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Selective laser melting (SLM) is a powder metallurgical (PM) additive manufacturing process whereby a three‐dimensional part is built in a layer‐wise manner. During the process, a high intensity laser beam selectively scans a powder bed according to the computer‐aided design data of the part to be produced and the powder metal particles are completely molten. The process is capable of producing near full density (∼98‐99 per cent relative density) and functional metallic parts with a high geometrical freedom. However, insufficient surface quality of produced parts is one of the important limitations of the process. The purpose of this study is to apply laser re‐melting using a continuous wave laser during SLM production of 316L stainless steel and Ti6Al4V parts to overcome this limitation.

After each layer is fully molten, the same slice data are used to re‐expose the layer for laser re‐melting. In this manner, laser re‐melting does not only improve the surface quality on the top surfaces, but also has the potential to change the microstructure and to improve the obtained density. The influence of laser re‐melting on the surface quality, density and microstructure is studied varying the operating parameters for re‐melting such as scan speed, laser power and scan spacing.

It is concluded that laser re‐melting is a promising method to enhance the density and surface quality of SLM parts at a cost of longer production times. Laser re‐melting improves the density to almost 100 per cent whereas 90 per cent enhancement is achieved in the surface quality of SLM parts after laser re‐melting. The microhardness is improved in the laser re‐molten zone if sufficiently high‐energy densities are provided, probably due to a fine‐cell size encountered in the microstructure.

There has been extensive research in the field of laser surface modification techniques, e.g. laser polishing, laser hardening and laser surface melting, applied to bulk materials produced by conventional manufacturing processes. However, those studies only relate to laser enhancement of surface or sub‐surface properties of parts produced using bulk material. They do not aim at enhancement of core material properties, nor surface enhancement of (rough) surfaces produced in a PM way by SLM. This study is carried out to cover the gap and analyze the advantages of laser re‐melting in the field of additive manufacturing.

The purpose of this paper is to visualise the activities of three solders; Sn‐37Pb, Sn‐9Zn and Sn‐3.5Ag on Cu substrates during reflow near their melting points and to relate them with reflow reactions between solder and substrate.

Melting activities of three solders near their melting points on copper substrates are visualised in an infrared reflow furnace.

Solder balls demonstrate different ways of melting and reflowing behaviours in dissimilar times and temperature intervals. Melting of Sn‐9Zn solder balls is initiated simultaneously at the surface and joint between solder balls. This is followed by the melting at the joint between solder balls and the Cu substrate. During melting, solder balls are first merged into each other and then reflow on the substrate from top to bottom. Opposite to Sn‐9Zn, Sn‐3.5Ag solder balls start to melt at the surface and the joint between the solder and substrate, simultaneously. Balls are first reflowed from top to bottom and, in the process, liquid solder is merged. Unlike Sn‐9Zn and Sn‐3.5 Ag, melting of Sn‐37Pb solder balls is initially commenced at the surface only. This is followed by simultaneous melting at both joints. Variation in melting activities of these solders is found to be closely related to the coalescence mechanism of solder balls and the reflow reactions between the solders and the Cu substrate.

The elementary melting activities of different solders on Cu substrates is related with their reflow behaviours. This provides better understanding of solder behaviour and selection of good lead‐free solder for applications in the electronic industry.

The purpose of this paper is to provide a theoretical foundation for improving the selective laser melting (SLM) surface roughness. To improve the part’s surface quality during SLM process, the upper surface roughness of SLM parts was theoretically studied and the influencing factors were analyzed through experiments.

The characteristics of single track were first investigated, and based on the analysis of single track, theoretical value of the upper surface roughness would be calculated. Two groups of cubic sample were fabricated to validate SLM parts’ surface roughness, the Ra and relative density of all the cubic parts was measured, and the difference between theoretical calculation and experiment results was studied. Then, the effect of laser energy density on surface roughness was studied. At last, the SLM part’s surface was improved by laser re-melting method. At the end of this paper, the curved surface roughness was discussed briefly.

The SLM upper surface roughness is affected by the width of track, scan space and the thickness of powder layer. Measured surface roughness Ra value was about 50 per cent greater than the theoretical value. The laser energy density has a great influence on the SLM fabrication quality. Different laser energy density corresponds to different fabricating characteristics. This study divided the SLM fabrication into not completely melting zone, balling zone in low energy density, successfully fabricating zone and excessive melting zone. The laser surface re-melting (LSR) process can improve the surface roughness of SLM parts greatly without considering the fabricating time and stress accumulation.

The upper surface roughness of SLM parts was theoretically studied, and the influencing factors were analyzed together; also, the LSR process was proven to be effective to improve the surface quality. This study provides a theoretical foundation to improve the surface quality of SLM parts to promote the popularization and application of metal additive manufacturing technology.

