Theoretical Study On The Stress Of Laser Cladding Layer Before And After Ultrasonic Impact In Metal Magnetic Memory Evaluation

Stress is an important factor that affects or even determines the service quality of laser cladding layers.Using ultrasonic impact treatment to control stress and metal magnetic memory technology to evaluate stress can provide guarantee for the quality of laser cladding products.This thesis was based on the metal magnetic memory evaluation stress technology with the laser cladding layer sample with 45 steel as the base material and Fe314 alloy powder as the cladding material as the experimental object,and the force-magnetic relationship was studied from both experimental and theoretical aspects.Firstly,the ultrasonic impact parameters and metal magnetic memory acquisition parameters were optimized.Then,the influence of the structure of the samples with different ultrasonic impact coverage on the results of the metal magnetic memory evaluating stress was studied in detail.Finally,combined with numerical analysis,the laser cladding layer was modeled and the physical mechanism of magnetic memory detection technology was discussed.Main results of this thesis are as follows:The influence of the lift-off height,scanning speed and scanning direction of the probe on the metal magnetic memory signal in the process of metal magnetic memory signal acquisition was studied.In this study,1mm was selected as the lifting height,2500mm/min was selected as the scanning speed and the north-south placement was selected as the detection direction for magnetic memory signal acquisition.The influence of stress on the metal magnetic memory signal of laser cladding samples was studied.By comparing the Hp(y)signals of laser cladding samples under different stresses,it can be seen that in the elastic deformation stage,as the stress increases,the Hp(y)signal magnetic field intensity gradient K increases,and the rate of increase is basically constant.The magnetic field intensity gradient reaches its peak during the yield phase.When the specimen reaches the stage of plastic deformation strengthening,the magnetic field intensity gradient K on the surface of the specimen decreases with the increase of stress.When the sample is broken,the Hp(y)signal on the surface shows an "inverted S" shape,and the polarity changes at the break.The influence of the coverage rate of ultrasonic impact on the stress evaluation of metal magnetic memory technology was studied.By comparing the Hp(y)signals of the samples after different ultrasonic impact coverage,the research results show that the relationship between the Hp(y)signal and the magnetic field intensity gradient K and the external stress after ultrasonic impact is similar to that without ultrasonic impact.With the increase of the ultrasonic impact coverage,the magnetic field intensity gradient K of the Hp(y)signal under each stress state gradually increases,and the increasing rate gradually decreases.After linear fitting,as the coverage of ultrasonic shock increases,the slope of the fitting equation gradually decreases,but the decreasing trend gradually slows down.However,the intercept gradually increases,but the increasing trend gradually slows down.Based on the theory of magnetic dipoles,the magnetic charge distribution of uniform/non-uniform magnetized ferromagnets was studied.By establishing the magnetic signal model of uniform/non-uniform magnetization ferromagnet,it is found that the uniform magnetization ferromagnet has only surface magnetic charge,but no body magnetic charge.The normal component of the magnetic signal is distributed in an oblique "S" shape,and the center is symmetric,which can be approximately regarded as an oblique straight line.The non-uniformly magnetized ferromagnetic magnetic charge has not only the surface magnetic charge but also the bulk magnetic charge.The normal component of the magnetic signal is symmetrically distributed with respect to the stress concentration area.There are wave crests and troughs,and its polarity is changed at the maximum stress concentration position.As the lift-off height increases,the amplitude of the magnetic signal decreases rapidly,the rate of decrease decreases,and the degree of signal mutation between peaks and valleys decreases significantly.The change of the depth and position of the stress concentration area will not change the law of the magnetic signal normal vector.As the depth of the stress concentration area increases,the amplitude of the magnetic signal normal vector increases significantly,while the increasing rate gradually decreases.As the stress concentration area moves from the surface to the inside of the structure,the amplitude of the magnetic signal normal vector decreases rapidly,but the range of the signal mutation does not change significantly.