Physical Simulation of Deformation and Microstructure Evolution During Friction Stir Processing of Ti-6Al-4V Alloy

The feasibility of using high-strain rate (1.475 to 3.942 s^?1) hot-torsion testing with a Gleeble^® thermomechanical simulator was demonstrated for simulating microstructures consistent with friction stir processing (FSP) of Ti-6Al-4V. The tests were performed on α/β-processed base material at temperatures both above and below the β-transus. Various phenomena including the refinement of α- and β-grains, deformation-induced heating, and deformation instabilities were observed. These tests reproduced the range of microstructures that are observed under FSP processing conditions. The testing methodology can be used for generating constitutive material property equations relevant to computational FSP/friction stir welding models.     

Microstructure and Texture Evolution during Friction Stir Processing of Fully Lamellar Ti-6Al-4V

Friction stir processing (FSP) was used to locally refine a thin surface layer of the coarse, fully lamellar microstructure of investment-cast Ti-6Al-4V. Depending on the peak temperature reached in the stir zone during processing relative to the β transus, three distinct classes of microstructures were observed. Using accepted wrought product terminology, they are equiaxed, bimodal, and lamellar, except for the case of FSP, the length scale of each was smaller by at least an order of magnitude compared to typical wrought material. The evolution of an initially strain-free fully lamellar microstructure to each of these three refined conditions was characterized with scanning electron microscopy, transmission electron microscopy and electron backscatter diffraction. The fundamental mechanisms underlying grain refinement during FSP, including both the morphological changes and the formation of high-angle grain boundaries, were discussed.     

DTA‐Measurements to Determine the Thixoformability of Steels

Semi‐solid metallurgy (SSM), also known as “thixoforming” or “thixoprocessing”, is of special interest as a new potential manufacturing technology for components in the automobile, machine and electronic industries. The aim of this technology is to produce complex shapes which cannot be produced with conventional processing methods. An important process step of semi‐solid processing (SSP) is the reheating and isothermal holding of the billet within the solid‐liquid range in order to obtain the required fraction liquid content and the desired globular microstructure. Aside from the investigation of billet heating and the development of a suitable tool design, the development and evaluation of adequate microstructures over a wide temperature area is very important. The focus of this paper is to determine the semi‐solid area of different steels through Differential Thermal Analysis (DTA) measurements. To determine a process window for handling the alloys in the semi‐solid state, the DTA‐results can be combined with microstructure parameters. Subsequent quenching experiments show the development of the microstructure parameters (e.g. grain size, phase distribution, volume fraction, shape factor, matrix character, contiguity, and particle density of the primary solid and liquid phases). A comparison of the slopes of the determined solid‐liquid areas for different steels show the width of the melting or freezing intervals to evaluate the possible process windows. DTA‐experiments performed at different heating rates show the influence of faster heating and cooling rates on the solidus‐liquidus interval. To evaluate the suitability for the thixoforming processes, this paper describes, and then compares, the semi‐solid intervals of different steel grades, which have been investigated in the Department of Ferrous Metallurgy at the RWTH Aachen University. The tool steel HS 6‐5‐3 and the cold work tool steel X210CrW12 have a wide semi‐solid area, which can be explained due to the dissolution of different carbides. In contrast to this, the steels C45, 42CrMo4, 16MnCr5, 34CrNiMo4, 100Cr6, X220CrVMo13‐4 and the Alloy 33 show a much smaller semi‐solid area.     

Finite‐Element Simulations of the Intercritical Deformation of a Low‐Carbon Steel

A mathematical model has been developed to describe the intercritical deformation of a low‐carbon steel under compression. It applies the finite‐element method in a 2D axially symmetric computation domain. By means of a special algorithm different types of microstructures to be used by the deformation model can be constructed, as far as ferrite and austenite phase fractions and distributions are concerned. Calculated results on the basis of constructed microstructures deviate significantly from mixture‐rule predictions. Compression curves exhibit a decreasing strength with increasing connected areas of either phase. The corresponding strain ratios demonstrate the difference in strain behaviour between the ferrite and the austenite phase. In the case of larger connected areas the contribution of the soft ferrite phase to the total strain is much more pronounced than that of the hard austenite phase.     

