Abstract
Hot work tool steels (HWS) are widely used for high performance components as dies and molds in hot forging processes, where extreme process-related mechanical and thermal loads limit tool life. With the functionalizing and modification of tool surfaces with tailored surfaces, a promising approach is given to provide material flow control resulting in the efficient die filling of cavities while reducing the process forces. In terms of fatigue properties, the influence of surface modifications on surface integrity is insufficiently studied. Therefore, the potential of the machining processes of high-feed milling, micromilling and grinding with regard to the implications on the fatigue strength of components made of HWS (AISI H11) hardened to 50 ± 1 HRC was investigated. For this purpose, the machined surfaces were characterized in terms of surface topography and residual stress state to determine the surface integrity. In order to analyze the resulting fatigue behavior as a result of the machining processes, a rotating bending test was performed. The fracture surfaces were investigated using fractographic analysis to define the initiation area and to identify the source of failure. The investigations showed a significant influence of the machining-induced surface integrity and, in particular, the induced residual stress state on the fatigue properties of components made of HWS.
Highlights
The functionality and applicabi of these modified surfaces is strongly dependent on the machining process-related pr erties in terms of topography design, residual stress state and fatigue behavior
As the surface integrity strongly affects the fatigue behavior of metals [13], the results of the surface characterization, the fatigue testing and the subsequent fractographic analysis of surface-modified specimens made of Hot work tool steels (HWS) AISI H11 are discussed
The depicted surface topography of the high-feed milled specimens (Figure 5b) shows a quasi-deterministic surface structure that resulted from the combination of the values for the lead angle, the feed per tooth and the width of the cut with the special geometry of the high-feed milling tool, which had adjusted cutting edges
Summary
Tool life may be limited to approximately 103 –104 produced parts [4] For those high-performance tools such as dies, which are often made of hot work tool steels with high levels of strength, hardness and toughness [5], considerable effort is dedicated to the surface finishing in order to reduce friction and to improve the forming quality. This can be provided by polishing or coating the tool surfaces to reduce the resulting surface roughness [6], which can affect the resulting fatigue behavior. Characteristics of the near-surface residual stress state have a significant effect on the fatigue strength of metallic materials [11]
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