Base Sheets Research Articles

Incremental sheet forming of dissimilar friction stir welded AA1100 and AA6061 blanks using single point tool

Recently, single point incremental forming (SPIF) gained popularity among researchers as well as in industry as a die - less flexible forming technique. In this work, two different metal sheets of Aluminium alloy, AA1100 and AA6061, each of thickness 1.2 mm is welded together to produce a tailor welded blank utilizing Friction Stir Welding (FSW) which is done by vertical milling machine and the forming operation is performed on the tailor welded blanks as well as base sheets. The main objective of this study is to identify the input factors affecting FSW process, compare the formability of the tailor welded sheets and to find out optimum step depth, feed and speed of SPIF. The single point incremental sheet forming is done using CNC Milling machine and optimized the experiment results using Taguchi method. After conducting the experiments, the spring back, surface roughness, wall angle, and formability is measured and Signal to Noise ratio, means were calculated. With the help of graphs, optimum parameter values were obtained using Minitab software. From analysing the results revealed that the step depth and spindle speed are the critical parameters affecting the incremental sheet forming process.

Investigation and optimization on the formability of aluminum 1050/stainless steel 304L bilayer sheets fabricated by cold roll bonding considering the Box-Behnken method

Composite metal sheets have found extensive applications thanks to their high corrosion and wear resistance, as well as greater strength, stiffness, and formability relative to the base sheets. Stainless steel composite sheets account for over 80% of the metallic composite sheets. Their higher performance and lower costs have increased their applications in the food and automotive industries. This study experimentally addressed the formability of the Al 1050/Stainless Steel 304 L bilayer sheets prepared through the cold rolling process. After designing the experiment using the response surface method, considering the layer thickness and reduction ratio of the rolling process, the bilayer sheets were prepared using the cold roll bonding process. Formability included the forming limit diagram, the magnitude of FLD0, and elongation, which were derived by Nakazima and tensile tests. Then, the formability was optimized as a function of input parameters considering the Box-Behnken method. To this end, a new criterion was presented to compare the formability of the sheets. This criterion can be used to assess the formability of a sheet considering various loading modes. Microstructural observations justified the variations in the formability of the bilayer sheets against the input parameters, i.e., thickness ratio and reduction ratio. The results indicated that the formability of the Al 1050/Stainless Steel 304 L bilayer sheet (i.e., safe forming region) reached its maximum value at tAl=4mm;tSS=0.8mm;andr=45%.

Consumable pin-friction stir spot welding of Al-Mg-Si alloy via pre-created hole and refilling: Microstructure evolution, defects, and shear/tensile failure load

Since Al-Mg-Si alloys are widely used in the transportation industry, it is important to produce a sound and robust weld between the sheets of these alloys. The focus of this work is on the tensile-shear and cross-tension strengths of the consumable pin-friction stir spot welds (CP-FSSWs) without an exit-hole between the Al-6061 aluminum sheets. Before welding, a hole was created at the joint region in the base sheets and then, it was filled using a rotating consumable pin. The tensile-shear, cross-tension, and microhardness tests were employed to evaluate the mechanical properties of the spot welds. The results showed that the pre-created hole was entirely filled during the welding process. While a complete bond was formed between the consumable pin and the lateral surface of the hole, there were three distinct regions at the interface of the pin and the bottom of the hole: complete bond, kissing bond, and defects. Enhancement of the tool rotational speed decreased the area of the complete bond in the weld compared to the other regions. A linear relationship existed between the bonding area and weld failure load in the cross-tension test. The proposed relationship approved the impact of the swirly region at the interface of the base sheets on the weld strength. While in the cross-tension test, the weld failure load decreased from ∼2800 to ∼1950 N, it improved from ∼10,500 to ∼12,000 N in the tensile-shear test with enhancement of the tool rotational speed from 700 to 2000 rpm. The hardness measurements demonstrated that there was no common heat affected zone softening after CP-FSSW.

