm);GH4169: 0.6R(500rpm)to 0.25R(1500rpm)).The rotation speed was found a criterion at about 900 rpm,above which the initiation location moves from 0.33R～R to 0～0.33 R and the width varies little(GH2132: 0.4～0.6R;GH4169: 0.2～0.4R).GH4169 has a highest redhardness,which concentrates the friction heat at center after 900 rpm.As a result,the limited initiation location of its corona-bond reaches 0.028 R,much smaller than 0.12 R of GH2132.Meanwhile,the width at initiation is stable at about 0.2R～0.4R smaller than 0.4R～0.6R of GH2132.Secondly,evolution law of corona-bond was achieved by taking stop actions to freeze the corona-bond morphologies in welding process.The experimental criterion of corona-bond spreading was established based on the models that describe the location and width of coronabond at initiation.Thereafter,experiments were conducted to reveal the correlation between the evolution of the normal stress distribution and the evolution of corona-bond at the friction interface,in which SUS304 was used as the base metal.Based on the results,mechanism of corona-bond spreading was revealed and evolution model of corona-bond was established.Then,formation mechanism of flash in rotary friction welding was investigated using GH4169 as the base metal.The driving force for the spreading of the corona-bond is the tendency of the normal stress distribution to evolve into a uniform distribution.The location and width of the corona-bond at initiation determine the form of the normal stress distribution after initiation.The normal stress distribution shows a ‘V’ shape when the corona-bond initiates at 0.33R～R,which concentrates the normal stress at periphery to drive the corona-bond spread to center.A shallow ‘M’ shape the normal stress distributed when the corona-bond initiates at 0～0.33 R with a width(29)0.4R,which drives the corona-bond spread from center to periphery.Whereas,the normal stress distribution is nearly uniform distributed when the corona-bond initiates at0～0.33 R with a width ≤ 0.8R,which makes it lacks the force for the corona-bond to spread.As a result,the corona-bond is closed at center without spreading,which causes the flash to be cracked.The GH4169 joint shows the flash appearance as smooth,arcuate microcracked and cracked.Smooth flash is the result of uniform flow and extrusion of plasticized metal,which is originated from the corona-bond initiating at 0.33R-R of the interface and then spreading.Whereas,arcuate microcracked and cracked flash are formed under rapid forging process of interface metal at welding temperature due to the lack of plasticized metal,which initiates at inner side of the interface(0-0.33 R with a width ≤ 0.4R)and then is closed without flow under the normal stress.Then,friction welding experiments were conducted at different rotation speeds to obtain different heat-patterns,formation mechanism of which was revealed based on the models as location and width of corona-bond at initiation,spreading criterion of corona-bond and evolution model of corona-bond.Thereafter,the grain structures inside different heat-patterns were characterized to analysis its effect on the mechanical properties’ distribution of the joints.The lens shape heat-pattern is formed when the corona-bond initiates at 0.33R～R and then spreads to center and periphery(SUS304,GH2132 GH4169 100-900rpm),inside which refined recrystallized grains are found,corresponding to the great performance of the joint.The line shape heat-pattern is formed when the corona-bond initiates at 0～0.33 R with a width(29)0.4R and then spreads to the periphery(GH2132 ≥ 900rpm),inside which a mixture of sub-grains and deformed grains is found.Line shape heat-pattern decreases the strength of GH2132 joint from 675 MPa to 640 MPa,compared with lens shape heat-pattern.The spindle shape heatpattern is formed when the corona-bond initiates at 0～0.33 R with a width ≤ 0.4R and without spreading(GH4169 ≥ 900rpm),which consisted of spindle body at center and friction line at periphery.The refined recrystallized grains are found in spindle body whereas a mixture of recrystallized grains and sub-grains is found in friction line.The friction line has little effect on the joint strength but significantly reduces the local plasticity,consequently increasing the unevenness of the joint elongation.The lens shape heat-pattern and spindle shape heat-pattern of GH4169 joint correspond to 17% and 9% elongation sampled at periphery respectively,where the corresponding inhomogeneity of joint elongation increases from 8% to 50%.Finally,the grain structures of pipe-structure samples under different welding conditions(characterized by the characteristic points of friction impedance and temperature)were analyzed to reveal the formation mechanism of friction welding,in which SUS304 was employed as the base metal.Then,the temporal and spatial distribution of the formation mechanism were obtained when it was generalized to the rod-structure samples.Meanwhile,evolution law of the critical recrystallization temperature with the friction linear velocity was revealed and the corresponding theoretical model was constructed.Based on the results,the phenomenon ‘the faster the friction speed,the lower the impedance peak value’ in friction welding was revealed.For the pipe-structure samples,the formation mechanism of was found a criticality as the impedance peak,before which the shear deformation governs the interface until it is taken over by recrystallization at the impedance peak;For the rod-structure samples,the temporal and spatial distribution of the formation mechanism of rod-structure sample were found a criticality as the impedance peak,before which the shear deformation governs the interface with the corona-bond spreads to 1/2R.The recrystallization takes over the interface from the periphery to the center and gradually occupies the interface after the impedance peak.In friction welding process,the critical recrystallization temperature decreases exponentially with the increase of friction linear velocity.As a result,the impedance peak decreases with the increase of friction linear velocity since a decreasing friction heat is needed to soften the interface when the critical recrystallization temperature decreases with the same impedance peak time.The research will enrich the fundamentals of process theory on rotary friction welding and support the further study on joint reliability. |
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