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The elasto-plastic material behavior, material strength and failure modes of metals fabricated by additive manufacturing technologies are significantly determined by the underlying process-specific microstructure evolution. In this work a novel physics-based and data-supported phenomenological microstructure model for Ti-6Al-4V is proposed that is suitable for the part-scale simulation of laser powder bed fusion processes. The model predicts spatially homogenized phase fractions of the most relevant microstructural species, namely the stable β -phase, the stable α s -phase as well as the metastable Martensite α m -phase, in a physically consistent manner. In particular, the modeled microstructure evolution, in form of diffusion-based and non-diffusional transformations, is a pure consequence of energy and mobility competitions among the different species, without the need for heuristic transformation criteria as often applied in existing models. The mathematically consistent formulation of the evolution equations in rate form renders the model suitable for the practically relevant scenario of temperature- or time-dependent diffusion coefficients, arbitrary temperature profiles, and multiple coexisting phases. Due to its physically motivated foundation, the proposed model requires only a minimal number of free parameters, which are determined in an inverse identification process considering a broad experimental data basis in form of time-temperature transformation diagrams. Subsequently, the predictive ability of the model is demonstrated by means of continuous cooling transformation diagrams, showing that experimentally observed characteristics such as critical cooling rates emerge naturally from the proposed microstructure model, instead of being enforced as heuristic transformation criteria. Eventually, the proposed model is exploited to predict the microstructure evolution for a realistic selective laser melting application scenario and for the cooling/quenching process of a Ti-6Al-4V cube of practically relevant size. Numerical results confirm experimental observations that Martensite is the dominating microstructure species in regimes of high cooling rates, e.g., due to highly localized heat sources or in near-surface domains, while a proper manipulation of the temperature field, e.g., by preheating the base-plate in selective laser melting, can suppress the formation of this metastable phase.

The laser-based powder bed fusion of metals (PBF-LB/M) offers a variety of advantages over conventional processing techniques and the possibility to recycle and reuse powder increases its sustainability. However, the process and resulting part properties are influenced by a variety of factors including powder recycling grade and residual oxygen content of the process atmosphere. Especially in terms of reactive materials like Ti-6Al-4V, oxidation during processing and recycling determines process stability and reproducibility. This work therefore focusses on the influence of the conventionally varied processing parameters as well as atmosphere residual oxygen content process and powder recycling on the microstructure and mechanical properties. For this purpose, the design of experiments approach is used and by evaluation of regression models, effect sizes and interactions are given. Additionally, two different etching techniques were employed to reveal different aspects of the microstructure. While no significant influence of powder recycling and residual oxygen on the microstructure could be observed, they both significantly influence the mechanical properties. A maximum hardness of 470 HV0.1, a maximum ultimate tensile strength of 1252.3 MPa, and a maximum elongation at break of 17.8 % were obtained. The results demonstrate the importance of the processing atmosphere’s residual oxygen content and of taking into account the changing powder characteristics during recycling as well as its effect on the part properties.

Additive manufacturing (AM) techniques, such as wire arc additive manufacturing (WAAM), offer unique advantages in producing large, complex structures with reduced lead time and material waste. However, their application in fatigue-critical applications requires a thorough understanding of the material properties and behavior. Due to the layered nature of the manufacturing process, WAAM structures have different microstructures and mechanical properties compared to their substrate counterparts. This study investigated the mechanical behavior and fatigue performance of Ti-6Al-4V fabricated using WAAM compared to the substrate material. Tensile and low-cycle fatigue (LCF) tests were conducted on both materials, and the microstructure was analyzed using optical microscopy and scanning electron microscopy (SEM). The results showed that the WAAM material has a coarser and more heterogeneous grain structure, an increased amount of defects, and lower ultimate tensile strength and smaller elongation at fracture. Furthermore, strain-controlled LCF tests revealed a lower fatigue strength of the WAAM material compared to the substrate, with crack initiation occurring at pores in the specimen rather than microstructural features. Experimental data were used to fit the Ramberg–Osgood model for cyclic deformation behavior and the Manson–Coffin–Basquin model for strain-life curves. The fitted models were subsequently used to compare the two material conditions with other AM processes. In general, the quasi-static properties of WAAM material were found to be lower than those of powder-based processes like selective laser melting or electron beam melting due to smaller cooling rates within the WAAM process. Finally, two simplified estimation models for the strain-life relationship were compared to the experimentally fitted Manson–Coffin–Basquin parameters. The results showed that the simple “universal material law” is applicable and can be used for a quick and simple estimation of the material behavior in cyclic loading conditions. Overall, this study highlights the importance of understanding the mechanical behavior and fatigue performance of WAAM structures compared to their substrate counterparts, as well as the need for further research to improve the understanding of the effects of WAAM process parameters on the mechanical properties and fatigue performance of the fabricated structures.

