| Friction stir spot welding is a new variant friction stir seam welding method.The high-speed rotating tool penetrate into the plates which are heated to the thermoplastic state by friction.The welding process forms a similar resistance point welding lap joint.As we know,the AZ31 magnesium alloy is a kind of high-performance lightweight structural materials.With the trend of automotive lightweight,it is important to study the principle of AZ31 friction stir spot welding process.In this paper,we use the AZ31 magnesium alloy friction stir spot welding process as the research object.The methods include the theoretical analysis,finite element numerical simulation and experimental testing.The study of the temperature field and the material thermoplastic flow is divided into three parts: theoretical system,temperature field finite element simulation,experimental data analysis and plastic flow analysis.The finite element model of the transient temperature field of AZ31 magnesium alloy based on the continuous solid mechanics Johnson-Cook model was established by using Deform-3D.The process parameters were as follows: the rotational speed of the tool was 1000 r/min and the penetrate speed was 10mm/s.The study had considered the influence of friction and plastic deformation during the process of the spot welding.By using the point tracking technique to obtain the temperature data in the bottom of the pin and the shoulder.The maximum simulation temperature of the plate was 436 ?,the maximum simulation temperature of the tool was 421 ?,and the experimental result was 428 ?.The results of numerical simulation were in good agreement with the experimental data.In the experimental part,we obtained the parameters such as temperature,torque and axial force during spot welding through sensor detection and the thermocouples which are inserted into the bottom of the pin and the shoulder.We had analyzed the influence of the rotational speed and the penetrate speed on the temperature field.Under the same rational speed,the heating rate of the shoulder and the pin increased with the penetrate speed,but the maximum temperature is basically unchanged.The higher the rational speed,the higher the maximum temperature of the spot welding.However,the maximum temperatures in different speed are all lower than the melting point of magnesium alloy 650 ?.The maximum temperature of the pin obtained in the experiment is 532 ?with the rational speed of 3000r/min,and the highest heating rate is 93.974 ?/ s..The spot welding is a kind of solid-phase connection.Based on the measured torque and axial force,a method for calculating the total input energy of the tool and the energy for warming the material.Under the rotational speed of 3000 r / min and the penetrate speed of 2.5mm / s press,the equipment total input energy was 7.49 KJ,the final conversion to material temperature was 49.835 J,the final translation ratio was 6.7‰.Based on the hydrodynamic Arrhenius constitutive equation,the relationship between the viscosity parameter and the deformation temperature and strain rate is deduced.The effect of three kinds of tool design on the flow of the material during the friction stir spot welding was analyzed by Fluent software.The results showed that the cylindrical screw tool was more effective in reducing the metal spillover,increasing the contact area,enhancing the flow of material and offering the downward force than the other two.We analyzed the typical flow field distribution pattern by using the cylindrical screw and the influence of the rotation speed on the flow of the material during the spot welding.The distribution of the flow field is symmetrical,and the shoulder can affect the flow of material in the area below 1mm.The higher the rotational speed,the larger the area affected by the shoulder and the pin,and the higher the fluidity of the material.The finite element model of AZ31 magnesium alloy friction stir spot welding based on continuous solid mechanics was established.The experimental data were used to verify the temperature variation of the shoulder and the pin during the spot welding.Based on the experimental data,we had established the material flowing finite element model.The temperature and flow field which were influenced by the welding parameters such as the design of the tool,the rotation speed,the penetrate speed and so on were analyzed in this paper.This paper can provide the theoretical technical support and analysis reference for establishing FSSW finite element analysis database and spot welding production under certain conditions,different materials and process parameters. |
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