S. S. Babu

The metallurgy and processing science of metal additive manufacturing

Additive manufacturing (AM), widely known as 3D printing, is a method of manufacturing that forms parts from powder, wire or sheets in a process that proceeds layer by layer. Many techniques (using many different names) have been developed to accomplish this via melting or solid-state joining. In this review, these techniques for producing metal parts are explored, with a focus on the science of metal AM: processing defects, heat transfer, solidification, solid-state precipitation, mechanical properties and post-processing metallurgy. The various metal AM techniques are compared, with analysis of the strengths and limitations of each. Only a few alloys have been developed for commercial production, but recent efforts are presented as a path for the ongoing development of new materials for AM processes.

Phase transformation dynamics during welding of Ti–6Al–4V

In situ time-resolved x-ray diffraction (TRXRD) experiments were used to track the evolution of the α→β→L→β→α/α′ phase transformation sequence during gas tungsten arc welding of Ti–6Al–4V. Synchrotron radiation was employed for the in situ measurements in both the fusion zone (FZ) and the heat-affected zone (HAZ) of the weld, providing information about transformation rates under rapid heating and cooling conditions. The TRXRD data were coupled with the results of computational thermodynamic predictions of phase equilibria, and numerical modeling of the weld temperatures. The results show that significant superheat is required above the β transus temperature to complete the α→β transformation during weld heating, and that the amount of superheat decreases with distance from the center of the weld where the heating rates are lower. A Johnson–Mehl–Avrami phase transformation model yielded a set of kinetic parameters for the prediction of the α→β phase transformation during weld heating. Corresponding TRXRD ...

Site specific control of crystallographic grain orientation through electron beam additive manufacturing

Abstract Site specific control of the crystallographic orientation of grains within metal components has been unachievable before the advent of metals additive manufacturing (AM) technologies. To demonstrate the capability, the growth of highly misoriented micron scale grains outlining the letters D, O and E, through the thickness of a 25·4 mm tall bulk block comprised of primarily columnar [001] oriented grains made of the nickel base superalloy Inconel 718 was promoted. To accomplish this, electron beam scan strategies were developed based on principles of columnar to equiaxed transitions during solidification. Through changes in scan strategy, the electron beam heat source can rapidly change between point and line heat source modes to promote steady state and/or transient thermal gradients and liquid/solid interface velocity. With this approach, an equiaxed solidification in the regions bounding the letters D, O and E was achieved. The through thickness existence of the equiaxed grain structure outlining the letters within a highly columnar [001] oriented bulk was confirmed through characterizing the bulk specimen with energy selective neutron radiography and confirming with an electron backscatter detection. Ultimately, this demonstration promotes the ability to build metal components with site specific control on crystallographic orientation of grains using the electron beam melting process.

Time-resolved X-ray diffraction investigation of primary weld solidification in Fe-C-Al-Mn steel welds

In situ time-resolved X-ray diffraction (TRXRD) using synchrotron radiation was used to monitor the phase evolution during welding of Fe-C-Al-Mn steel with 0.05 s resolutions. The primary solidification phase under rapid- and slow-cooling rate conditions was examined. The results showed nonequilibrium austenite solidification during rapid cooling; in contrast to the equilibrium δ-ferrite solidification that occurs under slow cooling conditions. These experimental results were analyzed using computational thermodynamics and interface-response function models. Results of the interface response function calculations considering the effect of carbon alone, predicted a change from primary δ-ferrite to primary austenite solidification with an increase in solid-liquid interface velocity.

Empirical model of effects of pressure and temperature on electrical contact resistance of metals

Abstract An important input property in the development of process models for resistance spot welding is electrical contact resistance. A model for the pressure and temperature dependence of electrical contact resistance was developed from established concepts of contact resistance. The key to developing the desired relationship is determining surface roughness characteristics, which is experimentally problematic. To overcome this difficulty the electrical resistance of contacting interfaces was measured as a function of the pressure applied across the interfaces. Using known information about the temperature dependence of bulk resistivity and mechanical properties, a curve fitting procedure was used to establish the desired relationship of contact resistance to pressure and temperature. This empirical model agrees well with experimental measurements in the regime of low applied pressure. At high pressures, predictions underestimate contact resistance, and this was attributed to strain hardening of asperities at the contacting interface. The model also predicts that the competing effects of bulk resistance and contact resistance will produce a peak in the variation of contact resistance with temperature. The model provides a suitable means for incorporating the pressure and temperature dependence of contact resistance into process models of the resistance spot welding process.