A transient, three‐dimensional mathematical model of a single‐pass laser surface alloying process has been developed to examine the macroscopic heat, momentum and species transport during the process. A numerical study is performed in a co‐ordinate system moving with the laser at a constant scanning speed. A fixed grid enthalpy‐porosity approach is used, which predicts the evolutionary development of the laser‐melted pool. It is observed that the melting of the added alloying element is not instantaneous in case its melting temperature is higher as compared to that of the base metal. As a result, the addition of alloying element at the top surface cannot be accurately modelled as a mass flux boundary condition at that surface. To resolve this situation, the addition of alloying elements is formulated by devising a species generation term for the solute transport equation. By employing a particle‐tracking algorithm and a simultaneous particle‐melting consideration, the species source term is estimated by the amount of fusion of a spherical particle as it passes through a particular control volume. Numerical simulations are performed for Ni as alloying element on Al base metal. It is revealed that the present model makes a distinctly different prediction of composition variation within the resolidified microstructure, as compared to a model that does not incorporate any considerations of distributed melting.

This paper aims to study the effect of processing parameters and the fundamental mechanism of surface morphologies during electron beam selective melting.

From the powder-scale level, first, the discrete element method is used to obtain the powder bed distribution that is comparable with the practical condition; then, the finite volume method is used to simulate the particle melting and flowing process. A physically reliable energy distribution of the electron beam is applied and the volume of fluid method is coupled to capture the free boundary flow. Twelve sets of parameters grouped into three categories are examined, focusing on the effect of scan speed, input powder and energy density.

According to the results, both melting pool width and depth have a positive relation with the energy density, whereas the melting pool length is insensitive to the scan velocity change. The balling effect is attributed to either an insufficient energy input or the flow instability; the hump effect originates from the mismatch between electron beam moving and the fluid flow. The scan speed is a key parameter closely related to melting pool size and surface morphologies.

Through a number of case studies, this paper gives a comprehensive insight of the parameter effects and mechanisms of different surface morphologies, which helps to better control the manufacturing quality of electron beam selective melting.

A model study of laser heating process including phase change and molten flow in the melt pool gives physical insight into the process and provides useful information on the influence of melting parameters. In addition, the predictions reduce the experimental cost and minimize the experimental time. Consequently, investigation into laser control melting of the titanium alloy becomes essential. The purpose of this paper is to do this.

Laser repetitive pulse heating of titanium surface is investigated and temperature field as well as Marangoni flow in the melt pool is predicted using finite volume approach. The influence of laser scanning speed and laser pulse parameter (defining the laser pulse intensity distribution at the workpiece surface) on temperature distribution and melt size is examined. The experiment is carried out to validate temperature predictions for two consecutive laser pulses.

The influence of laser scanning speed is significant on the melt pool geometry, which is more pronounced for the laser pulse parameter β=0. Temperature predictions agree with the thermocouple data obtained from the experiment.

Although temperature dependent properties are used in the simulations, isotropy in properties may limit the simulations. The laser canning speed is limited to 0.3 m/s, which is good for surface treatment process, but it may slow for annealing treatments.

The results are very useful to capture insight into the melting process. In addition, the influence of laser scanning speed and laser pulse intensity distribution on the melt formation in the surface vicinity is well presented, which will be useful for those working on laser surface treatment process.

The work is original and findings are new, which demonstrate the influence of laser parameters on the melt pool formation and resulting Marangoni flow.

Obtaining the required part top surface roughness and side roughness is critical in some applications. Each of these part properties can often be improved to the detriment of the other during selective laser melting (SLM). The purpose of this paper is to investigate the selective laser melting of Inconel 625 using an Nd:YAG pulsed laser to produce thin wall parts with an emphasis on attaining parts with minimum top surface and side surface roughness.

A full factorial approach was used to vary process parameters and identify a usable Inconel 625 processing region. The effects laser process parameters had on the formation of part surface roughness for multi‐layer parts were examined. Processing parameters that specifically affected top surface and side roughness were identified.

Higher peak powers tended to reduce top surface roughness and reduce side roughness as recoil pressures flatten out the melt pool and reduce balling formation by increasing wettability of the melt. Increased repetition rate and reduced scan speed reduced top surface roughness but increased side roughness. A compromise between attaining a relatively low surface roughness and side roughness can be attained by comparing part surface roughness values and understanding the factors that affect them. A sample with 9 μm top surface roughness and 10 μm side roughness was produced.

The research is the first of its kind directly processing Inconel 625 using SLM and investigating processing parameters that affect top surface and side roughness simultaneously. It is a useful aid in unveiling a relationship between process parameters and top/side roughness of thin walled parts.