Attachable/Detachable Sheets Containing Foam-Rubber Layers with Super Deodorant Functions

Two-layered sheets consisting of a cellulose-nanofibril-containing foam-rubber layer and a base film or nonwoven sheet were developed as attachable/detachable sheets with super deodorant functions. TEMPO-oxidized cellulose nanofibrils with copper carboxylate groups (TOCNs-Cu) were composited with foam-rubber layers, which were coated on base films and sheets. These TOCNs-Cu-containing foam sheets adsorb H2S and NH3 gases well because these gas molecules are chemically or physically trapped by the foam layers. The TOCNs-Cu moieties are primarily located at the air/rubber interfaces in the foam-rubber layer. The peel and tack strengths of the TOCNs-Cu-containing foam sheets are appropriate for manual foam sheet attachment to, and detachment from, flat glass plates and plastic/ceramic boards. The H2S- and NH3-adsorption behaviors are affected by the air permeability of the base film or nonwoven sheet. Transmission electron microscopy images show that TOCNs-Cu aggregates are located at the air/rubber interfaces, which results in super deodorant functions.

Investigations on formability of tailor laminated sheets in single point incremental forming

In this article, formability of tailor laminated sheets in single point incremental forming (SPIF) is investigated using forming limit curve (FLC) and maximum formable wall angle determined theoretically and incremental sheet forming tests. Two different sheet materials namely aluminum alloy AA5083 and polycarbonate were laminated in two different configurations, that is, AA5083/PC, and AA5083/PC/AA5083. FLCs were determined for the laminated sheets using deformation instability method and compared to that of the base sheets. It was observed that formability of the laminated sheets was higher than that of the PC and AA5083 sheets for plane strain case. The maximum formable wall angle was determined by varying the wall angle till the onset of fracture through deforming the sheet into square pyramid shapes. Deformation depths at the onset of fracture for the incrementally formed laminated sheets got decreased with the increase of forming wall angle. Optical macrographs of the cross-sections of the deformed laminated sheets with maximum forming wall angles were taken, and variations of sheet thicknesses along the center line of the deformed sheets were compared for the different cases. Higher triaxiality of AA5083/PC/AA5083 sheet results in lower formability compared to AA5083/PC sheet obtained from membrane analysis. Formation of bump structure due to delamination during SPIF of laminated sheets could be avoided using proper selection of tool path.

Formability study and metallurgical properties analysis of FSWed AA 6061 blank by the SPIF process

In this study, in order to obtain the maximum possible formability in tailor-welded blank AA6061 sheets connected by the friction stir welding (FSW) procedure, the incremental sheet forming process has been utilized. The results are presented both numerically and experimentally. To obtain the forming limit angle, the base and FSWed sheets were formed in different angles with conical geometry, and ultimately, the forming limit angle for the base metal and FSWed sheet is estimated to be 60° and 57.5°, respectively. To explore the effects of welding and forming procedures on AA6061 sheets, experimental studies such as mechanical properties, microstructure and fracture analysis are carried out on the samples. Also, the thickness distribution of the samples is studied to investigate the effect of the welding process on the thickness distribution. Then, the numerical process was simulated by the ABAQUS commercial software to study the causes of the FSWed samples failure through analyzing the thickness distribution parameter, and major and minor strains and the strain distribution. Causes of failure in FSWed samples include increased minor strain, strain distribution and thickness distribution in welded areas, especially in the proximity of the base metal area.

Ductility effect on clinching joint strength in lap-shear configuration loading

Clinching is a mechanical sheet metal joining method in which the sheets are geometrically interlocked using, punch and die, tools without any consumables. In this work, to improve the clinching joint strength in lap shear configuration, the base sheet material is heat-treated in a furnace over varying time durations and temperatures to improve the ductility. Ductility is the prime factor in deciding the amount of cold roll bonding (CRB). This CRB helps during the joining of heat-treated sample to eliminating the gap at the “S” interlock and increase in friction, otherwise the gap would be present in the untreated material. This elimination of gap and higher friction is necessary for the identified belt friction mechanism operating at the lap shear configuration to produce higher joint strength. These causes are confirmed by examining SEM images of the cross sections of the joint and fractography of the failed joint interface. The joints so formed are tested for its strength in monotonic lap-shear configuration loading. Experimentally it is observed that the heat-treatment of base sheets improves the strength in this configuration by more than 50% compared to the untreated material.

