Nils Ellendt

Porosity in spray-formed materials

Porosity in spray-formed materials is an important issue, but the formation of porosity is not completely understood. Many experimental results and some theoretical models have been presented in the past. Nevertheless, the prediction of porosity in a deposit is still not possible today. The paper will give some examples picked from literature, which show some general correlations between process parameters and porosity. These correlations can be helpful to form a basic understanding of the process. Finally it is necessary to know more about the conditions of the droplets and the deposit at the point of impingement. These impacting conditions have to be correlated to the porosity to improve the understanding of the process and to make a prediction possible. Determining the impact conditions is a challenge because usually they are not constant with time and some values are difficult to measure. Our experiments show a strong correlation between the surface temperature of the deposit and the porosity. For IN718 and U720 as-sprayed porosities below 1 vol.% were achieved if the deposit surface temperature is app. 1250 °C. The average impact angle weighted by the local particle mass flux is also an important parameter. The probability of low as-sprayed porosity is high if the average weighted impact angle is below 25° but decreases dramatically for higher impact angles.

Solidification Sequence of Spray-Formed Steels

Solidification in spray-forming is still an open discussion in the atomization and deposition area. This paper proposes a solidification model based on the equilibrium solidification path of alloys. The main assumptions of the model are that the deposition zone temperature must be above the alloy’s solidus temperature and that the equilibrium liquid fraction at this temperature is reached, which involves partial remelting and/or redissolution of completely solidified droplets. When the deposition zone is cooled, solidification of the remaining liquid takes place under near equilibrium conditions. Scanning electron microscopy (SEM) and optical microscopy (OM) were used to analyze the microstructures of two different spray-formed steel grades: (1) boron modified supermartensitic stainless steel (SMSS) and (2) D2 tool steel. The microstructures were analyzed to determine the sequence of phase formation during solidification. In both cases, the solidification model proposed was validated.

Cooling conditions for the generation of bulk metallic glasses by droplet deposition

Abstract The cooling rate during material processing until glass transition rate is the key parameter for the production of bulk metallic glasses. But in the past, little attention has been paid to advanced production techniques such as deposition of molten metal sprays or spray forming, which offer elevated cooling rates. In this work, cooling conditions during spray forming were investigated due to its utmost importance for producing amorphous structures. Spray forming is treated in this work as a three step cooling process consisting of droplet flight phase, splat phase and deposit phase. All cooling steps were simulated for different droplet sizes. The surface temperature of the deposit was found to play an important role in the production of metallic glasses via spray forming. The simulation model can be used to find suitable spray conditions for the generation of bulk metallic glasses.

Spray forming of Mg2Si rich aluminum alloys

Aluminium Alloys with a 22 wt.-% Mg2Si content were spray formed. This alloy features a low density and is therefore a superior material for lightweight applications. The main problem in the spray forming of this type of alloy was the occurrence of high porosities. First process optimizations have been performed to decrease porosity under a certain level, so that it can be closed by an extrusion process.

Exploring superior structural materials using multi-objective optimization and formal techniques

The continuous progress in engineering requires increasingly powerful materials being utilized in complex constructions. Commonly used methodologies are hardly capable to explore materials with improved properties, i.e., fulfilling a certain performance profile. An alternative high throughput screening approach exists which allows to process a high number of samples while operating on micro scale. For structural materials, it is not possible to project material properties from the micro to the macro scale. Due to the complex nexuses between the process parameters (applied for sample synthesis), the high-dimensional space of the screening data and the uncertainties concerning material properties while scaling, conventional algorithms are not capable to cope with these data. Thus, this work proposes a general data flow which orchestrates multi-objective optimization approaches as well as formal techniques to handle this challenging computational task. In the end, a framework is drawn that determines the resulting material properties on a macro level by using screening data of micro samples and compares them against a performance profile. For the case that only a slight correlation exists, the framework proposes a batch of alternate process parameters which are, highly probable, leading to superior structural material.

