Search

Titanium alloy Ti6Al4V is one of the most utilized alloys in the field of additive manufacturing due to the excellent combination of mechanical properties, density and good corrosion behavior. These characteristics make the use of this material particularly attractive for additively manufacturing components with complex geometry in sectors such as aeronautics and biomedical. Selective Laser Melting (SLM), by which a component is fabricated by selectively melting of stacked layers of powder using a laser beam, is the one of most promising additive manufacturing technologies for Ti6Al4V alloy. Although this technique offers numerous advantages, it has some critical issues related to the high thermal gradients, associated with the process, promoting the formation of a metastable martensitic microstructure resulting in high tensile strength but poor ductility of the produced parts. The formation of microstructural defects such as balling and porosity can occur together with the presence of residual stresses that may significantly affect the mechanical characteristics of the component. Specific process parameters and geometries can determine heat accumulation phenomena that result in a progressive decrease in thermal gradients between layers. These heat accumulation phenomena are influenced by the number of layers deposited, but also by the building orientation that, for a given geometry, determines a variation of the deposited surface for each layer.

Single Point Incremental Forming (SPIF) has recently introduced the concept of material formability enhancement through localized deformation. Since material is processed by means of a pin tool attached to spindle, physical interference (especially in vertical direction) limits attainable shapes with the conventional process. The aim of the following work is to increase the variety of achievable geometries with SPIF through in-process magnetic field assistance. An innovative configuration managing SPIF tool movement using magnetic force is proposed. With this in mind, a magnet configuration was designed to generate a vertical load able to plastically deform a 0.5 mm thick AA1100 aluminum sheet. Experiments were carried out to prove the concept by manufacturing a truncated cone; the results demonstrated the feasibility of Magnetic Field-Assisted SPIF.

Friction Stir Extrusion is an innovative direct-recycling technology for metal machining chips. During the process a specifically designed rotating tool is plunged into a cylindrical matrix containing the scraps to be recycled. The stirring action of the tool prompts solid bonding related phenomena allowing the back extrusion of a full dense rod. This process results to be particularly relevant because allows the reuse of the scrap without any previous treatment. Experiments have been carried out in order to investigate the influence of the process parameters on the extrudes quality and a numerical model has been developed in order to simulate the evolution of the material flow.

Titanium forging has been encountering a growing interest in the scientific and industrial communities because of the distinct advantages it provides with respect to machining, in terms of both mechanical properties of the product and material waste, thus significantly reducing the Buy to Fly ratio. In the paper, a numerical FE model, based on a tri-coupled approach and able to predict the microstructural evolutions of the workpiece during the process, is developed and set up. Calculated results are compared to experiments for a few industrial case studies. The final phases distribution in the forged parts is experimentally measured and compared to the FE model output finding satisfying overlapping.

Solid Bonding based welding processes allow to obtain defect free joints with low residual stress and low distortion. However, the engineering and optimization of solid bonding processes is difficult and requires a large number of time and cost consuming test trials. In this way, proper numerical models are essential tools permitting effective process design. The aim of this research was the comparison of the material process conditions during two different manufacturing processes taking advantage of the same metallurgical phenomenon, namely solid bonding. Linear Friction Welding, used to weld non-axisymmetric components and Accumulative Roll Bonding, used to increase the mechanical properties of sheet metals, were considered. Numerical models were set up, validated and used to design the process by studying the complex material behavior during the solid bonding of different aluminum alloys. An implicit approach was used for the Linear Friction Welding and Accumulative Roll Bonding processes, leading to the understanding of the main process variables influence on the field variables distribution and the occurrence of actual bonding.

In this paper, a critical analysis of the technical difficulties and numerical issues in running simulations of linear friction welding processes is carried out. The focus of the paper is the comparison of different modeling strategies of a numerical analysis for the LFW process of Ti-6Al-4V titanium alloy, for which the thermal aspect strongly influences the mechanical behavior due to the phase transformation, taking place over a definite range of temperature. A 3D simulation campaign, conducted using the FEA code DEFORMTM, was considered in order to show advantages and disadvantages of each approach, including the most critic limitations and complexity in a correct simulation design using two deformable objects.

