V.A.M. Cristino

Hole–flanging of metals and polymers produced by single point incremental forming

This paper presents recent developments in hole–flanging of metal and polymers produced by single point incremental forming (SPIF) to gaining insight into the deformation mechanics of the process and providing guidelines to foster its application in rapid prototyping and small batch production of customised sheet products. Experimentation with stainless steel AISI 304L and polyethylene terephthalate (PET) that comprised mechanical characterisation and utilisation of circle grid analysis in conjunction with the formability limit diagram gives support to the presentation and allows understanding the fundamentals of plastic flow and failure in hole–flanging produced by single point incremental forming and their main differences to hole–flanging produced by conventional pressworking.

Formability and Simulative Tests in Modern Sheet Metal Forming Education

This chapter provides the theoretical and experimental background to the utilization of circle grid analysis in conjunction with formability and simulative laboratory tests at undergraduate and graduate courses of metal forming with the objective of exposing students to machine-tools, tool systems and practical problems that are commonly found in daily production of sheet metal parts. The presentation starts with a description of strain measures and measurements techniques, extends this background to plastic instability and fracture, introduces the forming limit diagrams and systematizes the associated formability tests, and reviews the process simulation tests aimed at characterizing bendability, stretchability and drawability of blanks. Then, presentation proceed with a comprehensive description of a flexible, versatile and robust laboratory tool system that was developed by the authors to support education and basic research activities of sheet metal forming. The last part of the chapter presents the fundamentals of a simple finite element computer program developed by the authors that gives support to sheet metal forming laboratory tests and provides students with a strong link between experimentation and numerical simulation.

On the utilization of pin-on-disc simulative tests for the calibration of friction in metal cutting

Abstract This article proposes a new design for pin-on-disc machines and introduces a research methodology that aims at providing a new level of understanding for the influence of surface texture and roughness on the average value of the friction coefficient. The new design for pin-on-disc machines increases the overall stiffness, eliminates the need for counter weights, and allows tests to be carried out under variable loading conditions and includes a unit for producing and regenerating the desired texture and roughness in the surface of the discs after completion of each test. The comparison between the friction coefficient obtained with pin-on-disc tests and that acquired in metal cutting laboratory conditions allows concluding that pin-on-disc tests, when performed with an adequate control of texture and surface roughness, are capable of providing a good estimate of the average value of the friction coefficient in metal cutting applications.

The Role of Interfaces in the Evaluation of Friction by Ring Compression Testing

The role of interfaces in friction is most frequently not taken into account when performing the evaluation of the coefficient of friction by means of process and simulative tribology tests. Despite being difficult and time consuming to control surface roughness as well as the formation of oxide films through exposure to surrounding medium, it is very important to perform the experiments under conditions similar to those commonly found in real metal forming processes. As accuracy and reliability of the experimentally determined coefficients of friction depend on the similarity to real metal forming conditions, this paper is aimed to provide a comprehensive analysis on the influence of oxide films and surface roughness in the evaluation of friction by means of the ring compression test. The paper presents an innovative experimental approach for ring compression testing under controlled conditions of exposure to atmosphere and surface roughness. Quantitative data obtained in presence of inert and active gas shields and across typical values of surface roughness allow understanding the role of interfaces in friction and to build a conscience on its prospective source of modelling errors due to experimental drift from real metal forming. Results show an increase of the coefficient of friction up to 30% when active gas shields are employed.

Tribology in Metal Cutting

This chapter revisits tribology tests in metal cutting in order to obtain new fundamental knowledge on friction and to understand which technical modifications and operating parameters need to be developed and implemented in order to obtain good estimates of the coefficient of friction. The methodology draws from the development of new equipment and testing procedures focused on the interaction between surrounding medium, surface roughness and freshly formed surfaces to the independent determination of the coefficient of friction.