Quantitative Analysis of Micro‐Textures during Recrystallization in an Interstitial‐Free Steel

Two interstitial‐free steel samples were prepared by normal and by cross rolling. The effect of the resulting different deformation textures and microstructures on the subsequent recrystallization behavior was studied by micro‐texture analysis. The differences in recrystallization textures of the two differently rolled samples can be attributed to the microstructural differences in the as‐deformed state. The orientation distribution of the recrystallized grains forming at the early stages of the recrystallization dominated the final recrystallization textures, pointing to the importance of oriented nucleation in the formation of recrystallization textures of interstitial‐free steels.     

搅拌摩擦加工及后续热处理对7B04-O铝合金组织和硬度的影响

对7B04-O铝合金进行搅拌摩擦加工,对不同旋转速度参数下获得的7B04-O铝合金搅拌区的组织和硬度进行研究,同时利用后续热处理改善搅拌区的组织和硬度.研究表明,搅拌摩擦加工导致搅拌区的晶粒细化,硬度提升.不同的旋转速度会对后续热处理造成影响.采用低转速时,后续热处理不能进一步提高搅拌区的硬度;而采用高转速时,搅拌区的硬度则可以通过后续热处理得到提升.采用O态铝合金作为母材时,基体内的原始析出强化相尺寸粗大,热稳定性较高,难以在搅拌摩擦加工过程中充分溶解.通过提高转速来增加加工热输入可以增加析出相的溶解量,有利于后续热处理对组织和硬度的改善.     

Strengthening Mechanisms in NiAl Bronze: Hot Deformation by Rolling and Friction-Stir Processing

Microstructures produced by isothermal hot rolling of a NiAl bronze material were evaluated by quantitative microscopy methods and parameters describing the contributions of precipitate dispersions, grain size, solute content, and dislocation density to the yield strengths of the individual constituents of microstructure were determined. Models for the strengths of the individual constituents were combined to predict the temperature dependence of the yield strength as a function of hot rolling temperature, and the prediction was found to be in good agreement with measured yield strengths. The models were applied to microstructures in a stir zone produced by multipass friction-stir processing (FSP) and, again, found to predict measured yield strengths with high accuracy. Such models may aid in assessing the role of microstructure gradients produced during FSP and other processes.     

Micro Semi‐solid Manufacturing ‐ A New Technological Approach towards Miniaturisation

One of the most important prerequisites to meet the increasing demand for efficient technologies for micro‐part production is constituted by the ability to overcome existing process limitations by new innovative technological approaches. By the introduction of a new process variant based on a hybrid material condition between solid and liquid state, such an approach is presented. This micro semi‐solid manufacturing technology, so‐called Micro‐Thixoforming, was initiated, on the one hand, by being aware of the technological limits of existing micro‐forming and micro‐casting processes and, on the other hand, by a comprehensive understanding of the special rheological mechanisms of metallic materials in semi‐solid state finally establishing the desired potential to follow the trend towards miniaturisation with drastically reduced process restrictions. However, this promising potential can only be successfully exploited when the initial idea, which is based on phenomenological considerations, can be transferred to a process technology with sufficient practical relevance. Therefore, the presented new integrated process concept for Micro‐Thixoforming is particularly characterised by the application of unconventional solutions for the main process steps: raw material conditioning, thermal pre‐processing, semi‐solid forming and thermal post‐processing. To give an indication of the innovative character of the chosen practical solutions liquid metal jet technology, LASER‐induced plasma shockwaves and high pressure water jet should be mentioned. However, what is even more important in this context is the ability not only to realise a process concept but, beyond that, to recognise the further potential regarding new strategies for material design arising from the availability of this process. Such a strategy e.g. consists of utilising the well‐known segregation effect, which notably often is negatively associated with semi‐solid forming. However, for the envisaged technological approach, controlled segregation aims at a defined adjustment of functionally graded properties for the produced micro part.     

Producing of AA5083/ZrO2 Nanocomposite by Friction Stir Processing (FSP)

In this study, friction stir processing (FSP) was used to produce AA5083/ZrO₂ nanocomposite layer. Optical microscopy and SEM were used to probe the microstructures formed in the composite layer. In addition, the mechanical properties of each sample are characterized using both tensile and hardness tests. Results showed that FSP is an effective process to fabricate AA5083/ZrO₂ nanocomposite layer with uniform distribution of ZrO₂ particles, good interfacial integrity, and significant grain refinement. On processing, in the proper combination of process parameters, the metal matrix composite layer was observed to have increased tensile and hardness properties.     

