| At present,light-weight has become the future development trend of various manufacturing industries,and traditional steel is gradually replaced by high-strength light-weight metal materials,such as aluminum,magnesium and titanium.Both 5A06 and TA1 light-weight alloys have excellent corrosion resistance and mechanical properties.Self-pierce riveting(SPR),as a new,green and efficient mechanical connection technology,can realize the effective connection of homogenous or heterogeneous sheet materials 5A06 and TA1,and is widely used in aerospace,automotive and shipbuilding industries and other important fields.However,with the rapid development of society and the economy,the mechanical properties of the selfpierce riveting joints are required to be higher and higher in industrial production,and a single change in riveting process parameters has been unable to meet the proformance requirements of some special workpieces.The ultrasonic metal welding machine is used to perform ultrasonic composite connection on the self-pierce riveting joints.The high frequency and low amplitude ultrasonic vibration optimizes the forming quality of the self-pierce riveting joints,and solid phase welding is formed between the two thin plates of the joint,which effectively prevents the plate warping in the riveting area during the tensile process and significantly improves the mechanical properties of the self-pierce riveting joints.In this thesis,the influence of roll pattern of ultrasonic probe on the welding effect of SPR joints by composite ultrasonic metal welding is investigated,and the effect of ultrasonic metal welding on the mechanical properties of titanium and aluminum homogeneous or heterogeneous SPRjoints is also studied.At the same time,based on this experiment,a quality monitoring method for SPRjoints based on convolutional neural network is proposed.The results show that first of all,the solid phase welding between the joint plants is produced by ultrasonic welding with three types of roll patterns,which improves the forming quality and mechanical properties of joints,in which the C-type roll pattern has the best effect on the optimization of the mechanical properties of the joint.Secondly,ultrasonic metal welding improves the mechanical properties of aluminum alloy homogeneous SPR joints the most significant,while it is difficult to achieve reliable and effective optimization of titanium alloy homogeneous joints.In addition,the effect of ultrasonic metal welding on the failure form of the SPR joints has two sides,when ultrasonic welding can form a solid and effective solid-phase welding on the joint,the strength of the joint can be greatly improved,on the contrary,the strength of the joint will be reduced due to the defects formed at the joint by the pattern of the welding tool head and the bottom die,as well as the temperature rise during the welding process.Microscopic results show that the hardness distribution of the joint transverse line before and after ultrasonic welding show "M" type,after ultrasonic welding,the grain in the joint area is coarsened,resulting in softening of the plate.a large number of dimples appear on the fracture surface of SPR joint plate,which belong to typical plastic fracture,after ultrasonic welding,numerous black holes and structures like ’platform’ appear on the outer edge of the fracture surface of the joint plate.The fracture of the SPR joint rivet shows a stone-like shape,which is a typical intergranular fracture,but under the action of ultrasound,the outer edge of the joint rivet becomes brittle,while the plastic toughness of the inner edge of the rivet is enhanced.Finally,the highest accuracy of the SPR quality monitoring method based on the dynamic cavity convolution neural network has reached 95.7%,which is 3.6% higher than the standard convolution neural network.This is because the dynamic cavity convolution has significant advantages in suppressing background interference and identifying riveting defects with incomplete image edges. |
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