Blair E. Carlson

Study of friction stir joining of thin aluminium sheets in lap joint configuration

Abstract Thinning in friction stir lap joints and its relation with the process variables was investigated. Friction stir welds were made on 1 mm thick AA6111 aluminium alloy sheets in order to study the effects of rotation rate, traverse speed, plunge depth, tilt angle and pin height on faying surface defects. Sheet thinning on the advancing and retreating sides was quantified and the lap shear strength of the joints was evaluated. A decrease in the pitch of the runs at constant rotation rate increased the sheet thinning and reduced the joint strength in a linear manner. Process pitch and pin height were found to be the most critical factors in determining the faying surface lift up. A pin of the same height as the sheet thickness resulted in maximum lap shear strength. Tool tilt did not show a significant effect on the sheet thinning.

Study of top sheet thinning during friction stir lap welding of AZ31 magnesium alloy

Abstract Friction stir lap welding was applied to AZ31 magnesium alloy sheets to investigate the effect of tool designs and welding variables on top sheet thinning. Three tools with different shoulder designs were used. Sheet thinning aroused by the hooking or cold lap feature was quantified, and the lap shear fracture load of the joints was evaluated and correlated with the effective top sheet thickness. The tool geometry has a significant effect on the morphology and extent of both hooking and cold lap features, as well as welding variables such as welding pitch and plunge depth. The morphology and extent of these features influenced the effective top sheet thickness, which exhibited a linear relationship with the unit width fracture load via the welded material strength. High fracture loads can be achieved by suppressing the hooking and cold lap features as well as by strengthening the material in the nugget.

Cold Metal Transfer Spot Joining of AA6061-T6 to Galvanized DP590 Under Different Modes

In order to meet the upcoming regulations on greenhouse gas emissions, aluminum use in the automotive industry is increasing. However, this increase is now seen as part of a multimaterial strategy. Consequently, dissimilar material joints are a reality, which poses significant challenges to conventional fusion joining processes. To address this issue, cold metal transfer (CMT) spot welding process was developed in the current study to join aluminum alloy AA6061-T6 as the top sheet to hot dip galvanized (HDG) advanced high strength steel (AHSS) DP590 as the bottom sheet. Three different welding modes, i.e., direct welding (DW) mode, plug welding (PW) mode, and edge plug welding (EPW) mode were proposed and investigated. The DW mode, having no predrilled hole in the aluminum top sheet, required concentrated heat input to melt through the Al top sheet and resulted in a severe tearing fracture, shrinkage voids, and uneven intermetallic compounds (IMC) layer along the faying surface, leading to poor joint properties. Welding with the predrilled hole, PW mode, required significantly less heat input and led to greatly reduced, albeit uneven, IMC layer thickness. However, it was found that the EPW mode could homogenize the welding heat input into the hole and thus produce the most stable welding process and best joint quality. This led to joints having an excellent joint morphology characterized by the thinnest IMC layer and consequently, best mechanical performance among the three modes.

Friction Stir Blind Riveting for Joining Dissimilar Cast Mg AM60 and Al Alloy Sheets

A new one-sided joining method, friction stirring blind riveting (FSBR) was successfully implemented to form lap-shear joints for dissimilar metals from pairs of 3.05 mm thick cast Mg AM60, rolled 1.5 mm thick Al AA6022, and extruded 3.15 mm thick Al AA6082 specimens. The concept of this process is riveting the two workpieces with reduced force under frictional heat and fastening the workpieces through blind riveting once the rivet is fully inserted. In this research, the process was experimentally analyzed and optimized for four joint combinations. It was demonstrated that switching the positions of Mg and Al alloy specimens has a significant effect on the process window and maximum tensile load of the joints. Three quality issues of the FSBR joints were observed and discussed. During tensile testing, the sheet closer to the rivet tail work-hardens due to tail forming process but has worse loading condition than the sheet closer to the rivet head. For AA6xxx sheets, precipitate hardening due to frictional heat is another strengthening mechanism in FSBR compared to the conventional riveting process, which leads to higher tensile loads in FSBR joints.

