Laser Surface Modification of TC21 (α/β) Titanium Alloy Using a Direct Energy Deposition (DED) Process

Impact of thermal oxidation parameters on micro-hardness and hot corrosion of Ti-6Al-3Mo-2Nb-2Sn-2Zr-1.5Cr alloy

Protective oxide layers on Ti-6Al-3Mo-2Nb-2Sn-2Zr-1.5Cr (TC21) alloy with equiaxed microstructure considerably influence micro-hardness and hot corrosion resistance. The present work's thermal oxidation of TC21 alloy was performed at 600, 700, and 800 °C for 5, 20, and 50 h durations. Hot corrosion methods in NaCl and NaCl + Na₂SO₄ salt media were applied to raw (unoxidized) and oxidized samples at 600 and 800 °C for 50 h. Hot corrosion was conducted at 600 °C for 5 cycles with 10-h steps. The best oxide layer thickness was observed at 800 °C, which increased with increased oxidation time and temperature. The surface hardness of the oxide layer at 800 °C was 900 ± 60 HV_0.05 owing to the formation of TiO₂ and Al₂O₃ phases. Raw material hardness was 342 ± 20 HV_0.05, increasing threefold due to thermal oxidation. In the case of NaCl, weight loss dominated all samples except at 800 °C for 5 h. In the case of NaCl + Na₂SO₄, weight gain occurred at 600 and 800 °C for 5 h. Weight loss occurred for the raw samples and those processed at 800 °C for 20 and 50 h, where the oxide layer flaked off. Surface hardness increased upon hot corrosion testing because of the formation of brittle phases, such as TiO₂ and Na₄Ti₅O₁₂. Samples that oxidized at 800 °C for 5 h had the highest hardness and corrosion resistance.

Modeling of wear resistance for TC21 Ti-alloy using response surface methodology

This study investigated the effect of heat treatment processes on the dry sliding wear resistance of the TC21 Ti-alloy at several levels of normal load and sliding speed. Response Surface Methodology (RSM) has been used as a design of the experiment procedure. OM and FESEM besides XRD analysis were used for results justification. Highest hardness of 49 HRC was recorded for WQ + Aging specimens due to the plenty of α″ which decomposed to α_s and the more α_s, while the lowest hardness of 36 HRC was reported for WQ specimens. The results revealed that specimens subjected to water quenching and aging (WQ + Aging) under extreme load and speed conditions (50 N and 3 m/s), possessed the poorest wear resistance although they had the highest hardness. While those left in the annealed condition revealed the highest wear resistance although they had much lower hardness when compared to other conditions. A mathematical polynomial model for wear resistance expressed in wear rate was developed, validated then used to get the optimum parameters.

Turning parameters optimization for TC21 Ti-alloy using Taguchi technique

Background

The development of materials fabrication is an important trend in materials engineering. TC21 Ti-alloy is one of these materials’ trends. Investigations of different characteristics of TC21 Ti-alloy such as weldability, formability, and machinability will consume a large number of specimens. This work aims to study the machinability characteristics of TC21 Ti-alloy. The minimum number of experimental trials and optimal cutting conditions will be obtained by applying the orthogonal array (OA) L9 Taguchi technique. To achieve this aim, experimental work will be conducted under three varying cutting parameters, each one of them with three levels: cutting speeds (V) of 80, 100, and 120 m/min, feed rates (f) of 0.05, 0.10, and 0.15 mm/rev, and cutting depth (a) of 0.2, 0.4, and 0.6 mm.

Results

The results revealed that the cutting depth and cutting speed with percentages contribution of 40.8% and 48.6%, respectively, are the most significant parameters of surface roughness and wear of the tool insert. However, the least significant parameters are cutting speed and feed rate with percentages contribution of 20.2% and 2.3%, respectively.

Conclusions

Minimum surface roughness at V = 80 m/min, f = 0.10 mm/rev, and a = 0.4 mm is 0.16 µm, and maximum surface roughness at V = 80 m/min, f = 0.15 mm/rev, and a = 0.6 mm is 0.72 µm. Minimum tool wear at V = 100 m/min, f = 0.15 mm/rev, and a = 0.2 is 187.770 µm, and the maximum tool wear at V = 80 m/min, f = 0.10 mm/rev, and a = 0.4 mm is 274.896 µm.