Based on the software ABAQUS/Explicit, a finite element (FE) model for orthogonal cutting was established. The FE model was validated by comparing the cutting forces and serrated degree of chips obtained by orthogonal cutting experiments under the cutting speeds 40, 80, 120, and 160 m/min. Based on the developed FE model, the influence of thermal conductivity on the degree of chip segmentation and the adiabatic shear localization were investigated. Furthermore, the plot contours on undeformed shape of cutting simulation was used to investigate the temperature distribution, and the high temperature zone was identified, which can help enhance the understanding of the serrated chip formation. Finally, cracks located in the adjacent segments of chips were observed. The results show that with the increase in thermal conductivity, the degree of adiabatic shear decreases. It can be concluded that the poor thermal conduction performance should be primarily responsible for the formation of serrated chips during machining Ti-6Al-4V alloy. Due to the high temperature at contact surface between cutting tool and workpiece, the increasing of cutting speed facilitates the formation of serrated chips during machining.

The material of Ti–6Al–4V has been widely applied in various industries, such as automobile, aerospace, and medical due to its high specific strength, superior thermal stability and strong corrosion resistance. In the recent decades, selective laser melting (SLM) has become an attractive method to fabricate Ti–6Al–4V parts, thanks to its significant advantages in low material consumption, the high degree of freedom in design, low carbon footprint, etc. Predictability of SLM material fatigue properties is especially important for the safety-critical structures under dynamic load cases. The present research is aimed at evaluating the low cycle fatigue (LCF) performance of SLM Ti–6Al–4V under high loading states. LCF tests were performed for as-built and annealed SLM Ti–6Al–4V. Comparison between LCF properties of SLM Ti–6Al–4V and the wrought Ti–6Al–4V was also made. It was found that as-built SLM Ti–6Al–4V demonstrated a comparable LCF performance with the wrought material. The LCF life of as-built SLM Ti–6Al–4V was longer than that of wrought Ti–6Al–4V at lower strain amplitudes. However, the wrought Ti–6Al–4V had better LCF performance at higher strain amplitudes. The results revealed that the porosity in the as-built SLM material exerted much more impact on the degradation of the material at high strain amplitudes. Annealing deteriorated the LCF performance of SLM Ti–6Al–4V material due to the formation of coarser grains. The cyclic Ramberg–Osgood and the Basquin–Coffin–Manson models were fitted to depict the cyclic stress–strain and the strain–life curves for the SLM Ti–6Al–4V, based on which the LCF performance parameters were determined. In addition, the fatigue fracture surfaces were observed by using scanning electron microscopy (SEM), and the results indicated that fatigue cracks originated from the surface or subsurface defects.

Electron beam melting (EBM) has been recognized as an emerging additive manufacturing technology, which allows the production of fully dense parts from various metals. The technique builds parts using an electron beam to melt metal powder, layer by layer, in a powder bed. Thus, complex parts, including internal structures, can be made directly from 3D CAD models without special fixtures and cutting tools. Much research is conducted to study the microstructure and mechanical properties of EBM-built parts or to investigate the effects of EBM process parameters on the quality of parts. However, using EBM for building new features on an existing part has received little attention from the research community. Due to its performance, EBM seems able to transform an end-of-life (EoL) part/existing part into a new part without returning to the level of raw material. The aim of this study is to validate such principle. In this paper, an EBM machine was used to produce new Ti-6Al-4V features on a Ti-6Al-4V plate, which was considered as a useful core retrieved from an EoL product. The built samples, including EBM-built features and Ti-6Al-4V plate, were investigated in terms of their microstructure and mechanical properties. The results showed that EBM technique allows new features with a suitable microstructure and controlled mechanical properties to be built on an existing part. Moreover, the EBM-built features have a strong bond with the existing part leading to the same mechanical characteristics as original part. These demonstrate that the EBM technology has a promising potential for producing new parts from EoL parts/existing parts.