This paper aims to investigate laminar boundary layer flow and heat transfer from a warm laminar Casson liquid to a melting sheet moving parallel to a melting stream. The governing equations, i.e. continuity, momentum and heat transfer, are coupled non-linear partial differential equations. These equations are reduced to non-linear ordinary differential equations by means of similarity transformations, converted into first-order differential equations, and are solved numerically using the Runge–Kutta–Felhberg method with an efficient shooting technique. The velocity and temperature profiles are plotted for various values of the governing parameters, such as the moving parameter, Prandlt number, melting parameter and Casson parameter. It is found that the problem admits multiple solutions. The results of this study are validated by comparing them with the earlier published studies’ results. Thus, a good agreement is obtained.

This study carries out numerical solution of melting heat transfer analysis.

The findings of this study show the analysis of flow and melting heat transfer characteristics.

In this study, analysis of dual solution is carried out.

In this paper, the melting heat transfer analysis on Blasius flow of a Casson fluid is taken into consideration. To the best of the author’s knowledge, no investigations have been reported on this topic.

Tombstoning and voiding have been plaguing the surface mount assembly industry for decades. The recent global move toward lead‐free soldering and the extensive adoption of microvia technology further aggravate the problems. The present study investigates the impact of SnAgCu (SAC) alloy composition on these important issues.

In this study, tombstoning and voiding at microvias are studied for a series of SAC lead‐free solders, with an attempt to identify a possible “composition window” for controlling these problems. Properties which may be related to these problems, such as alloy surface tension, alloy melting pattern, and solder wetting behaviour, were investigated in order to assess the critical characteristics required to control these problems.

The results indicate that the tombstoning of SAC alloys is greatly influenced by the solder composition. Both the wetting force and the wetting time at a temperature well above the melting point have no correlation with the tombstoning frequencies. Because the tombstoning is caused by imbalanced wetting forces, the results suggest that the tombstoning may be controlled by the wetting at the onset of the paste melting stage. A maximum tombstoning incidence was observed for the 95.5Sn3.5Ag1Cu alloy. The tombstoning rate decreased with increasing deviation in Ag content from this composition. A differential scanning calorimetry (DSC) study indicated that this was mainly due to the increasing presence of the pasty phase in the solders, which result in a slower wetting speed at the onset of solder paste melting stage. Surface tension plays a minor role, with lower surface tension correlating with a higher tombstoning rate. The voiding rate at the microvias was studied by employing simulated microvias. The voiding level was lowest for the 95.5Sn3.8Ag0.7Cu and 95.5Sn3.5Ag1Cu alloys, and increases with a further decrease in the Ag content. The results indicate that voiding at microvias is governed by the via filling and the exclusion of fluxes. The voiding rate decreased with decreasing surface tension and increasing wetting force, which in turn is dictated by the solder wetting or spreading. Both low surface tension and high solder wetting prevents the flux from being entrapped within a microvia. A fast wetting speed may also facilitate reducing voiding. However, this factor is considered not as important as the final solder coverage area.

In general, compositions which deviate from the ternary eutectic SAC in Ag content, particularly with a Ag content lower than 3.5Ag, exhibit a greater solid fraction at the onset of melting, resulting in a lower tombstoning rate, presumably due to a slower wetting speed. The SAC compositions with an Ag content lower than 3.5 per cent, such as 2.5Ag, resulted in a lower tombstoning rate with minimal risk of forming Ag₃Sn intermetallic platelets. On the other hand, ternary eutectic SAC exhibits a lower surface tension resulting in an easier solder spread or solder wetting, and consequently exhibit a higher tombstoning frequency and a lower incidence of voiding.

Provides a solution to the tombstoning problem in lead‐free soldering.

The present study provided a solution to the tombstoning problem encountered in lead free soldering by controlling the SAC solder alloy compositions.

This paper aims to provide a review on the process of additive manufacturing of ceramic materials, focusing on partial and full melting of ceramic powder by a high-energy laser beam without the use of binders.

Selective laser sintering or melting (SLS/SLM) techniques are first introduced, followed by analysis of results from silica (SiO₂), zirconia (ZrO₂) and ceramic-reinforced metal matrix composites processed by direct laser sintering and melting.

At the current state of technology, it is still a challenge to fabricate dense ceramic components directly using SLS/SLM. Critical challenges encountered during direct laser melting of ceramic will be discussed, including deposition of ceramic powder layer, interaction between laser and powder particles, dynamic melting and consolidation mechanism of the process and the presence of residual stresses in ceramics processed via SLS/SLM.

Despite the challenges, SLS/SLM still has the potential in fabrication of ceramics. Additional research is needed to understand and establish the optimal interaction between the laser beam and ceramic powder bed for full density part fabrication. Looking into the future, other melting-based techniques for ceramic and composites are presented, along with their potential applications.