Search

Corrosion of one type or another occurs in almost every environment and high performance corrosion protection methods are often too expensive to be economically viable. This work examines the performance of protective layers of Inconel 617, laser clad in the form of salvaged machining chips, to protect a corrosive substrate from both mild and harsh environments. The clad layers are investigated for microstructure and phase composition and polarisation measurements used to determine their corrosion resistance in neutral and acidified NaCl electrolyte solutions. The clad microstructure was discovered to be columnar dendritic, with an atypical upper layer. However, neither this nor any dilution by the substrate compromised the corrosion resistance of the Inconel 617, and the clad layer provided very good corrosion protection. The results confirm the viability of using this method as a low cost corrosion protection method for both mild and harsh environments. © 2012 Institute of Materials, Minerals and Mining.

In this work, a single-stage, blown powder laser cladding process was used to deposit a protective layer of Ni-based alloy Inconel 617 on mild steel substrates. A Design of Experiments methodology is used to analyse the effects of the major laser cladding processing parameters on the deposited layer characteristics. Layer thickness, microstructure, dilution, and corrosion resistance are analysed and correlated with the processing parameters and the overall effectiveness of the protective coating assessed. The work is unique in that the protective material, usually clad in the form of costly powder, is in this case virtually cost-free and simply recycled from machining waste without any costly atomisation or similar process. The method has clear potential for material-efficient and cost-effective corrosion protection.

The laser metal deposition process continues to receive attention from researchers and industry due to its unique capabilities in applications such as surface coating or rapid manufacture. The development of numerical models has proven useful for improving the process. However, most models have focused on analyzing individual stages of the deposition process and have required the introduction of a number of assumptions at their limits. This paper describes a complete CFD model that, starting from particles in the deposition head, simulates all interactions that govern the dynamics of a deposition melt pool. Individual phenomena that are included in the gasphase stage of the model include the ricocheting of particles within the head, the flow of powder particles, their interaction with the laser and powder catchment/bouncing. Phenomena in the liquid phase (melt pool) stage of the model include particle enthalpy effects, buoyancy, temperature-dependant material properties and Marangoni forces. The model is demonstrated using the actual geometry and gas flows found in a typical coaxial nozzle. The method, using a single technique to capture all phenomena, allows simulation of the melt pool dynamics from input parameters in a single model.

Following recycling efforts to save cost, energy and ecological damage, use of metal scrap is gaining importance nowadays. This necessitates looking into the possibility of using machining waste (chips/swarf) in place of costly raw metal powders for laser direct metal deposition. This paper illustrates the viability of laser metal deposition of carbon steel machining swarf with non standard particle size and shape. Particles of unprecedented size and shape have been deposited and the feasibility of using this kind of build material has been established following analysis of different outcome parameters including clad geometry, microstructure and other physical properties.

Laser beam forming has emerged as a new and very promising technique to form sheet metal by thermal residual stresses. One of the advantages of laser beam forming compared to conventional flame bending is its ability to accurately control the size and geometry of the heat source. In the area of optics, many studies explain the methods and design approaches to produce a variety of beam shapes such as line, rectangular, star, D-shape, annular, cross, etc. The objectives of this work are to compare the orientation effect of the rectangular beam, and to compare circular and rectangular beam geometries, for laser bending of AISI 304 stainless steel sheets. To achieve this, a high power diode laser (HPDL) is used to obtain bending angle for sheets of various thicknesses. From the study of the orientation of the rectangular beam, it is found that a narrow beam orientation produces higher bending angle. This is due to higher temperatures generated because of longer beam-material interaction. Comparing the circular beam with the rectangular beam, for the same average beam intensity, the circular beam produces higher bending angle.

In certain laser welding applications, it is desirable to achieve net-shape (weld bead flat to the parent material surface) welding and minimum geometry distortion. In this paper, an investigation is reported on factors affecting weld bead geometry and feasibility of achieving net shape welding for bead-on-plate welding of mild steel sheets with a 1 kW single mode fibre laser. Design of Experiments (DoE) and statistical modelling were used in the experimental study to understand parameter interactions. The selected independent process variables explored in this study are laser power and welding speed. The weld bead geometry characteristics were analysed and the conditions to achieve net-shape welding were determined.