Effects of Friction Stir Processing Parameters and In Situ Passes on Microstructure and Tensile Properties of Al-Si-Mg Casting

Friction stir processing (FSP) was applied to modify the microstructure of an as-cast A356 alloy. The effects of rotation rate, travel speed, in situ FSP pass, FSP direction, and artificial aging on microstructures and tensile properties were investigated. FSP broke up the coarse eutectic Si phase into 2.5 to 3.5 μm particles and distributed them homogeneously, and resulted in the dissolution of the coarse Mg₂Si particles and the elimination of porosity, thereby improving both the strength and the ductility of the casting. Increasing the rotation rate was beneficial to breaking up and dissolving the particles, but it contributed little to eliminating the porosity. The travel speed did not affect the size of the particles apparently, but lower speed was beneficial to eliminating the porosity. 2-pass FSP showed an obvious advantage in the microstructure modification and tensile properties compared with the single-pass. However, a further increase of FSP passes only resulted in slight improvement. The FSP direction of the following pass did not show distinct effect on the microstructure and tensile properties. After post-FSP artificial aging, the strengthening phase (β″-Mg₂Si) precipitated, which increased the strength and decreased the ductility of the FSP samples.     

The influence of friction stir processing parameters on microstructure of as-cast NiAl bronze

The influence of friction stir processing (FSP) parameters on the evolution of microstructure in an equilibrium-cooled, as-cast NiAl bronze (NAB) material was evaluated by optical microscopy (OM) and transmission electron microscopy (TEM) methods. A threaded pin tool was employed and tool rotation and traversing rates were varied in order to examine the spatial variation of stir zone microstructures in relation to FSP parameters. For processing at low rotation and traversing rates, the microstructure throughout the stir zone consists of elongated and banded grains of the primary α and transformation products of the β phase. Such microstructures reflect severe deformation at temperatures up to ～900 °C in the α+β two-phase region for this NAB material. Increasing rotation and traversing rates, coarse Widmanstätten α near the surface in contact with the tool became apparent. The appearance of this constituent reflects nearly complete transformation to β during FSP with peak temperatures of ～1000 °C. Also, complex stir zone flow patterns, often referred to as onion ring structures, become distinct in the mid regions of the stir zones as rotation and traversing rates increase. Schematic representations illustrating the effect of FSP parameters on thermal cycles at various locations in stir zones were prepared based on microstructure observations. Thus, processing at higher rotation and traversing rates results in higher peak temperatures near the surface in contact with the tool but also in steeper temperature gradients when compared to lower rotation and traversing rates.     

Study of welding ultra‐fine grained ferrite steel by CO2 laser

Ultra‐fine grained ferrite steels have higher strength and better toughness than the normal ferrite steels because of their micrometer or sub‐micrometer sized grains. In this paper the ultra‐fine grained steel SS400 is welded by CO₂ laser. The shape of weld, cooling rate of HAZ, width of HAZ, microstructures and mechanical properties of the joint are discussed. Experimental results indicate that laser beam welding can produce weld with a large ratio of depth to width. The cooling rate of HAZ of laser beam welding is fast, the growth of prior austenite grains of HAZ is limited, and the width of weld and HAZ is narrow. The microstructures of weld metal and coarse‐grained HAZ of laser beam welding mainly consist of B_L + M (small amount). With proper laser power and welding speed, good comprehensive mechanical properties can be acquired. The toughness of weld metal and coarse‐grained HAZ are higher than that of base metal. There is no softened zone after laser beam welding. The tensile strength of a welded joint is higher than that of base metal. The welded joint has good bending ductility.     

Fabrication of A1050-A6061 Functionally Graded Aluminum Foam by Friction Stir Processing Route

Functionally graded (FG) aluminum foam consisting of A1050 and A6061 aluminum alloy was fabricated by using friction stir processing (FSP). The fabrication of the precursor and the bonding of the A1050 precursor to the A6061 precursor can both be conducted by FSP. According to the results of point analysis, the Mg content gradually changed in the bonding region, and it was shown that seamless FG aluminum foam can be fabricated by the FSP route.     

