Cranfield Institute of Technology - PhD, EngD, MSc, MSc by research theses, (CIT)
Permanent URI for this collection
Browse
Browsing Cranfield Institute of Technology - PhD, EngD, MSc, MSc by research theses, (CIT) by Author "Apps, R. L."
Now showing 1 - 3 of 3
Results Per Page
Sort Options
Item Open Access Fusion characteristics in P-GMAW of mild steel(1986-04) Quintino, Luisa; Apps, R. L.The influence of process parameters on deposition and fusion characteristics of mild steel with Pulsed Gas Metal Arc Welding has been investigated using a transistorized power supply. A simple model of melting behaviour has been developed which allows the prediction of dilution behaviour and explains the interplay between mass and heat in P-GMAW. In order to predict heat affected zone area, a model driven from the former one has also been developed. Deposition characteristics were studied taking the care of studying the influence of each welding parameter at each time, whenever it was possible. Emphasis was given to shielding gas mixture which was found to significantly influence metal transfer mode and arc stability. A systematic approach to choose process parameters in Pulsed Gas Metal Arc Welding with a constant current power supply is proposed. It was found that mean current and welding speed play the most important role in determining fusion characteristics, thus, based on a simple model, methods of controlling these aspects are suggested. Emphasis was placed on understanding dilution behaviour. It was found possibility to develop.procedures allowing the independent choice of deposition rate and dilution behaviour and to give an account of many observations with simple models of melting phenomena. The developed model is combined with calorimetric heat transfer measurements to investigate the result that only a small fraction of the total process power is required to melt the observed fusion areas. A "dual Heat" source fusion model is suggested where plate melting is largely in response to direct arc heating, providing means of optimising plate dilution and thereby reducing the risk of fusion defects. A generalized representation of fusion characteristics is given which allows independent selection of required fusion characteristics to be assessed.Item Open Access Metal transfer in MIG welding(1982-10) Ma, Jilong; Apps, R. L.The metal transfer process in MIG welding has been investigated. The heat balance of the melting process, forces acting on the wire tip and droplets, and droplet movement were examined quantitatively both under steady current and pulsed current conditions. A novel transistorised power source was employed for precision current adjustment which with the use of high speed cinephotography and careful metallographic techniques has allowed a re-assessment of current theories to be made. A new metal transfer mode designated as ’Drop Spray* has been discovered. This transfer mode is located between the well known globular and spray transfer modes and only occurs in a very narrow current range of 20A, but it has several important features. The relationship between metal transfer mode and the welding variables has been established quantitatively for the first time. It was found that the extension resistance and heat content of droplets are determined by current and hence metal transfer modes. The amount of spatter and fume was also found to be determined by transfer mode. Metal transfer under pulsed current was also investigated. It was found that the metal transfer modes under pulsed current are the same as that of steady current. It was also found that the first drop of every pulse is of drop spray mode and the subsequent droplets will be stream spray. Careful observations and measurements have been made at various stages of the current pulse to enable greater understanding of the influence of the pulse parameters. Based on the results mentioned above, a concept of controlled MIG welding was proposed, based on the control of metal transfer mode. By this concept any predetermined feature of conventional MIG welding can be achieved consistently and repeatedly. As an application of the proposed concept, drop spray transfer was reproduced over a wide current range by means of metal transfer control. A new controlled MIG process designated as ’controlled drop spray MIG' has been developed which features high efficiency, all positional ability, freedom from spatter, low fume generation and good bead appearance. The preliminary bead on plate trial shows that all the expected results have been achieved although many more trials are necessary to fully prove the process.Item Open Access A study of coarse grain heat affected zone of accelerated cooled structural steels(1990-11) Araujo, C. L. D.; Apps, R. L.Modern structural steels have significantly different (generally leaner) compositions than equivalent conventional steels developed over 20 years ago. To compensate for the lower carbon and alloy content more sophisticated thermomechanical treatments have been introduced to give very fine ferrite grain size and hence good strength, ductility and fracture toughness. Despite the fact that these changes were largely introduced to improve weldability, the modern structural steels have given problems of heat affected zone (HAZ) hydrogen induced coldcracking leading to considerable debate over the causes of this apparently increased susceptibility. The present work aimed to provide the basis for a better understanding of the metallurgical behaviour of the HAZ of these new modern steels. This was accomplished by employing three low carbon(C<0.15%) low sulphur (S<0.005%) content steels produced by the OLAC process which combines controlled rolling with accelerated cooling. A normalized low carbon (0.12%C) higher sulphur (0.031%S) steel was also included in the present project for comparison purposes. The starting point was to determine the transformation temperatures of the various microconstituents in the HAZ’s by Tn Situ’ thermal analysis. A new approach was developed in which thermal data obtained from real weld thermal cycle measurements were employed in the thermal analysis. The steels studied were analyzed in the light of the results of transformation temperatures, microstructural examinations and hardness measurements in the coarse grained region of the HAZ. This approach offers much greater accuracy and consistency than previous methods of determining thermal cycles and HAZ transformation temperatures, giving a better opportunity for studying the anomalous HAZ behaviour of modern steels. Between the OLAC steels, two steels presented very low hardness(<300HV5) within the entire range of heat inputs used (0.8-3.5kJ/mm),whereas the other gave hardnesses higher than those of the normalized steel. Higher carbon (0.13%C) and a poorly balanced alloy content was considered to be an explanation for the HAZ behaviour of this steel. An investigation on the accuracy of current carbon equivalent in predicting hardenability and hardness formulae was carried out but none of the existing formulae were completely satisfactory in indicating the trend in hardenability found in all the steels. An alternative formula for predicting the variation of hardness based on one put forward by Yurioka was developed, which proved to be suitable for three of the steels studied. However, the restricted nature of all empirical carbon equivalent formulae is demonstrated and the need to limit any such formula to a restricted range of steels is re-emphasized.