M. I. Khan

Effects of weld microstructure on static and impact performance of resistance spot welded joints in advanced high strength steels

Abstract Evaluating the impact performance of resistance spot welded joints in advanced high strength steels (AHSS) is critical for their continued integration into the automotive architecture. The effect of strain rate on the joint strength and failure mode is an important consideration in the design of welded structures. Recent results suggest that the failure mode is dependent upon the strength, chemistry, and processing of AHSS. Current literature, however, does not explain the effects of weld microstructure and a comprehensive comparison has yet to be conducted. The present study details the fracture paths within the joint microstructure of spot welded AHSS, including dual phase (DP), transformation induced plasticity (TRIP) and ferritic–bainitic (FB), in comparison to new high strength low alloy steels. Quasi-static and impact tests were conducted using a universal tensile tester and an instrumented drop tower respectively. Results for elongation, failure load and energy absorption for each material are presented. Failure modes were detailed by observing weld fracture surfaces. In addition, cross-sections of partially fractured weldments were examined to detail fracture paths during static loading. Correlations between the fracture path and mechanical properties were developed using observed microstructures in the fusion zone and heat affected zone. Results showed that good impact performance was obtained in DP780 and TRIP780 grades in relation to DP600, 590R and conventional high strength low alloy.

Recent progress in micro and nano-joining

Micro and nano-joining has been identified as a key enabling technology in the construction of micromechanical and microelectronic devices. The current article reviews recent progress in micro and nano-joining. In particular, laser micro-welding (LMW) of crossed 316 LVM stainless steel (SS) wire was compared to conventional resistance micro-welding (RMW) and was successfully employed in welding a Pt-Ir /SS dissimilar combination. Welding of Au nanoparticles was realized using femtosecond laser irradiation and its application in the surface enhanced Raman spectroscopy was investigated. Brazing between carbon nanotube (CNT) bundles and Ni electrodes was attained in vacuum, resulting in the development of a novel CNT filament of incandescent lamps.

Fabrication of a novel laser-processed NiTi shape memory microgripper with enhanced thermomechanical functionality

The thermomechanical properties of nickel-titanium shape memory alloys have sparked significant research efforts seeking to exploit their exotic capabilities. Until recently, the performance capabilities of nickel-titanium devices have been inhibited by the retention of only one thermomechanical response. In this article, the application of a novel laser-processing technique is demonstrated to create a monolithic self-positioning nickel-titanium shape memory microgripper. Device actuation and gripping maneuvers were achieved by thermally activating processed material regions which possessed unique phase transformation onset temperatures and thermomechanical recovery characteristics. The existence of each thermomechanical material domain was confirmed through differential scanning calorimetry analysis. Independent thermomechanical recoveries of each embedded shape memory were captured using tensile testing methods. Deployment of each embedded shape memory was achieved using resistive heating, and in situ resistivity measurements were used to monitor progressive phase transformations.

Dynamic actuation of a novel laser-processed NiTi linear actuator

A novel laser processing technique, capable of locally modifying the shape memory effect, was applied to enhance the functionality of a NiTi linear actuator. By altering local transformation temperatures, an additional memory was imparted into a monolithic NiTi wire to enable dynamic actuation via controlled resistive heating. Characterizations of the actuator load, displacement and cyclic properties were conducted using a custom-built spring-biased test set-up. Monotonic tensile testing was also implemented to characterize the deformation behaviour of the martensite phase. Observed differences in the deformation behaviour of laser-processed material were found to affect the magnitude of the active strain. Furthermore, residual strain during cyclic actuation testing was found to stabilize after 150 cycles while the recoverable strain remained constant. This laser-processed actuator will allow for the realization of new applications and improved control methods for shape memory alloys.