Investigation on Formability of Tailor-Welded Blanks in Incremental Forming

Steel laser tailor-welded blanks (TWBs) are produced by end-to-end joining of base sheets using different welding methods. In this article, the formability of laser TWBs of St12 and St14 with thicknesses of 1 mm and 1.5 in single point incremental forming process were experimentally and numerically investigated. First, the forming limit wall angle was experimentally determined for each of the base sheets. Then, SPIF of TWBs samples was carried out at the thinner sheet wall angle; 67°. For numerical investigation, the mechanical properties of the weld zone were obtained. For one combination of TWBs, the finite element (FE) simulation of incremental forming was performed by the use of ABAQUS/Explicit FE software. The simulation process was validated by comparing the results with those of experiments. Then, the effect of SPIF on thickness, stress and strain distribution of other combinations of TWBs was numerically investigated. The results showed that using the FE model, the SPIF of TWBs without performing high cost experimental tests can be properly investigated. Also, the results revealed that in steel laser TWBs with different thicknesses, the maximum and minimum principal strains are concentrated at the corners and the walls of the thinner sheet of TWBs, respectively. Hence, the maximum amount of effective strain is concentrated at the thinner side of TWBs corners and the weld zone of the two blanks. For the same reason, rupture is observed at these regions.

Multi-Objective Optimization of Friction Stir Spot-Welded Parameters on Aluminum Alloy Sheets Based on Automotive Joint Loads

By controlling various friction stir spot-welded (FSSW) factors, two base sheets AA 5052-H32 and 6061-T6 were selected to bond similar and dissimilar metal joints while considering dissimilar configuration orders. The effects of weld parameters on the sheer strength and peel strength were separately developed into empirical models utilizing the integrated central composite matrix design and response surface methodology (RSM). Meanwhile, the finite element (FE) analysis of the multi-axis load-bearing characteristics for automotive solder joints during service was carried out. As a result, the weights of the shear and axial stress, accounting for 90.5% and 9.5% respectively, were employed to restrict the relationship between multiple target properties, and the resulting security strength was applied to determine the feasible domain in subsequent parametric optimization. In order to enable the optimal multi-axis capacities in accordance with the load mode, the genetic algorithm NSGA-II was chosen to compute the Pareto front and further determine the best compromise solutions. The obtained optimums corresponding to each joining condition were validated by confirmation runs, indicating that this coupled multi-objective optimization approach based on working conditions was beneficial to the targeted improvement of post-weld mechanical properties.

Mechanical Performance of Resistance Spot Welded Steel Sheets

Mechanical performance of resistance spot welded steel sheets was evaluated under quasi-static and dynamic loading conditions. Numerical inverse calibration method was mainly utilized to characterize mechanical properties especially for flow curves and ductile fracture criteria of constituent zones of the welded joints. For the base sheets, standard tensile test and high-speed tensile test was conducted under quasi-static and dynamic loading conditions, respectively. Mechanical properties of the weld zones were characterized using the developed miniature tensile specimen. Implementing the mechanical properties and structure to the finite element model, finite element simulation was used to evaluate the mechanical performance of two distinct weld coupons.

Influence of dynamic loading on failure behavior of spot welded automotive steel sheets

Dynamic failure of the spot welded automotive steel sheets, similar and dissimilar combinations of TRIP980 and GMW2 sheets, was analyzed with finite element (FE) simulations. The strain rate-sensitivity of the base sheets was considered to investigate the influence of dynamic loading on failure behavior of the welded sheets. Mechanical properties were first identified under the quasi-static loading as the reference condition. Hardening behavior and fracture criterion were identified by numerical inverse method applied to the standard tensile test and a miniature tensile test for the base sheet and the weld nugget, respectively. Moreover, strain rate-sensitivities of the base sheets were inversely characterized as a function of strain and strain rate. Dynamic lap-shear and coach-peel coupon tests were performed to evaluate the dynamic failure behavior of the welded sheets. Peak strength, failure mode and load-displacement curves of the coupon tests were predicted with the FE simulations. The combined experimental and numerical approach showed that the strain rate-sensitivity of the base sheet played a key role on determination of failure characteristics in coupon tests.

