Advances in Plastic Deformation Technologies

1. Introduction

Material enhancement plays an important role in everyday life due to its impacts on the quality of goods, which we, consumers, buy and use. Advanced finishing technologies play a key role in automotive, aerospace, biomedical, and other industries and are an essential part of mechanical engineering. One of the most important groups of advanced finishing technologies are based on plastic deformation [1]. Although impressive progress in both instrumentation and theory was achieved in this field over the past decades, academic and industrial researchers still face a number of challenges, and a number of important issues are yet to be addressed, such as:

• How can we increase productivity and, at the same time, lower the production costs while maintaining a high quality of products?
• How can we make use of plastic deformation in machining products for additive technologies?
• Which materials are best to use in instrument design?

Answering these questions is critically important for gaining deep and insightful understanding of both new plastic deformation processes, such as, for example, incremental plastic deformation, shoot peening, nano-burnishing, wide burnishing, and low plasticity burnishing, as well as traditional plastic deformation processes, such as extrusion, drawing, and bending.

The importance of the topics covered by this Special Issue is well proven by the steadily growing number of articles on plastic deformation in mainstream international journals over the past few years.

2. Contributions

The Special Issue is composed of six full papers concerning plastic deformation processes and their applications. Four out of them are dedicated to burnishing, an advanced finishing operation widely used in various industries [2,3,4]. Burnishing is a plastic deformation process, in which a workpiece surface improves due to sliding contact with a tool called a burnisher.

The paper, authored by Bobrovskij et al. [5], discusses the impacts of the initial surface roughness Ra on the properties of the workpiece being burnished and the overall energy efficiency of the burnishing process. In the experimental part of their work, the nature of friction in the contact zone and impacts of the clamping force on the stability and energy efficiency have been investigated. The obtained results showed that the nature of friction accompanying the surface plastic deformation has a significant impact on the stability and energy efficiency of the burnishing process. While the clamping force was found to be equally important for burnishing with and without lubrication, the initial roughness (Ra) was shown to have a considerable impact on dry burnishing only. The application of minimum quantity lubrication (MQL) under experimental conditions typical for industrial burnishing is found not only to enhance the stability of the burnishing process, but also to increase its energy efficiency by more than 20%.

In the second paper, Llumà et al. [6] investigated superficial hardening, which appears after vibration-assisted ball burnishing and its influence on the tensile behavior of a carbon steel material. The authors found that ball burnishing affects the workpiece material via the plastic deformation to hundredths of micrometers in depth. They also showed that an increase in burnishing preload diminishes the ductile behavior of the material and increases its general strength, while the proportion of the affected material in the cross-section of the specimen is reduced with regard to the entire surface. They also found that the impacts of vibrations reduce as the preload increases, and, thus, the effect of the static preload acquires more relevance in modifying the macroscopic mechanical properties of the steel alloy.

Jerez-Mesa et al. [7] reported the impacts of ultrasonic vibration-assisted ball burnishing on the topological descriptors of the nickel-based alloy Udimet^®720. This material is of high interest for the transportation industry, especially for the aeronautical sector. In this paper, different surface descriptors were used to reveal how topology changes after ultrasonic-assisted ball burnishing, as well as how burnishing conditions influence observed changes. The burnishing preload and the number of passes appeared to be the key factors affecting the workpiece surface properties. The extent to which the workpiece surfaces can be successfully modified is highly variable and strongly depends on the original scale of the surface being machined. The results of vibration-assisted burnishing show that the resulting topologies are characterized by a periodical pattern of repetitive peaks and valleys that are present on the workpiece surface with a higher frequency in comparison to the non-assisted process.

Amini et al. [8] used the burnishing process to improve the material surface after being submitted to friction stir welding paths. The metallurgical and topological states of materials joined by this welding technique are typically detrimental to the ulterior performance of the workpiece. The improvement in the topology and deep hardness distribution was measured and discussed, and the evolution of mechanical properties was thoroughly assessed via tensile tests. In addition, the authors estimated the residual stresses by combining two pre-existing models of friction stir welding and burnishing developed using ANSYS^® software. Ball burnishing was shown to be an efficient method of enhancing the surface integrity of friction stir-welded joints, as evidenced by a reduction of 11% to 36% in average roughness and an increase of about 22% in hardness profile.

Bobrovskij et al. [9] developed a new mathematical model of coefficients in the Coulomb constant shear friction law for extruding a metal through narrow V-shaped channels with small convergence angles, which was evaluated and compared with laboratory measurements. The Coulomb friction coefficient µ and the constant friction factor m appear to be independent of the dimension ratio and are influenced mostly by roughness, and they range from µ = 0.363 (with lubricant) to µ = 0.488 (without lubricant) and from m = 0.726 (with lubricant) to 0.99 (without lubricant). The relative length dominated by the Coulomb friction law is less than 1%, and Coulomb’s coefficient of friction can be approximated as ½ the constant shear friction factor in all the cases studied in their work. The developed method and algorithm can be used in both the FEA of the manufacturing processes and in the efficiency tests of lubricants used in metalworking.

Finally, Zhou et al., in their work [10], discussed the deformation behavior of pure iridium during thermal compression testing with the help of Gleeble-1500D in the temperature range of 1200 °C~1500 °C and strain rate range of 10⁻¹ s⁻¹~10⁻² s⁻¹. Resistance to deformation, microstructural evolution, and hot workability of pure iridium have been used to analyze its deformation behavior in detail. A frictional coefficient has been used to modify the experimental stress–strain curve of a thermal compression test that has been found efficient in reducing the impact of friction during thermo–mechanical testing. On the basis of a constitutive model and processing map, optimum forming process parameters (temperature range of 1400 °C~1500 °C and strain rate range of 0.1 s⁻¹~0.05 s⁻¹) for pure iridium have been determined.