INTELLIGENT SPOT WELDING QUALITY MONITORING USING ADVANCED SIGNAL PROCESSING TECHNIQUES

Master'sMASTER OF ENGINEERIN

Challenges towards Structural Integrity and Performance Improvement of Welded Structures

Welding is a fabrication process that joint materials, is extensively utilized in almost every field of metal constructions. Heterogeneity in mechanical properties, metallurgical and geometrical defects, post-weld residual stresses and distortion due to non-linear welding processes are prime concerns for performance reduction and failures of welded structures. Consequently, structural integrity analysis and performance improvement of weld joints are important issues that must be considered for structural safety and durability under loading. In this study, an extensive experimental program and analysis were undertaken on the challenges towards structural integrity analysis and performance improvement of different welded joints. Two widely used welding techniques including solid-state “friction- stir- welding (FSW)” and fusion arc “gas tungsten arc welding (GTAW)” were employed on two widely utilized materials, namely aluminum alloys and structural steels. Various destructive and non-destructive techniques were utilized for structural integrity analysis of the welded joints. Furthermore, various “post-weld treatment (PWT)” techniques were employed to improve mechanical performances of weld joints. The work herein is divided into six different sections including: (i) Establishment of an empirical correlation for FSW of aluminum alloys. The developed empirical correlation relates the three critical FSW process parameters and was found to successfully distinguish defective and defect-free weld schedules; (ii) Development of an optimized “adaptive neuro-fuzzy inference system (ANFIS)” model utilizing welding process parameters to predict ultimate tensile strength (UTS) of FSW joints; (iii) Determination of an optimum post-weld heat treatment (PWHT) condition for FS-welded aluminum alloys; (iv) Exploration on the influence of non-destructively evaluated weld-defects and obtain an optimum PWHT condition for GTA-welded aluminum alloys; (v) Investigation on the influence of PWHT and electrolytic-plasma-processing (EPP) on the performance of welded structural steel joints; and finally, (vi) Biaxial fatigue behavior evaluation of welded structural steel joints. The experimental research could be utilized to obtain defect free weld joints, establish weld acceptance/rejection criteria, and for the better design of welded aluminum alloy and steel structures. All attempted research steps mentioned above were carried out successfully. The results obtained within this effort will increase overall understanding of the structural integrity of welded aluminum alloys and steel structures

Monitoring and characterization of abnormal process conditions in resistance spot welding

Resistance spot welding (RSW) is extensively used for sheet metal joining of body-in-white (BIW) structure in the automobile industry. Key parameters, such as welding current, electrode force and welding time, are involved in the RSW process. Appropriate welding parameters are vital for producing good welds; otherwise, undersized weld and expulsion are likely to be caused. For a specific type of sheet metal, an acceptable nugget is produced when an appropriate combination of welding parameters is used. However, undersized welds and expulsion are still commonly seen in the plant environment, where some abnormal process conditions could account for the production of the poor quality welds. Understanding the influence of abnormal process conditions on spot weld quality and other RSW related issues is crucial. A range of online signals, strongly related to the nugget development history, have attracted keen interest from the research community. Recent monitoring systems established the applied dynamic resistance (DR) signal, and good prediction of nugget diameter was made based on signal values. However, the DR curves with abnormal process conditions did not agree well with those under normal condition, making them less useful in detecting abnormal process conditions. More importantly, none of the existing monitoring systems have taken these abnormal process conditions into account. In addition, electrode degradation is one of the most important issues in the plant environment. Two major electrode degradation mechanisms, softening and intermetallic compound (IMC) formation, are strongly related to the characteristics of welding parameters and sheet metals. Electrode misalignment creates a very distinct temperature history of the electrode tip face, and is believed to affect the electrode degradation mechanism. Though previous studies have shown that electrode misalignment can shorten electrode life, the detailed mechanism is still not understood. In this study, an online-monitoring system based on DR curve was first established via a random forest (RF) model. The samples included individual welds on the tensile shear test sample and welds on the same sheet, considering the airgap and shunting effect. It was found that the RF model achieved a high classification accuracy between good and poor welds. However, the DR signals were affected by the shunting distance, and they displayed opposite trends against individual welds made without any shunting effect. Furthermore, a suitable online signal, electrode displacement (ED), was proposed for monitoring abnormal process conditions such as shunting, air gap and close edged welds. Related to the thermal expansion of sheet metal, ED showed good consistency of profile features and actual nugget diameters between abnormal and normal welds. Next, the influence of electrode misalignment on electrode degradation of galvannealed steel was qualitatively and quantitatively investigated. A much-reduced electrode life was found under the angular misalignment of 5°. Pitting and electrode softening were accelerated on the misaligned electrodes. δ Fe-Zn phase from the galvannealed layer that extends electrodes was found non-uniformly distributed on the worn electrode. Furthermore, electron backscatter diffraction (EBSD) analysis was implemented on the worn electrode, showing marked reduction in grain diameter and aspect ratio. The grain deformation capacity was estimated by the distribution of the Taylor factor, where the portion of pore grain was substantially weakened in the recrystallized region compared to the base metal region

