Michael Miles

Effect of friction stir welding conditions on properties and microstructures of high strength automotive steel

Abstract The use of high strength steels (HSS) and ultra high strength steels (UHSS) is increasing in automotive applications to reduce vehicle weight and to improve structural performance. This study focused on the properties and microstructures of friction stir welded steel sheets, as a function of welding conditions. Tooling feeds and speeds had a significant impact on mechanical properties and microstructures of the welds. For example, at constant tool speeds higher feedrates produced softer welds with large amounts of primary ferrite, compared with lower feedrates which produced harder welds and a greater fraction of martensite. These results illustrate the potential of adjusting weld properties and microstructures by using different friction stir welding process conditions for HSS and UHSS alloys.

Solid state spot joining of sheet materials using consumable bit

Abstract A new spot joining technology relying on a consumable joining bit has been developed and evaluated on dual phase (DP) 980 steel and a dissimilar combination of aluminium alloy 5754-O and DP 980. This new process, called friction bit joining (FBJ), uses a consumable bit to create a solid state joint in sheet materials by the action of cutting and frictional bonding. FBJ lap shear fracture loads were 14˙7 kN for 1˙4 mm DP 980 compared to 16˙6 kN for RSW, for the same spot diameters. FBJ of a dissimilar combination of aluminium alloy 5754-O and DP 980 produced joints with average lap shear fracture load of 6˙3 kN. This strength is greater than lap shear fracture load obtained by self-piercing riveting (SPR) in HSLA 350 bonded to AA 5754-O.

Spot joining of AA 5754 and high strength steel sheets by consumable bit

Abstract Spot joining of dissimilar metal alloys is difficult because of differences in melting temperature as well as differences in composition. If a fusion welding process like resistance spot welding is employed, then alloys mix in the fusion zone and can create brittle intermetallic compounds during solidification. When self-piercing riveting (SPR) is used, the alloys to be joined must be ductile enough to be formed around the rivet. In this paper, a new approach to joining dissimilar metal alloys has been studied, where a consumable bit has been used to create a spot joint between dissimilar alloys. The resulting joint has both metallurgical and mechanical bonding characteristics and can be made between very soft and very hard alloys, like dual phase (DP) 590 and DP 980 and light metals like AA 5754. Lap shear strengths of joints made by friction bit joining (FBJ) are similar to or better than comparable joints made by self-piercing riveting.

Impact of tool wear on joint strength in friction stir spot welding of DP 980 steel

Abstract Friction stir spot welding has been shown to be a viable method of joining ultra high strength steel, both in terms of joint strength and process cycle time. However, the cost of tooling must be reasonable in order for this method to be adopted as an industrial process. Recently a new tool alloy has been developed, using a blend of polycrystalline cubic boron nitride (PCBN) and tungsten rhenium (W–Re) in order to improve the toughness of the tool. Wear testing results are presented for two of these alloys: one with a composition of 60% PCBN and 40% W–Re, and one with 70% PCBN and 30% W–Re. The sheet material used for all wear testing was 1·4 mm DP 980. Lap shear testing was used to show the relationship between tool wear and joint strength. The Q70 tool provided the best combination of wear resistance and joint strength.

Methodology for Determining the Variance of the Taylor Factor: Application in Fe-3%Si

A method is proposed to determine the variance of an arbitrary material property based on the statistics of the texture of polycrystalline materials for a specified volume. This method is applied to determine the variance of the Taylor factor (i.e., measure of plastic deformation in crystal plasticity) and is compared to a random sampling method. The results from the random sampling method correlated well with the statistical variance relationship when the magnitude of the variance was greater than that of the numerical errors observed in the statistical calculation. An empirical relation was also shown to model the results, and the constants for this relationship were determined for pseudothree-dimensional Fe-3 % Si. Implementation of the statistical variance relationship in true three-dimensional microstructures is not limited by material opacity, since it depends only on the two-point pair correlation functions. The connection between the variance of the R-value and variance of the Taylor factor is considered. Although only a weak connection was found, it was observed that relatively small variations in the Taylor factor yield large variances in the R-value.