Characterization of the microstructure evolution in a nickel base superalloy during continuous cooling conditions

The solidification characteristics of the γ phase from the liquid and the subsequent decomposition of the γ phase control the evolution of the microstructure in nickel–base superalloy welds. The precipitation of the γ′ phase from the γ phase during continuous cooling conditions (0.17–75 K s−1) from the solutionizing temperature was characterized in a directionally solidified CM247DS alloy with thermomechanical simulator, and by transmission electron microscopy, atom probe field ion microscopy and atom probe tomography. The number density increased; size decreased and morphology of the γ′ precipitates changed with an increase in cooling rate. Under rapid water-quenched conditions, complex partitioning of the alloying elements between γ and γ′ phases was observed. Atom probe tomography on samples subjected to slower cooling rates showed different partitioning behavior compared to that of water-quenched samples and the presence of secondary γ′ precipitates in the samples subjected to a cooling rate of 1 K s−1.

Synchrotron X-ray studies of austenite and bainitic ferrite

High-resolution synchrotron X-ray diffraction has been used to conduct in situ studies of the temporal evolution of phases during the isothermal growth of bainite. Two populations of austenitic material were identified: one corresponding to the initial austenite and the other to the carbon-enriched austenite associated with the bainitic ferrite. The observed lattice parameters and the asymmetry of the peaks from the residual austenite have been interpreted in terms of the carbon partitioning due to the transformation. The results are contrasted with an earlier study in which the austenite unit cell appeared to split into two distinct densities prior to the onset of transformation.

Rationalization of Microstructure Heterogeneity in INCONEL 718 Builds Made by the Direct Laser Additive Manufacturing Process

Simulative builds, typical of the tip-repair procedure, with matching compositions were deposited on an INCONEL 718 substrate using the laser additive manufacturing process. In the as-processed condition, these builds exhibit spatial heterogeneity in microstructure. Electron backscattering diffraction analyses showed highly misoriented grains in the top region of the builds compared to those of the lower region. Hardness maps indicated a 30 pct hardness increase in build regions close to the substrate over those of the top regions. Detailed multiscale characterizations, through scanning electron microscopy, electron backscattered diffraction imaging, high-resolution transmission electron microscopy, and ChemiSTEM, also showed microstructure heterogeneities within the builds in different length scales including interdendritic and interprecipitate regions. These multiscale heterogeneities were correlated to primary solidification, remelting, and solid-state precipitation kinetics of γ″ induced by solute segregation, as well as multiple heating and cooling cycles induced by the laser additive manufacturing process.

Transition from bainite to acicular ferrite in reheated Fe-Cr-C weld deposits

Abstract Factors controlling the transition from acicular ferrite to bainite in Fe–Cr–C weld metals have been investigated. It appears that the presence of allotriomorphs of ferrite at austenite grain boundaries has the effect of suppressing the formation of bainitic sheaves. This in turn allows the acicular ferrite plates to develop on intragranular nucleation sites. A theoretical analysis indicates that bainitic transformation is prevented from developing at the allotriomorphic ferrite/austenite boundaries by the carbon concentration field present in the austenite at the allotriomorphic ferrite/austenite interface. This field does not homogenise within the residual austenite during the time scale of the experiments.MST/1217

Atom probe field ion microscopy study of the partitioning of substitutional elements during tempering of a low-alloy steel martensite

The partitioning behavior of Mn and Si at the cementite/ferrite interface during tempering of Fe-C-Si-Mn steel martensite has been studied by atom probe field-ion microscopy (APFIM). It has been shown that cementite can form without partitioning of Si and Mn during the early tempering stage at a low temperature. The atom probe compositional analysis shows no evidence of segregation or of concentration spikes of substitutional elements at the interface. This suggests that the early stage of cementite growth occurs by paraequilibrium mode and is controlled only by C diffusion in the matrix. In addition, significant C concentration fluctuations are measured in the as-quenched condition. The onset of partitioning of both Si and Mn occurs after prolonged time or by increasing the tempering temperature.