Friction Stir Welding (FSW) is a solid-state welding process used to weld difficult to be welded or unweldable materials as aluminum alloys. In the last years other materials have been successfully tested as magnesium, titanium and nickel based alloys. Mixed joints can be obtained by FSW but issues arise in the correct choice of the process parameters. In the paper a numerical model is presented for the prediction of the main field variables distribution and the occurring material flow in FSW of dissimilar AA6061 and AZ31 butt joints. Important insights are obtained on the effect of the mutual position of the two materials with respect to the advancing and retreating side of the joint has been highlighted.

Linear friction welding is a solid-state joining process developed for non-axisymmetric components in which the joining of the specimens is obtained through reciprocating motion and pressure. In the process, the friction forces work due to the high frequency oscillation and the pressure between the specimens is converted in thermal energy. In order to design an effective machine, relevant issues derive from the high frequency and the large inertial forces involved in the process. In this study, the authors describe the redesign of a preexisting prototypal machine for LFW processes. A machine redesign is needed when welding high resistant materials, i.e. steels or titanium alloys, with high frequencies, up to 72 Hz. The sensors equipping the machine allows in process measurements of key process variables as temperatures of the specimens, tangential forces, accelerations and speeds. At the same time through the acquired data, the main weaknesses of the machine can be highlighted allowing for effective redesign.

Linear Friction Welding (LFW) is a solid-state joining process used for non-axisymmetric components. LFW involves joining of materials through the relative motion of two components undergoing an axial force. In the process, the heat source is given by the frictional forces work decaying into heat and determining a local softening of the material and eventually the needed bonding conditions. In the paper, an experimental and numerical campaign is proposed for AA2011 aluminum alloys welding. Different case studies are considered with fixed oscillation frequency and varying pressure at the interface between the specimens. Constant oscillation amplitude and specimens geometry is used. The calculated results permitted to highlight the effects of the process parameters on the material flow determining the soundness of the weld.

Friction Stir Welding (FSW) is a solid state welding process patented in 1991 by TWI; initially adopted to weld aluminum alloys, is now being successfully used also for magnesium alloys, copper and steels. The wide diffusion the process is having is due to the possibility to weld materials traditionally considered difficult to be welded or “unweldable” by traditional fusion welding processes due to peculiar thermal and chemical material properties. Additionally, the process allows welding a wide range of sheet thickness (up to 50mm) avoiding typical fusion welding processes defects, like cavities and porosities, with no shielding gas, filling material or joint preparation. Recently, research is focusing on titanium alloys thanks to the high interest that such materials are getting from the industry due to the extremely high strength-weight ratio together with good corrosion resistance properties. Welding of titanium alloys by traditional fusion welding techniques presents several difficulties due to high material reactivity resulting in bonding with oxygen, hydrogen, and nitrogen with consequent embrittlement of the joint. In this way FSW can represent a cost effective and high quality solution. A few studies have been developed on the FSW of titanium alloys butt joints, while there is a complete lack of knowledge as far as different joint morphologies are regarded (lap joints, T joints, etc.). In the paper the results of an experimental campaign on lap joints made out of thin Ti-6Al-4V sheets are presented. The effect of the main process parameters on the micro and macro mechanical properties has been investigated and related to the microstructural transformations occurring during the process because of the thermo-mechanical action of the tool.

Solid state bonding recurs in several manufacturing processes, as Friction Stir Welding (FSW), Linear Friction Welding (LFW), extrusion of hollow profiles and Accumulative Roll Bonding (ARB). The former processes are nowadays of particular industrial interest because of the specific advantages with respect to the classic welding technologies. In FSW the solid state bonding is obtained between an undeformed cold material, already placed in the advancing side of the joint, and the hot material flow incoming from the retreating side. Proper conditions of pressure, temperature, strain and strain rate are needed in order to get the final effective bonding. In the paper experimental tests on butt joints made out of AA6061-T6 aluminum alloys are used to identify the sets of process parameters resulting either in sound or poor joints. The same process conditions have been simulated used an already developed model in order to highlight the actual bonding line and the values of the main field variables determining the soundness of the joints. Finally a correlation between process parameters values, field variables values and joint effectiveness is made.

