Analysis And Quality Inspection Of Nugget Characteristics For Dual Phase Steel Welds

The energy and environment crisis caused by the increase of automobileproduction are becoming more and more urgent. Weight reduction ofautomobile is considered as the one of most effective ways to overcome aboveproblems. Dual-phase steel (DP) is such a kind of material used in automobileproduction to realize automobiles’ weight reduction without decrease strengthand stiffness. Therefore, the traditional low carbon and low alloy steel used inautomobile industry are gradually replaced by high strength steels such as DP.Resistance spot welding process is still the major joining method inautomobile industry due to its advantages such as low cost and easyautomation. However, the special microstructure of DP will cause asymmetriccooling speed and result in crack and shrinkage voids during resistance spotwelding process. The strength of nugget will lower than base material andinterfacial fracture failure mode will appear during mechanical test. In addition,the weldability of the hot dip galvanized steel is poor and the effect of alloyingbetween zinc coating and electrode face result in change of character of nuggetface. How to recognize inside and outside weld nuggets defects and thenrealize on-line inspection is becoming urgent for automobile industries.Based on the background mentioned above, DP steel is selected as theresearch material in this paper. Firstly, tensile-shear are conducted to study thechange law of strength of nuggets and electrode wear; Secondly, in order toevaluate the weld quality by the data of characters of nugget face, A method ofimage pattern recognition is used to evaluate the quality of nuggets face;Thirdly, optimum range of the characteristic values of ultrasonic echo ofnuggets are determined. Then a pattern recognition method based on supportvector machine is developed. Lastly, a model of quality evaluation based ondynamic resistance and the weld’s indentation by character of feedback ofservo-gun is developed to realize on-line inspecting the spot quality of DP.The main tasks are listed as follows: (1) Experimental research on spot quality of DP and analysis ofnugget’s charactersIt is easy to produce martensite during the holding due to quick cooling inresistance spot welding DP steel. The high hardness will increase theoccurrence of brittleness and interfacial fracture in weld nugget zone. First, theexperiment system of spot welding based on servo-gun is developed to achievethe welding parameters and acquire the weld lobe curve under differentprocess parameters. Second, the effect of process parameters on tensile-shearstrength would be analyzed. The quantitative relation between welding processparameters and maximum tensile-shear force using multiple linear regressionmethods would be achieved. Moreover, the law of change of microstructure inthe nugget zone and HAZ is analyzed. The different fracture models of spotwelding are defined basic on the characteristics of martensite and the defectsof spot welding are classified. The results show that defects can affect thequality of spot welding obviously.(2) The characteristic recognition of weld nugget face by imageprocessingThe characteristics of weld nugget face, such as electrode pitting&crack,were complex, irregular and hard to measure accurately for the dual phaseresistance spot welds. The above characteristics would have an evident effecton joint quality. Therefore, an image recognition method was applied tomeasure the characteristics of pit. Firstly, an image collecting system wasdesigned based on CCD according to the requirement of image precision. Andthen an image processing procedure was developed to decrease noise of surfaceimages of nugget and to segment the image of defects. Then the quantitativeanalysis was done based on area algebra characteristic. Lastly, the procedure ofimage of nugget surface pattern recognition was developed to evaluate thequality of surface of nugget.(3) Inspecting defects of nugget by pattern recognition based onsupport vector machine and ultrasonic echo.In this section it selects five characteristic of ultrasonic echoes, such asattenuation, peak distance of bottom echo, valid numbers of echo, minimumgain and peak of middle echo based on different regular of ultrasonic echo corresponding to different defects of nugget. According to the differencebetween ultrasonic echo and different material and thickness of spot weld, onthe base of experiment of mechanical properties and metallography experimentthe optimum range of characteristic of ultrasonic echo can be made. Theintelligent method of inspecting defects of DP’s nugget based on SVM isdeveloped to realize recognizing the different quality of spot welds quicklyand accurately.(4) On-line inspection of spot welds quality based on weld indentationof spot weld.The on-line weld quality inspection of spot welded joints based ondynamic resistance and indentation depth are analyzed in this section. First, thechanging law of dynamic resistance is used to analyze the relationship betweendynamic resistances and weld nugget formation. The model of on-lineevaluating the quality of spot welds based on dynamic resistance is developedduring the welding process. The session of the dissertation proposed theon-line method of acquiring the indentation depth based on the controllabilityof servo-gun. The influence of process parameters on indentation and weldlobe based on indentation is developed.Based on above jobs, a comprehensive and systemic analysis oncharacteristics of DP’s nugget and quality of spot welds would be achieved.The dissertation shows the theoretical incomings and technical methods forinspecting and evaluating the joint quality. All these results would provide animportant theoretical reference for the wide use of dual-phase steel inauto-body manufacturing.