A novel convergent–divergent annular nozzle design for close-coupled atomisation

ABSTRACT Additive manufacturing processes such as selective laser melting and electron beam melting require small particle sizes. A widely used technique to produce suitable powders is close-coupled atomisation. To further decrease the achieved particle sizes, the annular geometry of the gas nozzle is changed to a convergent–divergent (CD) profile. This novel configuration is capable of operating stably at low pressures of 0.8 MPa and above. Beyond that, the unwanted effects of lick-back are avoided. Different nozzles with conventional and convergent–divergent annular geometry have been designed based on fluid-flow calculations. The aspiration pressure was measured to determine stable process windows. Powders from a CuSn alloy were produced using cold and hot gas atomisation to show the influence on the process stability, particle size and morphology. High-speed recordings are used to investigate the process conditions of the different nozzle configurations.

Generation of small batch high quality metal powder

Abstract As part of a project to develop a small batch metal atomisation system, based on free fall atomisation and close-coupled atomisation, the influence of the process parameters on various powder quality features has been investigated, using copper–tin alloys as feedstock material. Particle size distribution, appearance of satellite particles, particle circularity and the flowability were recorded as criteria of particle quality. From the data obtained in these experiments, the effects of the main process parameters (atomisation pressure, mass melt flow, and height of the spray chamber as well as the atomiser system) have been evaluated with respect to powder quality features. The atomisation system was optimised to produce high quality powder with narrow particle distributions (d84·3/d50·3 = 1·6 for free fall) and high circularity with mass melt flows in the range of 100 kg h−1 using melt volume between 100 and 1000 mL.

Experimental Investigation and Modeling of the Specific Enthalpy Distribution in a Spray Cone

In Spray Forming, specific enthalpy is a key parameter in the deposition process as it influences the thermal condition of the impinging droplets as well as that of the deposit surface. An empirical model for the distribution of specific enthalpy in the spray cone was developed as an easy to handle alternative to numerical models with which the descriptive partial differential equations are solved numerically. The model results were compared with the experimental data to validate its applicability.

Microstructural Evolution in Spray Forming

Spray forming is a casting process in which the molten metal is directly converted to a solid bulk with unique characteristics. When processed under optimum conditions, spray formed materials typically present microstructures composed of refined polygonal (non-dendritic) grains, uniformly distributed with low levels of micro- and macro-segregation. This set of characteristics is achieved regardless of the alloy system, making spray forming an attractive process to produce alloys where processing by conventional casting techniques is complicated. This chapter is dedicated to presenting the mechanisms that take place when the atomized droplets arrive at the deposit surface, and how the spray-formed microstructures evolve during deposition. It will be seen that spray forming is a self grain-refining casting process and cannot be considered a rapid solidification technique. Section 7.6 will address the main differences between the microstructural evolution in spray forming and other spray deposition or “thermal spray” processes. These processes include plasma spraying, high velocity oxy-fuel, wire arc spraying, detonation gun spraying, etc. In this way, it will show why spray forming is such a unique process. This chapter is also dedicated to presenting how the porosity—an intrinsic feature of spray-formed microstructures—is generated and how the processing parameters affect its type, size and distribution. Furthermore, the generation of other defects related to the solidification and/or to the cooling of the spray formed product after deposition—such as residual stresses and hot cracks—and their influence on the product quality and material properties will be presented. Finally, this chapter will also discuss the effect of the atomization gas (Ar, N2 or He) on the final product quality in terms of porosity and chemical composition of steels, superalloys, and copper alloys.

Processing Aspects in Spray Forming

Several atomization techniques have been used to spray form a bulk material. Detailed information about those techniques is described in Chapter 2. This chapter focuses on differences in process conditions in spray forming using single fluid and gas atomization as well. Specially, the conditions (e.g. mass flux and enthalpy distribution) in the spray cone of a free fall atomizer are reported in detail. Furthermore, the deposition process is described including topics like overspray, yield, sticking efficiency and temperature history of the deposit. These are important issues with relevance to processes similar to spray forming, such as current thrusts in Additive Manufaturing (AM).