The Use Of Complex, But Inexpensive, Thermo Mechanical Processing To Illustrate A Range Of Engineering Principles In An Integrated And Synergistic Manner

The events that occur in metallic materials heated to high temperatures in the dynamic processing environment associated with an arc or beam welding operation can be used to illustrate a broad range of fundamental scientific and engineering concepts in a holistic manner. Moreover, the process and resultant weld are of inherent interest to students. The specific application studied in this laboratory is the addition of small amounts of nitrogen to alter microstructure in the weld deposit. Understanding the process requires the integrated use of thermodynamics, kinetics, physics, chemistry, solidification, heat and mass transfer, phase stability and materials engineering. Few laboratory experiences allow engineers to explore the performance of real engineering materials at homologous temperatures greater than 0.8. Fewer still enable students to relate material performance at these temperatures to the microstructure of the material. Moreover, the relationship of material properties at lower temperatures are rarely graphically and directly connected to the conditioning of a material at higher temperatures. This paper describes the conception and execution of a laboratory to improve undergraduate students understanding of complex material behaviors. In addition it includes portions which enable the student to quantify phenomena often discussed only in a qualitative fashion. Laboratory procedure for the experiment is described in detail. The laboratory presents theory and application in natural fashion, linked and mutually supportive. The paper discusses the exceptionally positive impact that this immediacy has on student learning. Introduction There is renewed emphasis on laboratory experience and project based learning in undergraduate engineering education, coupled with widespread belief that interdisciplinary exposure is critical to the development of the prototype engineer for the 21 st Century. The Accreditation Board for Engineering and Technology, (ABET), has asserted as much in their newest criteria for engineering programs. Emphasis on a more holistic approach to engineering education has gained widespread acceptance. A new “premise” is evolving in education, simply stated this assertion could be phrased “It is much better to learn by doing something, even in a very controlled environment, than to learn by simply talking about something or talking about doing something, even in a very free and open environment. ” Furthermore, this “postulate” has a corollary associated with it – that the need to “learn by doing” becomes more critical as students progress through the curriculum, as does the need for interdisciplinary and multidisciplinary P ge 10335.1 Proceedings of the 2005 American Society for Engineering Education Annual Conference and Exposition Copyright © 2005, American Society for Engineering Education exposure. The closer students come to leaving the discipline-dominated world of academia and entering the function-driven world of corporate America, the more their academic experience and environment should resemble the world beyond the “ivy-covered” walls. Therefore, well-developed and well-conceived laboratories are a key component in student learning, underpin subsequent independent project based learning and support the development of engineering judgment. Moreover, laboratories and project based learning opportunities provide alternative means of accommodating the ever-increasing variety of learning styles represented in our classrooms. Laboratories can re-energize students and give them the skill-set required to demonstrate the outcomes mandated in ABET’s Engineering Criteria 2000. (1) This paper describes an inexpensive approach to providing a rich laboratory experience accessible to a broad cross-section of engineering majors. Indeed, this laboratory experience is embellished by the participation of students from different majors, who ostensibly would have different core competencies. Even in difficult economic times laboratories can provide a return much greater than the investment required to create them, and a value far in excess of their cost. To be effective laboratories must put students “in the moment”. That is, laboratories must have an apparent reality, they must be challenging and they must have verisimilitude to engineering practice. In creating the laboratory we must walk a very fine line. We must ensure that our students understand the goals of the laboratory, but not rob them of the joy of discovery. We cannot waste laboratory time, but must avoid being formulaic or plodding. Most of all, we must keep a novelty and a healthy uncertainty associated with the laboratory experience. Paradoxically, the willingness to embrace uncertainty, and the ability to make decisions when data is incomplete are key features in the make-up of successful engineers. Thus, laboratory experiences should hold the same attraction and delight for our students as research and applications laboratories possess for our graduates. Applied researchers go to the laboratory to entice truth from an impassive natural world. Their aim is to sense, to assess, and, eventually, to advance. A well planned instructional laboratory enables students to realize these same goals. Laboratories are a necessary interlude during which students discover the value of collective experience and collaboration, and develop skills in sharing and exchanging information. Laboratories, then, create a microcosm of, and a brief segue to, behaviors that are analogous to the stimulating milieu encountered in authentic occupational environments. They provide a prospect for legitimate eureka events that can rouse intellectual fires which can blaze for decades and which can afford illumination that reaches far beyond evident borders of the exercise. Laboratories that immerse students in significant tasks engender acceptance of ambiguity and apparent contradictions that place students on the path to the development of engineering judgment. Theoretical Background Importance Welding operations are used for fabrication of stainless steel materials employed in components of systems used in a variety of industries, most notably the power, nuclear, food processing and chemical industries. Welding alters the microstructure of the materials welded, through the rapid thermal cycling, the solidification processing and the alloying inherent to the process. The amount of delta ferrite present in weld deposits associated with welds in stainless steel is P ge 10335.2 Proceedings of the 2005 American Society for Engineering Education Annual Conference and Exposition Copyright © 2005, American Society for Engineering Education recognized to have a key role in hot cracking, fissuring, corrosion and embrittlement during service. Bead Shape Melting and solidification are two of the most important reactions involved in fusion welding. In a sense they may be regarded as competing processes which occur simultaneously, each at its own characteristic rate. Thus, at the melting temperature of a pure metal, both solid and liquid may coexist in equilibrium with one another, and one can say that the rates of melting and solidification are equal. If we cool the pure metal to slightly below its melting temperature, the rate of solidification becomes dominant, and the rate of growth of the solid phase is controlled by the rate at which the latent heat liberated can be conducted to the surroundings. Thus, one can say that the control of melting and solidification of a pure metal is strictly a heat flow problem. Most metals of commercial importance are alloys and, as such, impose an additional constraint on the melting and solidification processes. Except for eutectics and the comparatively rare congruent-melting alloys which exhibit discrete melting points, alloys exhibit a melting temperature range. In general, the composition of the solid and liquid phases in equilibrium with each other are a function of temperature within the melting temperature range. Thus, in addition to the necessity of dissipating the latent heat of fusion, solidification of an alloy involves the redistribution of solute as well. Therefore, the solidification of an alloy involves both heat transfer and mass transfer, and it is important to understand their roles in the solidification of a fusion weld. Solidification Mechanics (2,3) In the production of a fusion weld, a molten weld pool is established and, through control of the process variables, is made to travel at a constant rate without significantly changing its shape. Figure 1 is a schematic representation of an elliptical weld pool with a bounding surface defined by locus of the liquidus temperature of the alloy. Surrounding the weld pool in figure 1 is a dashed curve corresponding to the locus of the effective solidus of the alloy – which is always lower than the nominal solidus expected for any given composition. The volume of the molten phase present varies in a continuous fashion from 0 at the dashed line to 100% at the solid line bounding the weld pool. Unfortunately, the existence and practical significance of this region, called the partially melted zone, is not widely recognized. Nor is the presence of a completely liquid “umixed zone” between the partially melted zone and the weld pool where convection occurs. This unmixed zone forms as a result of basic fluid mechanics, it is the boundary layer in this fluid system. It presents a barrier where mass transfer occurs only by diffusion. Assume that the weld pool shown in Figure 1 is created by a heat source located at 0 and moving from right to left with a velocity, V. In an autogenous bead-on-plate weld, melting will be occurring along the leading edge, ABC, of the weld pool. This requires that the latent heat of fusion be supplied to convert the solid to a liquid at the liquidus temperature. At the trailing surface of the weld pool, CDEA, the latent heat of fusion must be liberated to ca