Enhanced thermomechanical functionality of a laser processed hybrid NiTi-NiTiCu shape memory alloy

The exciting thermomechanical behavior of NiTi shape memory alloys (SMAs) has sparked significant research effort seeking to exploit their exotic properties. The performance capabilities of conventional NiTi offerings are limited, however, by current fabrication technologies. In this study, a high power density laser source was implemented to locally alloy Cu into a conventional NiTi material. The effects of laser processing created a localized NiTiCu ternary material domain which possessed a set of unique thermomechanical properties. The combined active responses of the laser processed hybrid NiTi‐NiTiCu SMA represent an enhanced material functionality, which permits a multi-stage thermomechanical recovery and allows for unprecedented novel applications to be realized. (Some figures may appear in colour only in the online journal)

Resistance microwelding of crossed Pt–10Ir and 316 LVM stainless steel wires

Abstract Resistance microwelding of dissimilar materials such as Pt–10Ir and 316 low carbon vacuum melted stainless steel is becoming increasingly important for making electrical connections in medical devices. The joining of dissimilar materials increases flexibility in design while providing economic advantages, where more cost effective materials can be substituted for traditional materials. In this work, the performance of joints made using different electrode forces was studied by examining the surface morphology, cross-sections, joint break force and dynamic resistance measurements from resistance microwelding joints. Electrode sticking and excessive expulsion were observed with low electrode forces, whereas joints with undesirable cracks and notches were produced at higher electrode forces. Based on the analysis of single pulse welds, a new process variation using multiple pulses was developed, which improved the weld surface quality while obtaining a joint strength near 90% of the Pt–10Ir wire tensile strength.

Fabrication of a Novel Monolithic NiTi Based Shape Memory Microgripper via Multiple Memory Material Processing

The exciting thermomechanical behavior of nickel-titanium shape memory alloys have sparked significant research efforts seeking to exploit their exotic shape memory properties. The performance capabilities of conventional nickel-titanium alloys are currently limited, however, by the retention of only one shape memory geometry. In this paper we demonstrate the application of an unprecedented manufacturing process known as Multiple Memory Material technology to create a novel monolithic nickel-titanium shape memory microgripper. In our design, actuation and gripping maneuvers are achieved by thermally activating processed material regions which possess unique shape memory transformation temperatures and shape set geometries. The existence of multiple shape memory regimes is confirmed through differential scanning calorimetry analysis and in situ resistivity measurements.Copyright

Dynamic Actuation of a Multiple Memory Material Processed Nitinol Linear Actuator

Shape memory alloys such as Nitinol, which is a group of NiTi alloys composed of nearly equiatomic nickel and titanium, finds increasing applications in many industries because of its unique properties including the shape memory effect and pseudoelasticity. In past work simple linear actuators have been developed using Nitinol wire which are actuated and controlled using resistive heating. However, traditional Nitinol materials are batch processed and a monolithic component only possesses a single set of transformation temperatures, limiting the functionality of the actuator. In this work a linear actuator processed using the novel multiple memory material processing technology is presented showing multiple transformations and dynamic actuation by resistive heating. This dynamically controlled actuation greatly improves the functionality of the Nitinol actuator allowing for the realization of new applications and improved control methods. The different transformation temperatures embedded in the monolithic wire actuator following processing are identified using thermo-analytical analysis and the dynamic application of load and displacement are presented using a custom test set-up.Copyright

An Innovative Laser-Processed NiTi Self-Biasing Linear Actuator

The revolutionary multiple memory material technology allows local modification of shape memory alloy functional properties to create monolithic actuators that exhibit several different thermomechanical characteristics. In this work, high density laser energy was used to process a monolithic piece of NiTi shape memory alloy material to allow synergistic pseudoelastic and shape memory effect behavior. The resulting actuator contains self-biasing properties eliminating the need for a separate biasing mechanism for cyclic actuation. The characteristics of these different local behaviors were analyzed using tensile testing and differential scanning calorimetry. The stress and strain amplitude of the self-biasing linear actuation was characterized with relation to input current control. This work provides proof of concept for local modification of martensitic and austenitic phases; enabling self-biasing linear actuation.Copyright

Laser hole sealing of commercially pure grade 1 titanium

Hermetically sealed Ti capsules filled with electrolyte solutions are required in many medical device applications. The laser hole sealing process is well suited for this type of application. There is, however, a lack of understanding of the laser hole sealing mechanism, especially in the presence of an electrolyte medium. In this study, the mechanism of the laser hole sealing process was investigated by characterizing the surface morphology and cross-sections of welds made both with and without electrolyte. It was shown that the laser sealing mechanism transitions from (i) no sealing due to insufficient energy; (ii) the coalescence of the weld pool and the onset of sealing; (iii) increasing penetration depth up to full penetration; and (iv) laser ablation and drilling. Laser hole sealing in the presence of electrolyte decreases the size of the process window for suitable laser energies and affects the microstructure of the sealed hole.