Parametric Studies Based Mechanical and Thermal Modelling of Spot Welded Joints

This work has focused on formulating a experimental/numerical framework for the investigation of spot weld properties and performance. An Inverse temperature measurement approach has been established to predict the thermal history of a spot welded joints using remote thermocouples. This method incorporated the experimental data into an Artificial Neural Network (AAN) to predict cooling curves of the HAZ. Advanced modelling programs have been developed to simulate spot welded joints and thermocouples. Using the programs to investigate the effects of the key dimensional or material parameters on the mechanical or thermal response of spot welded joints of steels and different thermocouple joints relevant to their applications. Graphical User Interface Abaqus plug-ins of spot welded joints have developed using Python scripting and are used to investigate the effect of nugget size and sheet thickness on the stress and deformation of spot welded joints of steel. These works are important to establish an integrated approach to study the electrical, mechanical and thermal process of the spot welding process

Engineering Principles

Over the last decade, there has been substantial development of welding technologies for joining advanced alloys and composites demanded by the evolving global manufacturing sector. The evolution of these welding technologies has been substantial and finds numerous applications in engineering industries. It is driven by our desire to reverse the impact of climate change and fuel consumption in several vital sectors. This book reviews the most recent developments in welding. It is organized into three sections: “Principles of Welding and Joining Technology,” “Microstructural Evolution and Residual Stress,” and “Applications of Welding and Joining.” Chapters address such topics as stresses in welding, tribology, thin-film metallurgical manufacturing processes, and mechanical manufacturing processes, as well as recent advances in welding and novel applications of these technologies for joining different materials such as titanium, aluminum, and magnesium alloys, ceramics, and plastics

Investigation of the use of Dynamic Resistance in Adaptive Welding and Statistical Relation to Quality

The pursuit of real time quality detection of a resistance spot weld (RSW) in the automotive industry has progressed alongside technology. With the capability of modern controllers to collect real-time data from the welding procedure on a millisecond scale, the use of the dynamic resistance curves has become feasible. The controller used in this study was a Bosch Rexroth PSI63C0.120L1 with adaptive welding capabilities. The adaptive welding method used by Bosch is based on a reference dynamic resistance curve upon which the controller bases all of the adjustments to the welding current and/or time. In this study the dynamic resistance curves of a simple 2 high stack up of USIBOR® was welded under possible unideal process conditions such as shunting, edge welding, and shim/gap welding. It was found that under extreme cases for shunt and edge welds, the dynamic resistance curve changes significantly. For the particular set up in this thesis, it was found that at a center to center weld shunting distance of 10 mm and an edge distance of approximately half the electrode face is statistically detectable. For shim/gap welding, the change in the dynamic resistance curve was present at a 1mm gap and less at the 2mm gap due to the set-up of the robot arm. When performing the welds under the same process conditions with adaptive welding, there was little change to the adaptive dynamic resistance curves. In the shunted condition, the adaptive welding extended the time and was able to overcome the shunting effect to produce an ideal nugget size. During edge welding it resulted in expulsion when it would otherwise not occur, producing undersized nuggets. In an analysis of the quality indicators of the weld controller, it was discovered that the Stabilization Factor is based on the average values of the weld, while the UIP values are based on the deviation of the dynamic resistance of the weld to the reference. Both the quality indicator variables the controller output were found to have no correlation to either the nugget diameter or the tensile strength of the welds. Thus, an alternative to make use of the data the controller collects was explored through a statistical approach of large data and regression modelling. A model with a fit of 40% was made for the similar stack up and validated with production parts, but it lacked the robustness to capture all the data. When attempting to replicate the study on a dissimilar weld situation, the laboratory data and the data from the production part resulted in an unacceptable fit. This was due to the strong effect of different robot welders on the dynamic resistance curve, the lack of robustness in creating the model, and lack of logic found in the model parameters and coefficients

Friction Stir Welding Manufacturing Advancement by On-Line High Temperature Phased Array Ultrasonic Testing and Correlation of Process Parameters to Joint Quality

