Direct Selective Laser Sintering of Tool Steel Powders to High Density: Part B -The Effect on Microstructural Evolution

This paper describes recent progress on the Direct Selective Laser Sintering of M2 [Fe6W-5Mo-4Cr-2V-0.8C] high speed steel (HSS) and H13 [Fe-5Cr-1V-1Si-1.5Mo-0.4C] tool steel powders. Part B will focus on the microstructural evolution of laser scanned powder beds. It has been found that H13 powders are more amenable to Direct Selective Laser Sintering than M2 powders. Densities up to 90% are possible with H13 powder compared with 70% for M2. The relationship between alloy composition, microstructure, post-scanned density and scan conditions will be discussed for single track, single layer and multi-layer constructions.The research reported in this paper is a joint project between the Universities of Bradford, Leeds and Liverpool, funded by the UK Engineering and Physical Sciences Research Council under Grant Number GR/R32222.Mechanical Engineerin

Direct Selective Laser Sintering of Tool Steel Powders to High Density: Part A - Effects of Laser Beam Width and Scan Strategy

This paper describes progress on the Direct Selective Laser Sintering of M2 and H13 tool steel powders, comparing this with previous and further observations on stainless steel powders. The distinguishing feature is the melting of single tracks and layers in deep powder beds. The paper focuses on changing characteristics of the melt pool (mass, volume, aspect ratio, stability) and laser-powder interactivity as the laser beam width, power and scan speed change. It also compares the melt pool of neighbouring tracks during single layer construction. Simulations from a computer model to predict melt pool shape and dimension show reasonable agreement with experimental results at low scan speeds (0.5mm/s). But unexpected increases in melt depth above 1.0mm/s have been observed, suggesting higher values and more variability in laser absorptivity than expected, even approaching 1.0 for the CO2 laser radiation used in this work.Mechanical Engineerin

Wave evolution across the Louisiana shelf

Author Posting. © The Author(s), 2012. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Continental Shelf Research 52 (2013): 190-202, doi:10.1016/j.csr.2012.10.005.Observations and third-generation wave model hindcasts of ocean surface gravity waves propagating across the Louisiana shelf show that the effects of the mud environment on wave evolution are complex and episodic. Whereas low-frequency waves (0.04-0.20 Hz) show a consistent decay similar to earlier studies, the presence of mud also appears to suppress the development of short waves (0.20-0.25 Hz) under fetch-limited growth conditions. Significant suppression of wave development under wind-forced conditions is found to occur almost exclusively during easterly winds when satellite images show the Atchafalaya mud plume extends into the study area. These results suggest that episodic sediment suspension events with high mud concentrations in the upper water column can affect the evolution of wind waves.This work is supported by the U.S. Office of Naval Research (Littoral Geosciences and Optics Program, and Physical Oceanography Program), the National Oceanographic Partnership Program, the National Science Foundation, and a National Security Science and Engineering Faculty Fellowship

Thermal experiments in direct metal laser sintering

This paper discusses recent work undertaken to quantify energy absorptance and material conductivity during the laser processing of a powder bed. A carbon dioxide laser was used in experiments, directly sintering single lines and layers on the surface of a stainless steel powder bed. Thermocouples positioned under the bed’s surface measured temperature changes with time. Using adapted equations, experimental quantities were combined to calculate the proportion of incident laser energy absorbed and held by the bed. Response to different situations was tested, such as use of an inert atmosphere and change of scanning conditions. When scanning layers at various values of scan spacing, it was discovered that this change in scan spacing led to a change in energy absorptance

Morphology of Direct SLS-Processed Stainless Steel Layers

This paper discusses work done to analyse the shape of stainless steel layers generated by direct selective laser sintering (SLS). Laser power, scan spacing and scanning speed have been varied, to investigate their effect on geometry. The relationship between scanning parameters and the qualities of sintered parts (dimensional uniformity, porosity and scanned track shape) is described. A PC-based finite element code, developed to simulate SLS, has been modified to match the conditions of experiments discussed above. A comparison is made between computer-generated and experimentally-generated parts.Mechanical Engineerin

Simulation and Experimental Verification of Crystalline Polymer and Direct Metal Selective Laser Sintering 100

A previously developed two-dimensional finite element simulation of the SLS of amorphous polymers has been extended to the study of crystalline polymers and metals. For crystalline polymers, three developments have been needed: the treatment of latent heat of melting, the modifying of a viscous densification law to allow for the crystalline fraction of material during melting and, to obtain agreement with experiments, a consideration of the absorption depth of CO2 laser radiation into the powder bed; but the two-dimensional treatment remains sufficient. For metals, a different densification law and three-dimensional modelling have been needed for agreement with experiments on stainless steel powder beds.Mechanical Engineerin

Friction regimes in the lubricants solid-state regime

Friction measurements were performed in the lubricant's solid-state regime to study the transition from full-film lubrication, in which the separation is maintained by a solidified lubricant, to mixed lubrication. Special attention is paid to the influence of temperature (inlet viscosity) and roughness on this transition. The friction measurements showed that in the lubricants solid-state region three lubrication modes can be distinguished: A) full-film lubrication; separation is maintained by a solid film (S-EHL), B) mixed lubrication (ML); the load is carried by the interacting asperities as well as the lubricant (acting like a solid or as a liquid) and C) boundary lubrication (BL). Further, the dependency of the transition S-EHL to ML on temperature (inlet viscosity) depends on the lubricant used. The transition S-EHL to ML depends slightly more on roughness as found for L-EHL to ML transition. Finally, the film thickness formulas developed for EHL contacts in which the lubricant behaves as a liquid become doubtful when operating in the lubricant solid-state regime. This on the basis of the calculated film thickness over roughness ratios for the transition S-EHL to ML

Aspects of two-sided surface waviness in an ehl line contact

Applying a multigrid solver to the transient Reynolds equation, this paper investigates the influence of two-sided waviness on pressure and film thickness in a medium loaded EHL line contact. The contact properties are studied for one set of operating conditions varying the ratio of the surface velocities, as well as the amplitude and wavelength of the waviness. The characteristic quantities used to describe the transient problem are straightforward extensions of the ones describing the stationary problem such as H min , H ave and P max Where possible physical explanations of the observed relations are presented

Frictional heating of elliptic contacts

Wherever friction occurs mechanical energy is transformed into heat. The maximum surface temperature associated with this heat generation can have an important influence on the tribological behaviour of the mating components. For band contacts and circular contacts this temperature has already been studied extensively. However for elliptic contacts only approximate solutions exist. In this work a fast numerical algorithm is presented to calculate the steady state solution for the flash temperature for elliptic contacts with arbitrary entrainment angle. The heat generation may be due to either a uniform or a semi-ellipsoidal shaped heat source distribution, more or less representing EHL conditions and dry or boundary lubrication conditions, respectively. The asymptotic solutions for large and small Peclet numbers and numerical solutions will be presented. Function fits for the flash temperature will be proposed that are more reliable than the function fits in current use, even for circular contacts. Aspect ratios of the contact ellipse in the range of 0.20 - 5.0 are covered. Within this range the fits were found to be accurate within 5%