Laser direct metal deposition has wide applications in the areas of rapid manufacturing, surface coating and component repair. Defects of interlayer and intralayer porosity are often observed in laser deposited structures and repaired components. Interlayer porosity can be controlled to some extent by adjusting processing parameters, but there is still disagreement as to the source of intralayer porosity and whether processing conditions, parameters or initial powder materials are the dominant causal factors. In this work, two samples of Ti-6Al-4V powder, prepared using gas-atomization and plasma-atomization, were analyzed using laser diffraction and micro computed tomography with the Xradia Micro XCT machine for any initial porosity content. A 1.5 kW diode laser with a coaxial deposition head was then used to deposit a number of thin-wall structures with the different powders at different processing parameters. The deposited structures were characterized using scanning electron microscopy (SEM) and optical microscopy. The results show the effects of the processing conditions and initial powder characteristics on intralayer porosity and final deposit properties.

It is widely recognized that layered manufacturing techniques have the potential to make an important contribution in medicine. One of the most exciting areas of their application is the production of the bone-interfacing implants currently used in orthopaedics and dentistry. This article studies the laser sintering of a mixture of Calcium Polyphosphate (CPP) and Polyvinyl Alcohol (PVA) to make structures with possible applications in such areas. An experimental study of the creation of the green part is performed to identify the effects of different process parameters such as laser power and scanning speed. The work indicated that the CPP-PVA sintered layer thickness can be adjusted by controlling the laser sintering process parameters. The fabricated samples can be either utilized as composite bio-structures or considered as green parts for post-densification through conventional sintering.

Multiple analytical models of the laser metal deposition process have been presented, but most rely on sequential solution of the energy and mass balance equations presented. Discretization of the problem domain using finite element or similar code is typically used where a fully coupled mass-energy balance solution is required, but the multiphase nature of the process means it is difficult to establish an easily applied model of track geometry via this method. In this work a coupled analytical solution is presented. Sub-models of the powder stream, quasi-stationary conduction in the substrate, and powder assimilation into the area of the substrate above the liquidus temperature, allowing for surface geometry factors, are combined. An iterative feedback loop is used to ensure mass and energy balances are maintained at the melt pool. The model predictions show good agreement with experimental melt pool temperatures and Ti-6Al-4V deposited single tracks, produced with a coaxial nozzle and a Laserline 1.5 kW diode laser. The model is a useful industrial aid and alternative to finite element methods for selecting the parameters to use for laser direct metal deposition.

Welding had traditionally faced two major challenges - obtaining a flawless bond (high internal weld quality) and obtaining a smooth surface transition (high external weld quality). While the former has been, and is being, well researched, the latter has received much less attention, despite the fact that post-weld surface finishing treatments such as machining and grinding can be costly or sometimes impossible. This paper presents an investigation of the production of welds with extremely high external weld quality - Net Shape Welds. A fibre laser was used to perform bead-on-plate welding of mild steel plates at a range of parameters. The quality of the welds was studied and the combinations of parameters that led to the transition to net shape welding were identified. The results show the significant factors controlling external weld quality and help identify a process window for producing net shape welds. The work can open the door to the use of fibre laser welding in many novel applications, such as precision welding technology, and complex welding, with potential benefit to a range of industries.

During laser cleaving of brittle materials, with the controlled fracture technique, thermal stresses are generated which induce the crack and extend it along the cutting path subsequently causing material separation. One of the problems in laser cutting of glass with this technique is the cut path deviation at the leading and the trailing edges of the glass sheet. Previous work with a continuous beam diode laser has shown the deviation to be partly due the high magnitudes of thermal stresses generated near the edges of the sheet. This paper reports on the effects of using a pulsed diode laser to cut soda lime glass. The effect of pulse parameters and cutting speed on the quality output variables such as cut deviation angle and surface finish are studied. Finite element modelling is also used to simulate the effects of the moving beam on stress generations to facilitate the understanding of the process mechanisms and the results are compared with the experimental data. This work shows how to minimise the cut path deviation at the edges by reducing thermal stresses using optimum pulsed diode laser parameters and providing additional flexibility to the process.