Welding, a manufacturing process for joining, is widely employed in aerospace, aeronautical, maritime, nuclear, and automotive industries. Optimizing these techniques are paramount to continue the development of technologically advanced structures and vehicles. In this work, the manufacturing technique of friction stir welding (FSW) with aluminum alloy (AA) 2219-T87 is investigated to improve understanding of the process and advance manufacturing efficiency. AAs are widely employed in aerospace applications due to their notable strength and ductility. The extension of good strength and ductility to cryogenic temperatures make AAs suitable for rocket oxidizer and fuel tankage. AA-2219, a descendent of the original duralumin used to make Zeppelin frames, is currently in wide use in the aerospace industry. FSW, a solid-state process, joins the surfaces of a seam by stirring the surfaces together with a pin while the metal is held in place by a shoulder. The strength and ductility of friction stir (FS) welds depends upon the weld parameters, chiefly spindle rotational speed, feedrate, and plunge force (pinch force for self-reacting welds). Between conditions that produce defects, it appears in this study as well as those studies of which we are aware that FS welds show little variation in strength; however, outside this process parameter “window” the weld strength drops markedly. Manufacturers operate within this process parameter window, and the parameter establishment phase of welding operations constitutes the establishment of this process parameter window. The work herein aims to improve the manufacturing process of FSW by creating a new process parameter window selection methodology, creation of a weld quality prediction model, developing an analytical defect suppression model, and constructing a high temperature on-line phased array ultrasonic testing system for quality inspection

Numerical analysis of spot welds in sheet metal structures exposed to mechanical shock and vibration loading

Automobilska industrija jedna je od najvećih svjetskih industrija koja godišnje proizvodi otprilike 50 milijuna vozila. U novije vrijeme se, zbog ograničenja emisija ispušnih plinova te zahtjeva za povećanjem sigurnosti cestovnog prometa, ali i zahtjeva kupaca, suvremena autoindustrija suočava s izazovima smanjenja mase vozila, poboljšanja ekonomičnosti potrošnje goriva te optimiranja u svrhu povećanja kompatibilnosti tijekom sudara. Elektrootporno točkasto zavarivanje je najzastupljeniji postupak spajanja metalnih limova u autoindustriji, a zavar nastaje uslijed topline koja se razvija prolaskom struje kroz elektrode te otpora materijala koji se zavaruje. Dakle, točkasti zavar nastaje kao rezultat kompleksnog međudjelovanja topline, pritiska elektroda i naglih faznih pretvorba u materijalu. Budući da točkasti zavar predstavlja geometrijski i materijalni diskontinuitet, oko zavarenog spoja javlja se koncentracija naprezanja, što ima negativan utjecaj na pojavu pukotina uslijed cikličkih opterećenja te na integritet konstrukcije tijekom dinamičkih opterećenja, tj. sudara. Nadalje, numerička analiza pomoću metode konačnih elemenata te razvoj metoda za proračun zavara na temelju sila omogučuju brz i jednostavan proračun točkasto zavarenih spojeva uslijed cikličkih i dinamičkih opterećenja, čime se smanjuju troškovi i vrijeme eksperimentalnih ispitivanja u fazi izrade prototipa i konstruiranja. Stoga je cilj ovog diplomskog rada razviti numeričke procedure za proračun točkasto zavarenih spojeva uslijed cikličkih opterećenja s konstantnim amplitudama i stohastičkih uzbuda te uslijed dinamičkog šoka.Occupant safety, weight reduction, fuel efficiency and vehicle crashworthiness remain the most challenging design objectives of the modern automotive industry. Recent improvements in sheet metal joining processes and the use of advanced high strength steels have increased fuel-efficiency and enhanced vehicle durability, as well as the integrity of passenger compartment, simultaneously increasing weight reduction. Resistance spot welding is the most widely used metal joining process in automotive industry intended for joining of light gauge overlapping metal sheets. Although resistance spot welding includes complex interaction between mechanical loading, heating generated by the electric current and rapid microstructural transformations, compared to other joining processes, resistance spot welding is fast, easily automated and does not require the additional filler material. Since the material heterogeneity and geometric discontinuity around the weld nugget circumference cause stress concentration, spot-welded components are prone to premature failure under fatigue and crash loading conditions. Hence, it is crucial to understand the welded region microstructure and the mechanical behavior of spot-welded joints to balance competing design objectives and thus enhance vehicle durability and crashworthiness. In recent times numerical analyses have had a major impact on design optimization and cost reduction of full-scale experiments during prototype testing. Moreover, various numerical methods have been proposed to evaluate the fatigue strength of spot welds under constant amplitude loading. Generally, the fatigue analysis of spot-welded joints is classified into stress-based and force-based approaches. In comparison with the stress-based method, the force-based approach offers a quick solution and an accurate fatigue life estimation. Therefore, one of the objectives of the conducted research was to evaluate the existing methods for the fatigue analysis of spot-welded structures under constant amplitude loading and to study the effect of geometric characteristics on the fatigue behavior of spot-welded specimens. However, vehicle components are seldom subjected to the constant amplitude cyclic loading, thus the method of random vibration fatigue life prediction for spot-welded structures is evaluated. Finally, quasi-static damage analysis of spot-welded specimens is performed to evaluate the coupled force-based damage initiation criterion, which captures the complex behavior of spot-welded joints under general loading conditions, yet greatly simplifying the analysis