Mitigation of Pore Generation in Laser Welding of Magnesium Alloy AZ31B in Lap Joint Configuration

Magnesium, as the lightest structural metal, has been widely used in the automotive and aerospace industries. Porosity is the main issue in the welding of magnesium alloys and can be caused by surface coatings, hydrogen gas, pre-existing porosity, the collapse of an unstable keyhole and vaporization of alloying elements. In this study, the effect of the oxide layer on pore generation in the welding of AZ31B-H24 magnesium alloy is investigated. A fiber laser with a power of up to 4 kW is used to weld samples in a lap joint configuration. Two groups of samples are studied: as received (AR) surfaces (where an oxide layer remains on the surface) and treated surfaces. The surface treatment includes two techniques: mechanical removal (MR) and the use of a plasma arc (PA) as a preheating source. Also, a separate set of experiments are designed for preheating samples in a furnace in order to investigate whether the pore mitigation effect of a plasma arc is caused by preheating. Observations include a weld bead profile achieved through optical microscopy, chemical compositions tested by Electron Dispersive Spectroscopy (EDS), and mechanical properties measured with a tensile test. The results obtained show that the preheating effect of a plasma arc procedure can effectively mitigate pore generation. The tensileshear results reveal that PA samples have a higher strength than other groups of samples.

Mitigating Zinc Vapor Induced Weld Defects in Laser Welding of Galvanized High-Strength Steel by Using Different Supplementary Means

© 2012 Ma et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Mitigating Zinc Vapor Induced Weld Defects in Laser Welding of Galvanized High-Strength Steel by Using Different Supplementary Means

Formation and Structure of Work Material in the Friction Stir Forming Process

There has been significant work on the application of ultrahigh strength steels and aluminum alloys for automotive body mass reduction. Integration of these materials into the automotive structure is non-trivial and advanced methods are required to effectively join the dissimilar materials with the properties required for automotive applications. Friction stir forming is a new environmentally benign manufacturing process for joining dissimilar materials. Fundamentally this process is based on frictional heating and mechanically stirring a plasticized material into a mechanical interlocking joint with a second material. In this research, the significant process parameters were identified and their optimized settings for the current experimental conditions defined using a design of experiments methodology. The overall joint structure and grain microstructure were mapped along different stages of the friction stir forming process. Then, friction stir forming of aluminum work material into different cavity geometries was mapped to study geometrical effects. Two layers were found to form within the aluminum, the thermo-mechanical affected zone that had been deformed due to the contact pressure and angular momentum of the tool, and the heat affected deformation zone that deformed into the interlocking cavity. The geometrical study produced interesting results about the direction and magnitude of material forming under different areas of the tool.

Friction Stir Scribe Welding of Dissimilar Aluminum to Steel Lap Joints

The use of dissimilar material combinations such as aluminum to steel has been increasing in automobile and aerospace industries due to its potential for energy savings. Achieving an acceptable joint quality with fusion welding can be problematic due to the significant differences in physical and thermal properties between materials. One alternative to conventional fusion welding is friction stir scribe welding and because of the nature of the process, it can be a viable option for joining dissimilar metal combinations. The primary emphasis of this work is to investigate the feasibility of using FSS welding as a possible option for joining 1.0 mm thick 6022 aluminum to 0.7mm electro galvanized steel sheets in a dissimilar lap weld configuration. An H13 steel pin tool featuring a tungsten carbide scribe insert was used. An investigation on the optimum size of the scribe insert was conducted to evaluate the effects of microstructure and mechanical properties.

Numerical analysis of Al vapour effects in gas metal arc welding of Al alloys

Abstract Using an in-house three-dimensional (GMAW) process simulation tool, the effects of Al vapour in the arc welding of aluminium alloys for various welding currents have been studied. The mathematical model, which takes into account the effects of metal vapour, is described. A bead-on-Al-plate problem is then modelled, with and without consideration of the Al vapour for various welding currents. The effects of the Al vapour on the energy flow of the system and the resulting weld profile are examined. The calculated plasma temperatures are compared with measured temperatures. The change of heat input to the workpiece due to the presence of Al vapour is examined. Finally, the calculated weld profiles with and without consideration of Al vapour are compared with actual weld profiles. It is found that the presence of Al vapour reduces the arc temperature and the weld depth significantly.

Comparing Laser Welding Technologies with Friction Stir Welding for Production of Aluminum Tailor-Welded Blanks

A comparison of welding techniques was performed to determine the most effective method for producing aluminum tailor-welded blanks for high volume automotive applications. Aluminum sheet was joined with an emphasis on post weld formability, surface quality and weld speed. Comparative results from several laser based welding techniques along with friction stir welding are presented. The results of this study demonstrate a quantitative comparison of weld methodologies in preparing tailor-welded aluminum stampings for high volume production in the automotive industry. Evaluation of nearly a dozen welding variations ultimately led to down selecting a single process based on post-weld quality and performance.