Forming of metastable austenitic stainless steel tubes with axially graded martensite content by flow-forming

Abstract

One of the main objectives of production engineering is to reproducibly manufacture (complex) defect-free parts. To achieve this, it is necessary to employ an appropriate process or tool design. While this will generally prove successful, it cannot, however, offset stochastic defects with local variations in material properties. Closed-loop process control represents a promising approach for a solution in this context. The state of the art involves using this approach to control geometric parameters such as a length. So far, no research or applications have been conducted with closed-loop control for microstructure and product properties. In the project on which this paper is based, the local martensite content of parts is to be adjusted in a highly precise and reproducible manner. The forming process employed is a special, property-controlled flow-forming process. A model-based controller is thus to generate corresponding correction values for the tool-path geometry and tool-path velocity on the basis of online martensite content measurements. For the controller model, it is planned to use a special process or microstructure (correlation) model. The planned paper not only describes the experimental setup but also presents results of initial experimental investigations for subsequent use in the closed-loop control of α’-martensite content during flow-forming.

Innovative Online Measurement and Modelling Approach for Property-Controlled Flow Forming Processes

“…The characteristic curve is parameterized with experimental data presented in [15]. The data contains different feeds 𝑓𝑓(𝑡𝑡) from 0.1 mm/s to 1 mm/s, infeed depths Δ𝑟𝑟(𝑧𝑧(𝑡𝑡)) from 0.5 to 1 mm and wall thicknesses between 1.5 mm and 4.0 mm.…”

“…9 includes the absolute value due to the sign of 𝜑𝜑 𝑠𝑠 . The equation is parameterized with experimental data presented in [15]. The data results from the same experiments as used for Eq.…”

A flow forming process model to predict workpiece properties in AISI 304L

“…The experiments from preliminary work pursued an empirical approach to process modeling. Correlation models (see section 2.3) were developed to quantify the influence of isolated process parameter variations, such as the feed rate or the wall thickness reduction per pass (infeed of the roller tools), on the resulting true strain 𝜑 𝑟 (true strain in thickness direction) as well as the α'-martensite content, like mentioned in the introduction [6]. In addition to the process analysis, the correlations are used for model-based control design purposes.…”

“…The investigations were performed jointly by Forming and Machining Technology (LUF, Paderborn University), the Chair of Materials Test Engineering (WPT, TU Dortmund University) and the Fraunhofer Institute for Mechatronic Systems Design (IEM, Paderborn). These served to identify correlations between flow forming process parameters and the resulting strain-induced α'-martensite content as well as the true strain 𝜑 𝑟 (true strain in thickness direction) of the formed seamless tubes [6].…”

Cryogenic reverse flow forming of AISI 304L

“…The first strategy employs a multi-stage process that utilizes process parameters such as the feed rate and infeed, which has been demonstrated to be effective in earlier research. Our previous investigations have shown a correlation between the selection of these parameters and the α'-martensite content, while maintaining the external dimensions [3]. The second strategy is part of current work and involves thermomechanical forming, which locally affects the formation of α'-martensite in a single-stage process through the active manipulation of temperature (cooling/heating).…”