Single-point incremental forming (SPIF) is a quite new sheet-forming process which offers the possibility to deform complex parts without dedicated dies using a single-point tool and a standard three-axis CNC machine. Although the process mechanics enables higher strains with respect to traditional sheet-forming processes, research has been focused on further increasing the maximum forming angle. In the paper, a new approach is used to enhance the material formability through a localized sheet heating as a consequence of the friction work caused by high speed rotating tool. Numerical simulation was utilized to relate the effect of temperature with the main field variables distribution in the sheet.

In the last decade the industrial use of solid state welding processes based on frictional forces work decaying into heat is continuously increasing due to their strong advantages with respect to traditional fusion techniques. Several advances have been proposed by the scientific community regarding process mechanics, material flow and also the computer aided engineering of the operation with the aim to maximize the mechanical performances of the welded joints. In the paper Friction Stir Welding (FSW) and Linear Friction Welding (LFW) operations are considered and a review of the most relevant research issues and results is provided.

Light weight construction is a major task within automotive and aircraft industry due to lower fuel consumption or increase the possible payload. Structural or exterior shell components are more and more manufactured out of aluminum alloys for this reason. A further weight reduction could be achieved by the substitution of aluminum alloys by magnesium alloys. Also the application of blanks with a varying thickness is a possibility to realize light weight design. To combine the advantages of weight reduction by the use of magnesium alloys and tailored welded blanks (TWBs), an effective joining technique is required. Friction Stir Welding can be used for difficult to be welded magnesium alloys to manufacture magnesium TWBs. During this process the different thickness of the two sheets can cause an unequal heat distribution below the tool, affecting material flow and therefore components strength. In the paper the main results of a numerical and experimental campaign on Friction Stir Welding of AZ31 magnesium alloy tailored blanks are presented. The numerical simulation was validated by experimental observations and the occurring bonding conditions have been analyzed in both approaches.

The present investigation aims at studying the effect of different tool geometries and process parameters on FSW of thin sheets in AZ31 magnesium alloy. In particular two properly designed tools, with shoulder diameters equal to 8 and 19 mm, were used; each of them was manufactured both in pin and pinless configurations. The effect of the different tool configurations and sizes, and welding parameters on mechanical properties of FSWed joints were analyzed in detail. The results were compared with those obtained on the base material. It was shown that FSWed joints are characterized by strength and ductility values lower than those of base material. Furthermore, the pin tool configuration, with a shoulder diameter of 8 mm, leads to the obtaining of strength and ductility values higher than those provided by the pinless one. A strong beneficial effect is obtained by increasing the shoulder diameter from 8 to 19 mm using the pinless configuration, whilst the FSW with the pin tool is critically affected by the welding conditions. The experimental work was joined to a numerical investigation based on finite element method (FEM) in order to study the material flow occurring during the welding process as well as the distribution of temperature, with the aim to identify a input window of the process parameters within which sound joints can be obtained.

Solid state bonding recurs in several manufacturing processes, as extrusion of hollow profiles and solid welding processes. Among the latter, Friction Stir Welding (FSW) is nowadays of particular industrial interest because of the specific advantages with respect to the classic welding technologies. Proper conditions of pressure, temperature, strain and strain rate are needed in order to get the final effective bonding. In the paper the authors compare different solid state bonding conditions obtained at the varying of the main process parameters in FSW of butt joints of AA5754 aluminum alloys. The experimental results are compared with the numerical ones from a FEM model previously developed by the authors, in order to investigate an effective bonding criterion to be implemented into the FSW numerical model.