Predicting the Effect of Tool Configuration during Friction Stir Welding by Cellular Automata Finite Element (CAFE) Analyses

The present work aims at selecting the pin geometry for single-sided friction stir welding of steel, and modelling the double-sided FSW process, both through cellular automata finite element analyses. Cellular automata cells are used to hold initial grain size of the base sheets. The stirring action during the process is not modelled physically. Instead the heat flux and strain-rate attained are incorporated by models. In the first objective, by developing heat flux models, strain-rate and strain models for different pin profiles, and the final grain size distribution across the weld zone is predicted. The final pin profile predicted from the present work is not the same used by authors in the existing literature, as observed from grain size prediction. In the second objective, during double-sided friction stir welding, the grain size predictions are agreeing well with the existing data. The efficiency of the approach for tool selection has been demonstrated successfully. [Submitted 8 December 2016; Accepted 5 December 2017]

Simple and effective failure analysis of dissimilar resistance spot welded advanced high strength steel sheets

The failure of welded structures was analyzed considering dissimilar combinations of advanced high strength steels (AHSS) and a conventional mild steel. A method to characterize the mechanical properties of hardening behavior along with the deterioration associated with micro-void development and fracture criterion based on a modified damage model was applied by employing simple but effective experimental and numerical inverse procedure. The proposed procedure was solely based on standard and miniature simple tension tests for base sheets and weld nuggets, respectively. The identified plastic and failure properties were applied to the analysis of the failure mode and strength in the two well-accepted coupon tests of lap-shear and U-shape tension tests for DP980-TRIP980 and GMW2-TRIP980 dissimilar welded sheets. The analysis confirmed that the distinct failure behavior in the coupon tests for the two dissimilar weld cases was mainly due to the competition between the element with high strength/low ductility and the element with low strength/high ductility.

Practical failure analysis of resistance spot welded advanced high-strength steel sheets

In this study, a practical numerical procedure to analyze the failure behavior of resistance spot welded structures, especially for structures composed of advanced high-strength steels (AHSS) sheets, was developed for the first time. The root cause of the difference in failure modes between the welded structures of AHSS and conventional mild steel sheets was also elucidated. Given that the characterization of detailed mechanical properties of base sheets and weld nuggets is extremely complex, considerable effort was exerted to identify and properly simplify critical mechanical properties that are essential for failure analysis. This was achieved by developing a simple but effective experimental procedure to characterize the key properties by mainly utilizing standard and miniature-specimen tensile tests that are commonly available in any laboratory. In this study, two AHSS sheet grades, TRIP980 and DP980, as well as a conventional mild steel (GMW2) sheet grade were considered. The study involved the macroscopic characterization of mechanical properties including hardening behavior and fracture criterion. Furthermore, electron backscattered diffraction analysis of the base sheets and weld nuggets was also performed. The failure mode and strength of the lap-shear and U-shape tension tests were then analyzed for similar welded sheets under quasi-static loading conditions.

Steady-state thermal measurement of moist granular earthen materials

A technique based on the heat flow meter method is proposed for measuring the thermal conductivity of moist earthen and granular loose-fill materials. Although transient methods have become popular, this steady-state approach offers an uncertainty that can be reliably estimated and a test method that is widely accepted for building certification purposes. Variations to the standard method are proposed, including the use of a rigid holding frame with stiff base and silicone sponge buffer sheets, in conjunction with difference measurement to factor out the contributions from base, buffers and contact resistance. Using this approach, results are presented for green-roof substrates based on scoria, terracotta and furnace-ash at different moisture contents. Thermal conductivity ranged from 0.13 to 0.80 W/m K and fitted well to linear regression plots against moisture content. Further comparative measurements of a single specimen showed that direct measurement was less consistent than difference measurement and thus indicated that thermal resistance was higher by 0.023 m2 K/W, attributable to the presence of contact resistance.

Predicting the tensile behaviour of adhesively bonded sheets using equivalent geometrical heterogeneities

In the present work, the formability of adhesively bonded sandwichsteel sheets is predicted during tensile tests, with the help of equivalent geometrical heterogeneities in the base sheets, without incorporating adhesive layer and properties during modelling simulations. This will help us mainly to overcome the difficulties while incorporating adhesion/adhesive properties during formability simulation and prediction. The limit strains during the tensile test are predicted and validated with experiments. The effect of hardener to resin (H/R) ratios of adhesive on the forming limit strains has been predicted with a set of thickness heterogeneity factor ‘f’ and size (w) of the defect, located in the base material. In other words, a relationship between defect morphology (f, w) and adhesive properties has been evaluated. For example, in the case of rectangular-infinite groove of width, w = 0.5 mm, the limit strains of DDQ steel evaluated during tensile tests at f = 0.912, f = 0.9137, f = 0.924, f = 0.927, and f = 0.95 are equivalent to the H/R ratios of 0.6:1, 0.7:1, 0.8:1, 0.9:1, and 1:1 of adhesive, respectively. The limit strains are predicted in the same manner with a square hole and finite square defect in the base material. It is suggested that the overall forming (tensile) behaviour of adhesively bonded blanks (ABB) can be predicted with the help of a thickness heterogeneity factor (or similar geometrical heterogeneities) that is equivalent to all the geometrical and structural heterogeneities in the whole bonded sheets.