The use of porous engineered materials is gaining popularity in biomedical implant manufacture due to its capability to promote increased osseointegration and cell proliferation. A method to produce such parts is laser direct metal deposition (LDMD) - a rapid manufacturing technique that has much flexibility. In this work, a 1.5 kW high power diode laser with coaxial powder feed nozzle has been used to manufacture a number of porous multilayer structures from titanium alloy, Ti-6Al-4V. It is found that by optimizing the deposited track geometry and track offset distance, the overall porosity of the structure and pore size can be controlled. Micro computed topography (MicroCT) has been used to examine the internal morphology of the porous structures. An analytical model for direct laser deposition to account for the interaction of multiple tracks in a porous structure has also been developed and is shown to realistically model the multiple layer deposition process.

This study considers welding of Ti-6Al-4V, which is widely used in the chemical, automotive, aerospace and medical industries due to its high strength-to-weight ratio and corrosion resistance. Bead-on-plate melt on 2.15 mm thick plates with a 1 kW single mode fibre laser and a continuous wave 4 kW Nd:YAG laser are investigated and compared. The fibre laser was found to produce narrower welds with finer microstructure using less than 30% the laser power required by the Nd:YAG laser. However, levels of porosity were generally higher meaning the Nd:YAG laser welds were classified as higher quality according to American Welding Standard D17.1:2007. In both cases there was a trend for lower porosity at a higher laser power when the welding speed was higher. The difference in porosity levels may have been partly or wholly due to the different inert gas shielding systems used.

Laser Direct Metal Deposition, alloying and similar additive processes are recognised as difficult to model because of the complicated mass and heat flow. This paper compares analytical and finite element approaches via a case study of the coaxial laser direct metal deposition of Inconel 718. The analytical models used are based on superposition of the quasi-stationary temperature fields of multiple moving heat sources, modified to account for the incoming mass. The finite element models used are based on element birth methods and utilize the anisotropic enhanced thermal conductivity method. The simulations are compared with experimentally results and results show that both methods can predict depth more accurately than width. The analytical method generally underestimates width and the finite element method overestimates it. The model inaccuracies can be explained in part by considering the increased effective conductivity within the melt pool.

Metal powder delivery systems have been used widely in Direct Laser Deposition (DLD) processes. Most current commercial DLD systems use a gas to carry the metallic powders to the point of delivery through a feed tube and then a nozzle. The major drawback of this method is that wastage of the expensive powder is very high. To overcome or mitigate this process deficiency, a gas-free Powder Delivery System (PDS) has been developed. The system uses sonic/ultrasonic vibration to exert a distributed driving force on the powder and assist its deliver to the laser generated melt pool. Three different PDS configurations (off-axial, coaxial and multiple-stream) were designed and evaluated. Experimental tests showed that the powder flow rate from these nozzles is both highly stable and has fast dynamic response to the vibration. Trials were carried out with a 1.5 kW diode laser and a 1 kW single mode fibre laser to deposit copper, steel and Inconel 718. They showed that 100% deposition efficiency can be achieved by using the developed gas-free PDS. The deposition qualities in term of surface roughness, microstructure and porosity are studied and compare favourably with the results produced by a conventional gas feed system operating at the same parameters.

The potential of engineered porous materials for aiding the osseointegration of metallic dental and orthopedic implants is well established. A range of techniques, such as powder metallurgy, plasma-spraying and titanium fiber meshing have been tried to produce the implant structures with control on porosity and other pertinent characteristics. In the present work, direct laser deposition of pure titanium powder (particle size: -100/+63 microns) was carried out to form porous structure. A 1 kW fiber laser, integrated with lateral powder feed nozzle and workstation, was used at different primary process parameters (laser power, powder mass flow rate, transverse speed, hatch parameter) to fabricate a number of multi-layer structures. The relevant structural parameters of these fabricated structures were studied as a function of the process parameters using various techniques. The result of these studies are presented.