Determining the fracture forming limits in sheet metal forming: A technical note:

This technical note describes an experimental method to determine the formability limits by fracture in sheet metal forming. The method makes use of laboratory test specimens commonly utilized in the mechanical, fracture and formability characterization of sheet materials and involves determination of the gauge length strains at the cracked regions of the specimens after testing. The presentation explains how measurements are made and what calculations need to be performed in order to determine the fracture forming limit by tension in the principal strain space. The method is applied to Titanium grade 1 sheets, and the fracture forming limit is validated by subsequent experiments with single point incremental forming.

Revisiting the Calibration of Friction in Metal Cutting

This article reexamines pin-on-disc and ring compression tests in order to understand which technical modifications and operating parameters have to be revised and controlled in order to obtain valid estimates of the coefficient of friction for metal cutting applications. The methodology draws from the development of new equipment and testing procedures focused on the interaction between surrounding medium, surface roughness, and freshly formed surfaces to the independent determination of the coefficient of friction. Assessing the coefficient of friction obtained from experimentation with pin-on-disc and ring compression tests against that acquired in orthogonal metal cutting conditions allows concluding that simulative tribology tests, performed in dry friction conditions, with adequate control of surface morphology and under a protective shield of argon, are capable of modeling contact with friction in good agreement with real metal cutting. The identification of operative testing conditions that are capable of merging the coefficient of friction supplied by different tribology tests ensures a unified view of tribologists and metal cutting experts on the accuracy, reliability, and validity of simulative tribology tests for metal cutting applications.

Design of Flexible Bulge Testing System for Evaluating the Influence of Size Effect on Thin Metal Sheets

Formability in sheet forming processes are usually analyzed by standardized tests, which often requires different test equipment associated with high initial investment cost. The present study purposes a flexible test tooling system for hydraulic bugle test apparatus that allows to evaluate the impact of size effect on the formability of thin metallic sheets. Finite Element Method was used for concept and design of the tooling system and experimental tests were carried out with thin sheets of SUS316L stainless steel to assess the overall performance of the tooling system.

Effects of ultrasonic vibration on SUS304 stainless steel subjected to uniaxial plastic deformation

ABSTRACT In this study, a series of experiments was conducted in order to investigate the mechanisms of tensile force reduction and martensitic transformation in SUS304 stainless steel during ultrasonic vibration assisted tensile tests (UAT). An independent analysis of the impact of the stress superposition effect in the ultrasound assisted tensile tests was done by finite element simulation, and metallographic analysis on the specimens showed that the increase of the amplitude of the ultrasonic vibration has a great influence on the martensite transformation in the material during deformation, reducing the elongation of the specimens. The results also pointed out that the tensile force reduction is caused by a combination of the effects of stress superposition and the energy absorption of dislocations over the hardening of the material due to the increase of dislocation density and induced martensitic transformation.

Cutting Under Gas Shields: Phenomenological Concepts Versus Industrial Applications

This chapter is focused on the interaction between cutting medium and freshly formed surfaces with the aim of providing a new level of understanding on the mechanics of chip flow in orthogonal metal cutting and analyzing its influence on the cutting forces and tool life in conventional milling. The first part of the chapter presents experimental results from an investigation performed with a specially-designed orthogonal metal cutting apparatus and a model material under dry conditions, with the objective of analyzing the influence of active and inert gas shields in the kinematics of chip flow, the friction coefficient, the chip-compression factor and the cutting forces. In particular, authors provide a correlation between surrounding medium, tribological conditions, surface roughness, freshly cut surfaces and chip curling at the tool-chip contact interface. The second part of the chapter extends the investigation to conventional milling of an engineering material in order to study the influence of cutting in the presence of air or under an argon atmosphere in the overall cutting forces and tool wear. Results from the investigation based on orthogonal metal cutting show that cutting in the presence of oxygen leads to higher values of friction, chip compression factor and chip curl radius, and to lower values of the shear plane angle. The presence of oxygen is also responsible for increasing the cutting forces and tool wear in conventional milling.