Wear properties of Al–Al2O3/TiB2 surface hybrid composite layer prepared by friction stir process

In this study, surface hybrid composite was fabricated on Al6061 base sheets using friction stir processing (FSP). Samples were subjected to various numbers of FSP passes from one to four with Al2O3/TiB2 powder. Influence of number of FSP passes was studied on distribution of Al2O3/TiB2 particles in aluminium matrix, microstructure, hardness, and wear properties of specimens. The results showed that increase in the number of passes causes a more uniform dispersion of composite particles and thus, decreases particles clustering. In addition, an increase in the number of FSP passes was found to decrease the matrix grain size of the surface hybrid composite. It was observed that the hardness increased as the number of passes increased due to presence and pinning effect of hard Al2O3/TiB2 particles. Also, at higher number of passes, the surface hybrid composite wear resistance was increased.

Measurement of the Bauschinger behavior of sheet metals by three-point bending springback test with pre-strained strips

In this paper, a novel approach is proposed to measure the Bauschinger effect, transient behavior and permanent softening of metallic sheet subjected to reverse loading. The hardening parameters related to the Bauschinger effect, transient behavior and permanent softening are optimized from the springback profiles of three-point bending tests with pre-strained sheets. Taking the dual phase steel sheet DP780 as an example, its Bauschinger effect, transient behavior and permanent softening determined from the new approach are compared with those of the cyclic simple shear method. Finite element simulations are also performed for three-point bending and U-draw/bending of base (as-received) and pre-strained DP780 steel sheets to validate the suggested approach. The results show that the aforementioned hardening behavior determined from the new approach shows good agreements with those of cyclic simple shear tests. Moreover, reasonable springback predictions for three-point bending and U-draw/bending tests are obtained as well. In particular, the proposed approach is quite suitable for the industrial applications because only uni-axial tension and three-point bending tests are required.

Influences of Thickness Ratio of Base Sheets on Formability of Tailor Welded Blanks

The influences of thickness ratio of base sheets on formability of tailor welded blanks were examined based on Erichsen cupping tests. Three-dimensional finite element (FE) models, using Hill’48 yield criterion and iso-hardening rule, were built under the environment of the FE analysis software ABAQUS. The failure onset sites of tailor welded blanks were analysed and the quantitative relationships between the Erichsen index values and base thickness ratio were established. Based on these relationships, the formability of tailor welded blanks made from dissimilar base sheets are evaluated, provided that the Erichsen index values of tailor welded blanks made from identical ones are known and that the above two different types of tailor welded blanks undergo a similar stamping process. The experiments were then carried out to validate this method.

Effect of different combinations of tailor-welded blank coupled with change in weld location on mechanical properties by laser welding

Using tailor-welded blanks (TWB) in automotive industry is on the rise. This is done due to advantages such as reduction in weight leading to more effective operational cost and lower energy consumption of vehicles. The main objective of this investigation was to study the effect of location change of weld zone and differences in thickness combination of TWB sheets on their tensile characteristics and forming capabilities. Quality evaluations of weld zone metallographic and tensile as well as ball punch tests have been conducted. Tension characteristics of welded samples have been determined by conducting a uni-axial tensile test perpendicular on the weld line in those samples. Forming capability of TWB samples were also studied by using sphere head chisel test. By moving weld line toward thick sheets direction and increasing thin sheets share of the weld in TWB, an increase in relative elongation in tensile test and in chisel test increase in cups height was observed. This indicates that forming capability of TWB samples by moving weld line toward thick sheet increases and weld zone does not have much effect on forming capability of TWB. By using the derived recommended relation induced from this study, it is conceivable to obtain the amount of increase in relative length of TWB from its base sheets. Results of this relation are confirmed with obtained results from tensile test. Also by reduction in thickness difference of TWB sheets, their formability increases.