A Review on Friction Stir Processing of Titanium Alloy: Characterization,Method, Microstructure,Properties

Metallurgical and Materials Transactions B - In the past two decades, friction stir processing (FSP) technology has received considerable attention. FSP can be used to adjust and control the...     

Analysis of Microstructure and Mechanical Properties of Different Hot Stamped B‐bearing Steels

Hot stamping is a technique to produce ultra high strength automobile components. The common material used in hot stamping process is coated and/or uncoated 22MnB5 boron alloyed steel. Ferritic‐pearlitic microstructure in as‐delivered sheets is transformed to fully lath martensitic after hot stamping. In the present research, hot stamping under water or nitrogen cooling media was investigated using different boron alloyed steel grades. Microstructural analyses, linear and surface hardness profiling as well as tensile tests of hot stamped samples were performed. Various microstructures of fully bainitic and/or fully martensitic were produced. The resulting microstructures provided yield strengths of 650–1370 MPa and tensile strengths of 850–2000 MPa. There is an optimum carbon equivalent content for which the highest formability index value, UTS × A₂₅, is achieved. Using a nitrogen cooled punch resulted in higher yield strength without significant changes in ultimate tensile strength. It is concluded that a wide range of B‐bearing steels having an extended carbon equivalent range with an acceptable formability index value can be used by increasing the cooling rate in the die assembly.     

Thermo‐elastic Homogenization of 3‐D Steel Microstructure Simulated by the Phase‐field Method

A multi‐scale approach based on the asymptotic homogenization method of periodic material structures is applied here to determine the effective thermo‐elastic properties of 3D steel microstructures, which have been calculated by phase‐field simulations. A multiphase‐field model, coupled to thermodynamic databases, is used to evaluate the microstructure evolution during the austenite to ferrite phase transformation of low carbon Fe‐C‐Mn steel. In order to derive effective mechanical properties, geometrical information about the grains, their phase properties and crystallographic orientations are transferred to the homogenization tool. Effective cubic Young and shear modules and Poisson coefficients are predicted for different ferrite volume fractions. Moreover, the volume change is derived as function of the phase fractions, leading to a calculated dilatometer curve. The effects of the thermal shrinkage and the volume expansion caused by the phase transformation are taken into account.     

Prediction of bending limits in friction-stir-processed thick plate aluminum

Friction-stir processing (FSP) was used to modify surface microstructures, to enhance the bending of thick-plate 6061-T6 and 7050-T7451 aluminum alloys. Plates were bent at room temperature into a V-shaped die, to various angles. Bending performance in the friction-stir-processed plates was significantly better than that in the base plates, where processing caused localized softening of the pretensile surface of the plate. A finite-element model of the plate-bending process was developed, to predict the bending limits of both the unprocessed base plates and of the friction-stir-processed plates. For the friction-stir-processed plates, the model employed a mesh divided into two or more zones; one zone was for unprocessed base material and other zones were for the processed material or for material that was affected by the heat of processing. The model used both the von Mises and the Latham and Cockroft criteria to predict bending limits. The bending-limit predictions were reasonably accurate, provided the gradient in true stress-strain behavior through the plate thickness was well characterized.     

Microstructural modification of as-cast NiAl bronze by friction stir processing

The application of friction stir processing (FSP) to a cast NiAl bronze (NAB) material is presented as a means for selective modification of the near-surface layers by converting as-cast microstructures to a wrought condition in the absence of macroscopic shape change. This may enable selective surface hardening of cast components. The complex physical metallurgy of the NAB is reviewed, and microstructure changes associated with FSP for a selected set of processing parameters are examined by optical microscopy (OM) and transmission electron microscopy (TEM) methods. Direct temperature measurement in the stir zone is infeasible and, so, these microstructure changes are used to estimate peak temperatures in the stir zone. The persistence of a Fe₃Al phase (κ _ii) indicates that peak temperatures are below the solvus for this phase, while the presence of transformation products of the β phase, including fine Widmanstätten α, bainite, and martensite, indicates that peak temperatures exceed the eutectoid temperature for the reaction β → α+κ _iii throughout the stir zone.