Functional Surface Generation by EDM-A Review

Electro-discharge machining (EDM) removes electrically conductive materials by high frequency spark discharges between the tool electrode and the workpiece in the presence of a dielectric liquid. Being an electrothermal process and with melting and evaporation being the mechanisms of material removal, EDM suffers from migration of materials between the tool and the workpiece. Although unwanted surface modification was considered a challenge in the past for many applications, this inherent nature of the EDM process has recently become of interest to the scientific community. As a result, researchers have been focusing on using the EDM process for surface modification and coating by targeted surface engineering. In order to engineer a surface or generate functional coatings using the electro-discharge process, proper knowledge of the EDM process and science of electro-discharge surface modification must be understood. This paper aims to provide an overview of the electro-discharge surface modification and coating processes, thus assisting the readers on exploring potential applications of EDM-based techniques of surface engineering and coating generation. This review starts with a brief introduction to the EDM process, the physics behind the EDM process, and the science of the surface modification process in EDM. The paper then discusses the reasons and purposes of surface modification and coating practices. The common EDM-based techniques reported in the literature for producing coatings on the surface are discussed with their process mechanisms, important parameters, and design considerations. The characterization techniques used for the analysis of modified surfaces and coating layers, as well as the tribological and surface properties of modified surfaces or coatings are discussed. Some of the important applications of EDM-based surface modification and coating processes are generating surfaces for protective coating, for aesthetic purposes, for enhancing the biocompatibility of implants, for improving corrosion resistance, for improving wear resistance, and for improving tribological performance. The current state of the research in these application areas is discussed with examples. Finally, suggestions are provided on future research directions and innovative potential new applications of the electro-discharge-based surface engineering and coating processes.

Effect of oxide layers formed by thermal oxidation on mechanical properties and NaCl-induced hot corrosion behavior of TC21 Ti-alloy

In the current study on TC21 Ti-alloy (6.5Al-3Mo-1.9Nb-2.2Sn-2.2Zr-1.5Cr), the thermal oxidation formed oxide layers that considerably influenced mechanical properties (hardness and wear). TC21 specimens were oxidized at 600, 700, 800, and 900 °C for 5, 20, and 50 h. NaCl-induced hot corrosion testing was carried out on raw (un-oxidized) and oxidized specimens at 600 and 800 °C for 50 h. The cyclic testing was performed at 600 °C for durations of 5, 10, 20, 30, 40, and 50 h. The average thickness of the layer grew with increasing oxidation time and temperature. A thin oxide layer (average 0.16 µm) was generated by oxidation at a temperature of 600 °C for a duration of 5 h, and at 800 °C, a large oxide layer of 10.8 µm thickness was formed. The most significant surface hardness of 1000 ± 150 HV0.05 was produced for the layer oxidized at 900 °C. On the other hand, the lowest hardness of 360 ± 150 HV0.05 was recorded for the raw materials. Best wear resistance had been achieved for specimens oxidized at 800 °C. During NaCl hot corrosion test, the weight loss of the raw specimen was 6.4 mg/cm2 due to the flaking off of the corrosion product. However, for specimens oxidized at 600 °C for 50 h, weight loss after corrosion testing was 0.54 mg/cm2, less than that of the specimen before corrosion. Oxidized specimens at 800 °C exhibited the best mechanical characteristics and corrosion resistance.

Removal Mechanism and Electrochemical Milling of (TiB+TiC)/TC4 Composites

Titanium matrix composite (TiB+TiC)/TC4 has excellent physical properties and is a completely new composite material with great application prospects in the next generation of the aerospace field. However, there are problems, such as tool loss and material overheating, when using conventional processing methods. Electrochemical milling is a low-cost, high-efficiency processing method for difficult-to-machine metal materials with no tool wear. In this research, the feasibility of the electrochemical milling of (TiB+TiC)/TC4 and removal mechanisms during processing was reported for the first time. The feasibility of electrochemical milling is verified by the current efficiency experiment and basic processing experiment. Through the adjustment of the processing parameters, the final material removal rate increased by 52.5% compared to that obtained in the first processing, while the surface roughness decreased by 27.3%. The removal mechanism during processing was further performed based on the current efficiency experiment; three stages were observed and concluded during the electrolytic dissolution. This research proved that electrochemical milling is an excellent low-cost method for roughing and semi-finishing (TiB+TiC)/TC4 composites and provides guidance for better electrochemical milling in the titanium matrix composites.