Numerical analysis of spot welds in sheet metal structures exposed to mechanical shock and vibration loading

Automobilska industrija jedna je od najvećih svjetskih industrija koja godišnje proizvodi otprilike 50 milijuna vozila. U novije vrijeme se, zbog ograničenja emisija ispušnih plinova te zahtjeva za povećanjem sigurnosti cestovnog prometa, ali i zahtjeva kupaca, suvremena autoindustrija suočava s izazovima smanjenja mase vozila, poboljšanja ekonomičnosti potrošnje goriva te optimiranja u svrhu povećanja kompatibilnosti tijekom sudara. Elektrootporno točkasto zavarivanje je najzastupljeniji postupak spajanja metalnih limova u autoindustriji, a zavar nastaje uslijed topline koja se razvija prolaskom struje kroz elektrode te otpora materijala koji se zavaruje. Dakle, točkasti zavar nastaje kao rezultat kompleksnog međudjelovanja topline, pritiska elektroda i naglih faznih pretvorba u materijalu. Budući da točkasti zavar predstavlja geometrijski i materijalni diskontinuitet, oko zavarenog spoja javlja se koncentracija naprezanja, što ima negativan utjecaj na pojavu pukotina uslijed cikličkih opterećenja te na integritet konstrukcije tijekom dinamičkih opterećenja, tj. sudara. Nadalje, numerička analiza pomoću metode konačnih elemenata te razvoj metoda za proračun zavara na temelju sila omogučuju brz i jednostavan proračun točkasto zavarenih spojeva uslijed cikličkih i dinamičkih opterećenja, čime se smanjuju troškovi i vrijeme eksperimentalnih ispitivanja u fazi izrade prototipa i konstruiranja. Stoga je cilj ovog diplomskog rada razviti numeričke procedure za proračun točkasto zavarenih spojeva uslijed cikličkih opterećenja s konstantnim amplitudama i stohastičkih uzbuda te uslijed dinamičkog šoka.Occupant safety, weight reduction, fuel efficiency and vehicle crashworthiness remain the most challenging design objectives of the modern automotive industry. Recent improvements in sheet metal joining processes and the use of advanced high strength steels have increased fuel-efficiency and enhanced vehicle durability, as well as the integrity of passenger compartment, simultaneously increasing weight reduction. Resistance spot welding is the most widely used metal joining process in automotive industry intended for joining of light gauge overlapping metal sheets. Although resistance spot welding includes complex interaction between mechanical loading, heating generated by the electric current and rapid microstructural transformations, compared to other joining processes, resistance spot welding is fast, easily automated and does not require the additional filler material. Since the material heterogeneity and geometric discontinuity around the weld nugget circumference cause stress concentration, spot-welded components are prone to premature failure under fatigue and crash loading conditions. Hence, it is crucial to understand the welded region microstructure and the mechanical behavior of spot-welded joints to balance competing design objectives and thus enhance vehicle durability and crashworthiness. In recent times numerical analyses have had a major impact on design optimization and cost reduction of full-scale experiments during prototype testing. Moreover, various numerical methods have been proposed to evaluate the fatigue strength of spot welds under constant amplitude loading. Generally, the fatigue analysis of spot-welded joints is classified into stress-based and force-based approaches. In comparison with the stress-based method, the force-based approach offers a quick solution and an accurate fatigue life estimation. Therefore, one of the objectives of the conducted research was to evaluate the existing methods for the fatigue analysis of spot-welded structures under constant amplitude loading and to study the effect of geometric characteristics on the fatigue behavior of spot-welded specimens. However, vehicle components are seldom subjected to the constant amplitude cyclic loading, thus the method of random vibration fatigue life prediction for spot-welded structures is evaluated. Finally, quasi-static damage analysis of spot-welded specimens is performed to evaluate the coupled force-based damage initiation criterion, which captures the complex behavior of spot-welded joints under general loading conditions, yet greatly simplifying the analysis