Titanium and its alloys are attractive materials due to their low density and resistance to high temperature and corrosion. However, these materials are also known for their low machinability that leads to poor surface finish and premature tool wear. When machining such materials, serrated chips are often generated. According to the literature, this is generally due to the material damage and microstructure transformation phenomena. A flow stress modeling that takes into account material damage and dynamic recrystallization (DRX) is proposed to obtain a more realistic cutting process simulation of the titanium alloy Ti-6Al-4V. The Johnson-Mehl-Avrami-Kolmogorov (JMAK) model is used to predict the recrystallized volume fraction involved in the proposed flow stress law. The microstructure evolution influences the material damage and the JMAK DRX initiation criterion is used to introduce this effect. A 2D Lagrangian finite element (FE) formulation is adopted to simulate the orthogonal cutting process. The cutting forces and chip morphology, obtained with the proposed behavior model, are analyzed and compared to those obtained with known tangent hyperbolic (TANH) and Johnson-Cook (JC) behavior models. A good accordance between the proposed model simulations and the experimental results is noticed. The link between recrystallization, damage and chip segmentation has been deeply analyzed.

In this paper, a milling force prediction model considering the Taylor factor is established, and the Ti-6Al-4V milling force predicted by the model under different milling parameters is presented. In the study, the milling experiment of Ti-6Al-4V was carried out, the milling force was collected by the dynamometer, and the microstructure evolution of the milling surface before and after milling was observed by EBSD. Through the comparative analysis of the experimental results and the model prediction results, the reliability of the prediction model proposed in this study was verified, and the influences of the milling parameters on the milling force were further analyzed. Finally, based on the EBSD observation results, the effects of the milling parameters on the microstructure evolution of the milling surface were studied. The results show that both the tangential milling force and normal milling force increase with the increase in the milling depth and feed rate. Among the milling parameters selected in this study, the milling depth has the greatest influence on the milling force. The average errors of the tangential milling force and normal milling force predicted by the milling force model are less than 10%, indicating that the milling force prediction model established in this paper considering Taylor factor is suitable for the prediction of the Ti-6Al-4V milling force. With the change in the milling parameters, the grain structure, grain size, grain boundary distribution, phase distribution, and micro-texture of the material surface change to varying degrees, and the plastic deformation of the milling surface is largely coordinated by the slip.

Resistance spot welding (RSW) is one of the most effective welding methods for titanium alloys, in particular Ti-6Al-4V. Ti-6Al-4V is one of the most used materials with its good ductility, high strength, weldability, corrosion resistance, and heat resistance. RSW and Ti-6Al-4V materials are often widely used in industrial manufacturing, particularly in automotive and aerospace industries. To understand the phenomenon of resistance spot weld quality, the physical and mechanical properties of Ti-6Al-4V spot weld are essential to be analyzed. In this study, an experiment was conducted using the Taguchi L9 method to find out the optimum level of the weld joint strength. The given optimum level sample was analyzed to study the most significant affecting RSW parameter, the failure mode, the weld nugget microstructure, and hardness values. The high heat input significantly affect the weld nugget temperature to reach and beyond the β-transus temperature. It led to an increase in the weld nugget diameter and the indentation depth. The expulsion appeared in the high heat input and decreased the weld nugget strength. It was caused by the molten material ejection in the fusion zone. The combination of high heat input and rapid air cooling at room temperature generated a martensite microstructure in the fusion zone. It increased the hardness, strength, and brittleness but decreased the ductility.

Recent studies have shown that the mechanical properties of Ti alloys produced by additive manufacturing (AM) methods are sensitive to AM process parameters. The mechanical threshold stress (MTS) model is capable of predicting the flow stress behavior of materials; however, the parameters needed in the MTS model are affected by the microstructure that originates from the AM process parameters. To find a relationship between the AM process parameters and the MTS parameters, the effect of process parameters on the mechanical properties of selective laser-melted (SLM) Ti-6Al-4V samples was studied. As the MTS model is sensitive to the microstructure, only near fully dense samples were considered.