Lunes 27 Octubre 2003

Friction Stir Welding of Aluminum Alloys. G. Bendzsak, and A. Gerlich, University of Toronto, Canada.

This paper reviews the research carried out on friction stir welding of aluminium alloys. The basic features of friction stir welded joints will be described. Satisfactory modeling of the FSW joining process depends on being able to determine the properties of the fully-plasticised material immediately adjacent to the rotating pin. Research will be described where the viscosity of this fully-plasticised material is found using plunge testing. During plunge testing the forces that acting on a rotating steel pin as it penetrates the aluminum alloy base material are measured and converted into effective viscosities and temperature output. The results produced during 3-D modeling of the friction stir welding process and the proposition that material mixing during welding depends on chaotic flow will be discussed in detail.

Concepts for Innovative Cutting Tools Based on Modern Joining Technologies. Institute of Materials Engineering, University of Dortmund, Dortmund, Germany.

Diamond as a cutting material plays a crucial role for machining stone and other mineral material. Diamond-based tools are employed in all types of cutting, drilling and grinding tools. In particular, diamond impregnated segments show a high flexibility regarding different substrates. Concrete, not matter whether it is reinforced or not, can be machined with diamond tools without large problems as long as the segment is not exposed to high temperatures. Due to this the application of diamond-impregnated cutting is generally accompanied by a water cooling equipment that has several disadvantages if it is employed in inhabitated buildings.

The history of diamond tools dates back to the early 1950’s when the high-temperature/high-pressure synthesis of synthetic had been developed. Over the years the synthesis process has been further optimized and today synthetic diamonds can be regarded as a commodity. Over the last decades further interesting developments have come up within the material group of artificial diamonds such as polycristaline diamonds (PCD) and diamond layers deposited by chemical vapour deposition (CVD).

Martes 28 Octubre 2003

Weldability Assesment for Repair Welding of Aluminum Casting Dies. E. Mendieta, C. Maldonado and J. Castillo, Instituto de Investigaciones Metalúrgicas, UMSNH, México.

The effects of heat input and preheating temperature on the weldability of an H13 steel for die applications are evaluated. The microstructure and properties of the weld metal and heat affected zone of H 13 steel in the as-weld condition are examined. In the present work the Tekken weldability test was applied and different electrodes and procedures were tested. The Tekken test was selected because of its high displacement restraint. SMAW and GTAW processes are applied altogether with a selection of several electrodes and wires. Three different preheating temperatures were considered. All the testing samples were examined using ultrasonic and liquid-penetrant tests, and some samples show the presence of cracks. The information of this work allowed to select the best procedure for repair welding of aluminum alloy casting dies.

Weldability of Aluminum Matrix Composites by Pulsed MIG Process. P. Lean, Área de Tecnología de Materiales, Pontificia Universidad Católica del Perú, Lima, Perú.

Resumen no disponible.

Microstructural Control in High Strength, High Toughness Steel Welds. S. Liu, Colorado School of Mines, Golden, USA.

Due to the excellent mechanical properties of High Strength Low Alloy (HSLA) plate steels, fabricators of naval and marine structures have great interest in increasing the application of these materials. In the last two decades, these steels have also spread into the pipeline industry. Nowadays, steels of strength levels between 400 and 550 MPa (80 and 100 ksi) can be found in many projects. The challenge is to produce weld deposits of comparable mechanical properties to the base metal.

Alloying elements such as Mn, Ni, Cr, Mo, Cu, Nb, V, and Ti (these latter three in microquantities) are added to improve the strength and toughness of the steels and their weld metals. The microstructure of these steels can vary from almost exclusively acicular ferrite, in the case of steels with strengths around 500 and 600 MPa (72 and 81 ksi), to a mixed ferrite/martensite or ferrite/bainite microstructure, in the case of steels of strength above 690 MPa (100 ksi).

Experimental shielded metal arc (SMA) welding electrodes showed that copper, together with a small quantity of niobium, and proper amounts of carbon, nitrogen, and titanium were required to produce high strength welds with acceptable impact toughness.

Transmission electron metallography revealed that a single needle of lath martensite contained an array of parallel sub-laths. Ferrite with second phase (FS) laths are also made up of several smaller parallel sub-laths. Acicular ferrite (AF) exhibits sub-laths, but in almost 90° interlocking patterns. Finally, a dispersed phase, known as granular bainite (GB) is a body-centered cubic (BCC) matrix with retained austenite (RA) islands. A microstructure consisted of predominantly AF with some coarse GB presented the best combined strength and impact toughness.