Mechanical properties of dissimilar metal joints composed of DP 980 steel and AA 7075-T6

Abstract A solid state joining process, called friction bit joining, was used to spot weld aluminium alloy 7075-T6 to dual phase 980 steel. Lap shear failure loads for specimens without adhesive averaged ∼10 kN, while cross-tension specimens averaged 2·8 kN. Addition of adhesive with a thickness up to 500 μm provided a gain of ∼50% to lap shear failure loads, while a much thinner layer of adhesive increased cross-tension failure loads by 20%. Microstructures of the welds were martensitic, but the hardness of the joining bit portion was greater than that of the DP 980, owing to its higher alloy content. Softening in the heat affected zone of a welded joint appeared to be relatively small, though it was enough to cause nugget pullout failures in some lap shear tension specimens. Other failures in lap shear tension were interfacial, while all of the failures in cross-tension were interfacial.

Friction stir spot welded joints of 409L stainless steels fabricated by a convex shoulder tool

Spot joints of ferritic 409L stainless steel are successfully fabricated by friction stir spot welding (FSSW) using a convex shoulder tool. The welding process, microstructure and failure of the FSSW joint are investigated experimentally. During the FSSW process, the Z-force history shows significant variations depending on the contact phenomena between the tool and the joined sheets, while the Z-torque history shows a rather steady increase without pronounced changes in the trend until the initiation of dwelling. Electron back-scatter diffraction suggests that both continuous dynamic recrystallization and recovery occurred in the stir zone during the FSSW process. Observation of the FSSW joint that failed under the given lap shear load shows that the cracks, which are the result of the interfaces between the upper and lower sheets, propagated into the weld along the interfacial surfaces, after which a necking/shear failure occurred. Finally, the rupture of the joint, which was initiated by the necking/shear failure, propagated along the circumference of the weld.

Spatial Control Charts for the Mean

Developments in metrology provide the opportunity to improve process monitoring by obtaining many measurements on each sampled unit. Increasing the number of measurements may increase the sensitivity of control charts to detection of flaws in local regions; however, the correlation between spatially proximal measurements may introduce redundancy and inefficiency in the test. This paper extends multivariate statistical process control to spatial-data monitoring by recognizing the spatial correlation between multiple measurements on the same item and replacing the sample covariance matrix with a parameterized covariance based on the semivariogram. The properties of this control chart for the mean of a spatial process are explored with simulated data and the method is illustrated with an example using ultrasonic technology to obtain nondestructive measurements of bottle thickness.

Corrosion behaviour of friction-bit-joined and weld-bonded AA7075-T6/galvannealed DP980

ABSTRACT Joining of aluminium alloys 7075-T6 and galvannealed dual phase 980 steel was achieved by friction bit joining (FBJ) and weld-bonding (FBJ + adhesive) processes. Accelerated laboratory-scale corrosion tests were performed on both FBJ only and weld-bonded specimens to study joint strength under a corrosive environment. Static lap shear tests showed that both FBJ only and weld-bonded cases generally retained more than 80% of the joint strength of non-corroded specimens at the end of corrosion testing. The presence of Zn/Fe coating on the steel substrate resulted in improved corrosion resistance for FBJ specimens, compared to joints produced with bare steel. An optical microscopy was used for cross-sectional analysis of corroded specimens. Some corrosion on the joining bit was observed near the bit head. However, the joining bit was still intact on the steel substrate, indicating that the primary bond was sound.

Failure behaviours of friction stir spot welded joints of dissimilar ferrous alloys under quasi-static shear loads

Joining process and failure behaviours of friction stir spot welded (FSSW) joints of two dissimilar ferrous alloys, cold rolled carbon steel (SPCC) and 409L stainless steel (SUS 409L), were investigated. Spot joints with four different material combinations were successfully fabricated by the FSSW process. The failure behaviours of the joints were investigated by quasi-static lap shear tests. Among the dissimilar material combinations tested, FSSW joints with the softer and more ductile material as an upper sheet (SPCC/SUS) provided better joint properties, including a slightly higher failure load and a significantly higher toughness, in comparison to having the softer material as the lower sheet. The different toughness properties of the SPCC/SUS and SUS/SPCC FSSW joints may be attributed to a dependency on the failure characteristic of the upper sheet material.