Single Point Incremental Forming (SPIF) is a quite new sheet forming process which offers the possibility to deform complex parts without dedicated dies using only a single point tool and a standard 3-axis CNC machine. The process mechanics enables strains much higher than traditional sheet forming processes, but particular attention must be given to the final part geometrical accuracy. In this paper the capabilities of a dedicated explicit numerical model are quantitatively analyzed on pyramid-shaped parts. In particular a comparison between experimental and numerical results is reported. Three different shapes at the varying of the stamping angle were considered and the final shape was acquired, through a white light triangulation based optical scanner, after the removing of the punch and of the clamping fixture. LS-DYNA commercial code was used to simulate the three different case studies, and an explicit loading-implicit unloading approach was implemented in order to simulate also the shape distortions due to springback phenomenon. The effect of several numerical parameters on the final shape and the thickness distribution were investigated. In particular numerical results were analyzed at the varying of the material constituve law, of the shell element type and of the number of integration points along the thickness.

Experimental and numerical investigations have been performed in order to study the effect of welding parameters on properties of FSW-ed AZ31 magnesium alloy sheets. The results, presented in terms of tensile strength and numerical field variables distributions, allow to understand the behaviour of such material when FSW-ed using different rotational and welding speeds for a given tool geometry.

Spot welding can be considered a very common joining technique in automotive and transportation industries as it permits to obtain effective lap-joints with short process times and what is more it is easily developed through robots and automated systems. Recently the Friction Stir Spot Welding (FSSW) process has been proposed as a natural evolution of the already known Friction Stir Welding (FSW) process, allowing to obtain sound spot joints that do not suffer from the insurgence of typical welding defects due to the fusion of the base material. In the paper, a modified Friction Stir Spot Welding (FSSW) process, with a spiral circular movement given to the tool after the sinking stage, is proposed. A continuum based numerical model for Friction Stir Spot Welding process is developed, that is 2D Lagrangian implicit, coupled, rigid-viscoplastic. This model is used to investigate the distribution of the main field variables, namely temperature, strain and strain rate, as well as the Zener-Hollomon parameter which, in turn, strongly affects the Continuous Dynamic Recrystallization (CDRX) process that takes place in the weld nugget. Numerical and experimental results are presented showing the effects of the process parameters on the joint performances and the mechanical effectiveness of the modified process.

Friction Stir Welding (FSW) is an energy efficient and environmentally "friendly" welding process. The parts are welded together in a solid-state joining process at a temperature below the melting point of the workpiece material under a combination of extruding and forging. This technology has been successfully used to join materials that are difficult-to-weld or ‘unweldable’ by fusion welding methods. In the paper a neural network was set up and trained in order to predict the final grain size in the transverse section of a FSW butt joint of aluminum alloys. What is more, due to the relationship between the extension of the “material zones” and the joint resistance, the AI tool was able to furnish indications for the design of the welding process. Experimental tests and subsequent microstructure observations were developed in order to verify the numerical predictions.

Friction Stir Welding (FSW) has been arousing a continuously increasing interest among joining processes since its invention in 1991. Although mainly used for aluminum alloys, it can also be applied to other light alloys. In the present work, experimental and numerical campaigns have been performed with the aim to study the effect of the tool geometry on the mechanical properties of FSW-ed AZ31 magnesium alloy sheets. The results, presented in terms of tensile strength, ductility, micro-hardness values and numerical field variables distributions, allow to reach a deeper knowledge on the behaviour of such relatively new material when FSW-ed, and can be used for a full optimization of the joints.

In the paper, a variation of the Friction Stir Spot Welding (FSSW) process has been considered. In particular, a particular tool path is given after the sinking phase nearby the initial penetration site. The process mechanics was highlighted and the joint strength was considered at the varying of the most relevant process parameters. Furthermore macro and micro analyses were developed in order to highlight the process mechanics and the local material microstructure evolution. The investigated technology appears a promising joining technique in order to develop effective spot joints.

In the paper the results of a wide experimental activity on friction stir welding (FSW) of aluminum alloys are reported. In particular the butt joints of two different materials, namely AA1050-O and AA6082-T6 were considered. Grains dimensions and precipitates density were investigated both in the parent materials and after the welding processes. Furthermore post-welding heat treatments effects on the joint strength were studied.