The prediction of weld metal and HAZ microstructure and their cracking susceptibility can be made as a function of carbon equivalent type predictors, in particular, the martensite start temperature and alloy hardenability.

Advances in Hydrogen Management for High Strength Steel. D.L Olson, S. Liu, Y.D. Park, Colorado School of Mines, Center for Welding, Joining, and Coatings Research, Golden, Colorado, USA.

High strength low alloy (HSLA) steels are known to be susceptible to hydrogen cracking. Also, the hydrogen assisted cracking in HSLA steel welds is considered to take place when all the necessary conditions for cracking are satisfied simultaneously. These conditions include the combination of unacceptable diffusible hydrogen content, high tensile stress, high hardness or a susceptible microstructure, and a temperature ranging between –50 and 100 °C. One of the main practices to make a HAC resisting high strength steel weldment is to reduce the amount of diffusible hydrogen. Another common practice is the pre- or post- weld heat treatment. However, heat treatments are cost intensive and, in some critical cases, not effective. Alternative methods based on metallurgical principles have been studied and offer both for technological and economic merits.

This paper focuses on the recent advances in hydrogen management in high strength steel, particularly on three strategies. First of all, proper selection of weld metal martensite (ferrite) start temperature is introduced. Second, Irreversible weld metal hydrogen traps, such as yttrium addition, have been demonstrated to be effective in managing diffusible hydrogen content, and thus the susceptibility for hydrogen-assisted cracking. The influence of the welding parameters on yttrium transferability across the welding arc and on hydrogen trapping behavior in the weld deposition is presented to assist in transferring this advancement to welding practice. Finally, the use of fluoride additions to the welding consumables also has been shown to effectively reduce the weld metal hydrogen content. This paper also discussed about the role of retained austenite in the higher strength steel weld metal in storing and supplyingusible hydrogen in the weld deposit and its behavior during changes in service temperature is reported.

Miércoles 29 Octubre 2003

Material Science Aspects of Weld Corrosion. D.L. Olson, A.N. Lasseigne, M. Marya, B. Mishra, Colorado School of Mines, Center for Welding, Joining and Coatings Research; Golden, Colorado, USA.

Corrosion is an environmentally assisted damage that professionals face daily, particularly with welded structures. Fusion welds result from solidification and solid-state transformations induced by well-localized thermal cycles. A fusion weld joint inherently exhibits an irregular surface as well as gradients in chemical composition, microstructure, properties and residual stress, depending upon process parameters and part geometry. This article analyzes the roles of surface topography, alloy chemical compositional variation, hydrogen distribution, and stress on weld corrosion.

Current Problems in Hot Cracking Research Described on the Example of PVR Test. H. Herold, P. Chennikov and M. Streitenberger, Universitat Magdeburg, Germany.

The latest state of the art of experimental and theoretical investigation for defining the hot cracking sensitivity of austenitic materials, filler metal and applied welding procedures is reported. On the basis of hot cracking theories both detailed theoretical statements as well as hot cracking criteria for the assessment of various stages of hot cracking are developed. The expressiveness of hot cracking tests (varestraint test, transvarestraint tests, PVR test und hot tensile test) and their special test criteria are different. The thermal, mechanical and temporal influences on the stages of hot cracking initiation are observed and classified by its origin of material, welding procedure and specimen size.
The Institute of Joining and Beam Technology has at its disposal the largest PVR testing equipment in Europe, for the time being. During the test a linear increasing tension speed is superposed by a bead on plate TIG welding in welding direction. Diverse applications of the PVR test are introduced. The sensitivity against solidification cracking in weld metal and liquation cracking and ductility dip cracking in heat-affected base metals or weld metals are quantified for a weldment during a definite welding procedure on one specimen only. The tested materials include austenitic stainless steels in various compositions and alloys on the bases of Ni, Al and Mg. The original standard test procedure of the PVR test is developed for the filler material selection as well as the optimisation of welding procedure. For the optimisation of welding processing any variation of welding procedures (gas-shielded metal-arc welding, TIG welding, laser welding) can be applied.
The limits of the quantifying criteria for hot cracking sensitivity by test procedures are proved by the critical hot cracking initiation during welding of large welding components. An assessment diagram for hot cracking initiation of weldment is developed by a substantial experiment package during fabrication welding of large components