2,173 research outputs found
Improved micro-contact resistance model that considers material deformation, electron transport and thin film characteristics
This paper reports on an improved analytic model forpredicting micro-contact resistance needed for designing microelectro-mechanical systems (MEMS) switches. The originalmodel had two primary considerations: 1) contact materialdeformation (i.e. elastic, plastic, or elastic-plastic) and 2) effectivecontact area radius. The model also assumed that individual aspotswere close together and that their interactions weredependent on each other which led to using the single effective aspotcontact area model. This single effective area model wasused to determine specific electron transport regions (i.e. ballistic,quasi-ballistic, or diffusive) by comparing the effective radius andthe mean free path of an electron. Using this model required thatmicro-switch contact materials be deposited, during devicefabrication, with processes ensuring low surface roughness values(i.e. sputtered films). Sputtered thin film electric contacts,however, do not behave like bulk materials and the effects of thinfilm contacts and spreading resistance must be considered. Theimproved micro-contact resistance model accounts for the twoprimary considerations above, as well as, using thin film,sputtered, electric contact
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
Experiments and Simulation for 6061-T6 Aluminum Alloy Resistance Spot Welded Lap Joints
This comprehensive study is the first to quantify the fatigue performance, failure loads, and microstructure of resistance spot welding (RSW) in 6061-T6 aluminum (Al) alloy according to welding parameters and process sensitivity. The extensive experimental, theoretical and simulated analyses will provide a framework to optimize the welding of lightweight structures for more fuel-efficient automotive and military applications. The research was executed in four primary components. The first section involved using electron back scatter diffraction (EBSD) scanning, tensile testing, laser beam profilometry (LBP) measurements, and optical microscopy(OM) images to experimentally investigate failure loads and deformation of the Al-alloy resistance spot welded joints. Three welding conditions, as well as nugget and microstructure characteristics, were quantified according to predefined process parameters. Quasi-static tensile tests were used to characterize the failure loads in specimens based upon these same process parameters. Profilometer results showed that increasing the applied welding current deepened the weld imprints. The EBSD scans revealed the strong dependency between the grain sizes and orientation function on the process parameters. For the second section, the fatigue behavior of the RSW’ed joints was experimentally investigated. The process optimization included consideration of the forces, currents, and times for both the main weld and post-heating. Load control cyclic tests were conducted on single weld lap-shear joint coupons to characterize the fatigue behavior in spot welded specimens. Results demonstrate that welding parameters do indeed significantly affect the microstructure and fatigue performance for these welds. The third section comprised residual strains of resistance spot welded joints measured in three different directions, denoted as in-plane longitudinal, in-plane transversal, and normal, and captured on the fusion zone, heat affected zone and base metal of the joints. Neutron diffraction results showed residual stresses in the weld are approximately 40% lower than the yield strength of the parent material, with maximum variation occurring in the vertical position of the specimen because of the orientation of electrode clamping forces that produce a non-uniform solidification pattern. In the final section a theoretical continuum modeling framework for 6061-T6 aluminum resistance spot welded joints is presented
Welding Processes
Despite the wide availability of literature on welding processes, a need exists to regularly update the engineering community on advancements in joining techniques of similar and dissimilar materials, in their numerical modeling, as well as in their sensing and control. In response to InTech's request to provide undergraduate and graduate students, welding engineers, and researchers with updates on recent achievements in welding, a group of 34 authors and co-authors from 14 countries representing five continents have joined to co-author this book on welding processes, free of charge to the reader. This book is divided into four sections: Laser Welding; Numerical Modeling of Welding Processes; Sensing of Welding Processes; and General Topics in Welding
Index to 1984 NASA Tech Briefs, volume 9, numbers 1-4
Short announcements of new technology derived from the R&D activities of NASA are presented. These briefs emphasize information considered likely to be transferrable across industrial, regional, or disciplinary lines and are issued to encourage commercial application. This index for 1984 Tech B Briefs contains abstracts and four indexes: subject, personal author, originating center, and Tech Brief Number. The following areas are covered: electronic components and circuits, electronic systems, physical sciences, materials, life sciences, mechanics, machinery, fabrication technology, and mathematics and information sciences
NASA Tech Briefs, September 1990
Topics covered include: New Product Ideas; NASA TU Services; Electronic Components and Circuits; Electronic Systems; Physical Sciences; Materials; Computer Programs; Mechanics; Machinery; Fabrication Technology; Mathematics and Information Sciences; Life Sciences
- …