Analytical modelling of a quasi-stationary laser melt pool without mass addition can be achieved using relatively simple moving surface heat flux solutions. However, including mass addition from a coaxial powder stream alters the laser flux and energy and mass flow pathways and often leads to the problem being modelled using numerical methods. The model described in this paper combines an analytical beam attenuation model to account for beam powder interaction above the melt pool with series of standard solutions for a moving Gaussian heat source to calculate melt pool size and substrate isotherms. A negative enthalpy method is used to compensate for the mass addition to the melt pool. The model is verified using a variety of methods and can predict powder stream mass and temperature distribution at the substrate and final melt pool shape in three dimensions from the major laser direct metal deposition process variables. The model highlights the role of beam-powder interaction in the process.

It is well established that the Marangoni flow dominated circulation within a laser melt pool significantly modifies the pool profile and temperature distribution. Detailed computational fluid dynamics models are required to accurately predict this but these are complicated and computationally expensive. Many researchers have in the past used an enhanced thermal conductivity approach, but the validity of this approach for accurately predicting the melt pool geometry and temperature distribution is largely unproven. This paper presents an analysis of the widely-used isotropic enhanced thermal conductivity approach and compares it with a more advanced anisotropic approach for modelling the laser melting of Inconel 718. Experimental and modelled results for the geometry of a melt pool created by a moving laser beam are compared. It is found that the conventional enhanced thermal conductivity approach does not change the melt pool size and shape; it only reduces the maximum surface temperature. The anisotropic enhanced thermal conductivity approach on the other hand is able to modify the melt pool size and geometry and yields a better agreement with the experimental results.

During layered manufacturing of functional metallic parts by laser direct metal deposition (LDMD) residual stresses are generated by the transient thermal fields. Various methods to reduce or control the development of these stresses have been previously attempted. This paper reports a novel method using the pulse parameters of a diode laser deposition system as a tool for residual stress control. Stresses in LDMD manufactured Waspaloy blocks are calculated from strain measurements in three directions carried out at the neutron diffraction beam line SALSA at the European ILL facility, Grenoble, France. The results show tensile stresses close to the top of the structures of similar magnitude for all parameters and large compressive stresses normal to the deposition surface close to the base.

This work describes a study of laser direct metal deposition (LDMD), using a diode laser in pulsed (discontinuous beam) mode and a coaxial powder nozzle. An experimental study analyses the effect of pulse parameters on the microstructure of thin-wall Waspaloy (nickel superalloy) parts and a simple model is used to explain some of the trends seen. Findings indicate that varying pulse parameters is potentially a useful way of controlling laser deposited superalloy part properties.

Laser direct metal deposition has the potential to increase the material efficiency during the manufacture of superalloy aerospace components and also to reduce the need for welded joints to produce the necessary geometries. However, a possible problem is inconsistent microstructure, and therefore physical properties, in the deposited part. In this work, the factors that affect microstructure formation and evolution were examined in simple Waspaloy parts deposited using a high power diode laser and coaxial powder nozzle across a range of process parameters. Thermal imaging and embedded thermocouples were used to record the thermal cycle during deposition at different positions within the part and this is correlated with the wall characteristics and the final microstructures formed. Results allow the extent of the variations in sample properties with process parameters and position to be determined and indicate that intra melt pool factors such as local fluctuations in temperature gradients due to Marangoni flow and changes in nucleation density due to injected powder are significant.

The residual stress fields generated within a laser deposition metal component or added feature have the potential to significantly affect its properties and performance. Stresses play a major role in determining the initial levels of distortion, in both the added part and the initial substrate material, and affect both short-time properties, such as ductility, and long-time properties, such as creep and corrosion resistance. Their control is therefore essential for the production quality demanded in high performance aerospace components. In this work, a series of geometrically simple components was additively manufactured from Inconel 718 powder using a coaxial diode laser deposition system under a range of process parameters. The residual surface stresses arising were measured using x-ray diffraction techniques and the variations in them with surface position and process parameters are shown. The formation of these stresses and their relationship to Direct Laser Deposition process parameters are discussed.