Study on Densification of Laser Ignited Reaction Sintering of Ni-Al-Cu Powder

The mechanisms of densification during laser ignited reaction sintering of Ni-Al-Cu powder materials were studied. With the addition of Cu, densification of Ni-Al-Cu specimens made of big particles was realized under laser rapid ignition and sintering without signification change of the final products. Filling of the mixture of liquid and disintegrated small grains or particles was regarded as the dominant mechanism for the final densification.


Introduction
Combustion synthesis or self-propagating high temperature synthesis (SHS) provides an attractive practical alternative to the conventional methods for producing advanced materials, such as ceramics, ceramic-composites and intermetallic compounds, since SHS offers advantages with respect to process economics and process simplicity [1,2].Powder compact laser sintering is another laser sintering process in which powder compacts are sintered by а laser [3][4][5][6].Compared with conventional sintering processing, the most obvious advantages of combustion synthesis are [1]: (1) the generation of a high reaction temperature which can volatilize low boiling point impurities and, therefore, result in higher purity products; (2) the simple exothermic nature of the SHS reaction avoids the need for expensive processing facilities and equipment; (3) the short exothermic reaction times result in low operating and processing costs;(4) the high thermal gradients and rapid cooling rates can give rise to new non-equilibrium or metastable phases; (5) inorganic materials can be synthesized and consolidated into a final product in one step by utilizing the chemical energy of the reactants.These advantages have intrigued researchers to become more active in exploring the combustion synthesis of new and improved materials with specialized mechanical, electrical, optical and chemical properties.However, it is difficult to obtain density higher than 50% using combustion synthesis [7].
Several studies on laser sintering (LS) of some Fe-Cu and Cu-Sn powder metallurgy (PM) parts have been conducted in recent years, and some results were reported [5,6].The purpose of this paper is to study the densification of Ni-Al-Cu powder alloy obtained by laser sintering.

Experimental Procedures
Samples were prepared from mixtures of elemental Ni and Al powders according to their atomic 1:1 composition.Cu powders used were used in the proposed proportion of 0-20wt%.Both the Ni and Cu powders were -200 mesh and Al powder was -300 mesh.The homogeneously mixed powders were compacted into cylindrical samples with a diameter of 18mm and a height of 10mm.A high energy CO 2 laser generator was employed to ignite the reaction.The rectangle spot of the laser beam was adjusted to about 16mm×22mm, nearly covering the top surface of the sample.The output laser power was 1600W.In order to increase absorption of the laser power, the top surface of the samples was pre-coated with a very thin carbon paste.Temperature was measured using Pt/Pt-Rh thermocouples.The laser irradiation stopped as soon as a flame was observed because the reaction could be selfsustaining.The ignition time or delay time was defined as a period of time from starting laser irradiation to flame production.
Microstructures and phase analysis were examined by electron microscope (SEM) (Model JSM-5310, Japan, equipped with EDS).X-ray diffraction was carried out on a diffractometer, model D/2500PC Rigaku.The microhardness of laser treated zone was measured by an HX-1000 type micro Vickers loaded at 200g and loading time set at 15s.The given values of hardness were average values taken from five to eight measurement points at the same depth along the cross-sectional plane of the sample.

Temperature profile
Typical temperature of the Ni-Al-3wt%Cu compacts was plotted in Fig. 1 as a function of time for a given laser power.The ignition time is very short 1-3s.The temperature rises rapidly after ignition.The average heating rate is estimated to be up to 25°C/s to 30°C /s according to Fig. 1 (on account of the temperature gradient between the top and the bottom), higher than that of conventional sintering, thus providing insufficient time for solid particles to build bridges.
The compacts achieve maximum temperature 1480°C; the continuance time is about 20s at the Cu melting point (1083°C).So the ignition process is a transitory liquid sintering process.Where s ρ is the sintered density of the compact, g ρ is the green density of the compact, t ρ

Influence of Cu content on properties
is the theoretical density of the alloy, ϕ is the densification parameter, and ε is the relative density.Parameters ϕ and ε can be employed to evaluate densification of sintered samples.It is more objective to use the densification parameter to express densification than to use the sintered density because the effects of the constituent content on the theoretical density and pressed density are taken into consideration.The effect of Cu content on densification is indicated in Fig. 2. It is obvious that the densification parameter ϕ and the relative density ε increase with increasing Cu content.Fig. 3 shows the influence of Cu content on microhardness.With the increase of Cu content, the hardness increases.Liquid Cu can flow into the pores at the temperature over Cu melting point and at the same time small solid particles can be also move with liquid Cu to fill the relatively large pores, leading to the occurrence of the densification.Hardening is mainly dependent upon the increase of the sintered density and the effect of the second phase precipitated on the matrix [8].In this study, the reason for the enhanced hardness of materials with higher Cu content can be attributed to the enhanced density and the synthesized NiAl phase confirmed by X-ray examination.

Effect of the Cu content on the microstructure of the samples
Fig. 4 shows the typical pore morphologies in laser-sintered samples.These pores are characterized by a dark-net structure and dark spot, respectively.It can be seen that the amount and size of pores reduce as the Cu content increases.5 shows typical morphologies of laser-sintered samples.Two kinds of structure can be found in these figures.One is dentrite-like crystal grains which make up the main part of sintered samples and the other is a net-like structure with a light color.As can be seen, the amount of the net-like structure increases with increasing Cu content.When the Cu content rise to 15wt% or 20wt%, a large amount of net-like structure can be found as shown in Fig. 5c  and d.The reason for the formation of the net-like structure distributed on the grain boundary may be the result of Cu flowing into pores during sintering.2) the dissolution and precipitation stage [9].Particle rearrangement is prerequisite for effective densification [10].It is believed that without rearrangement, no considerable shrinkage can be obtained.Laser ignited reaction sintering has a very fast heating rate and particle rearrangement can be completed within less than 1 minute, because there is no sufficient time for solid particles to build bridges.The time is too short to form solid bridges between the particles during short sintering time.The Ni and Al particles were almost exhausted in the fast laser ignited reaction sintering.So the amount of Cu liquid phase will affect further progress.
As shown in Fig. 1, the fast reactive heating rate and sintering at the temperature of 1480°C favors Cu liquid penetration so densification of the sintered density takes place during sintering.It is possible for Cu particles to melt and quickly flow along the particle boundaries or into the pores at such a high temperature.

Conclusions
Reaction laser sintering of Ni-Al-Cu compacts is automatically maintained by chemical reaction heat.The densities of the sintered samples increase with increasing Cu content, and the relative density is improved.The densification of the sintered samples increases with the increase of Cu content.

Fig. 1
Fig. 1 Temperature variation of Ni-Al-3wt%Cu during laser ignition sintering, the measured point is about 3mm above the bottom surface.

Fig. 2
Fig.2 shows the properties of various samples.The variation in sintered densities and densification parameter for sintered samples of the sintered Ni-Al-Cu alloys is shown in Fig.2.

Fig. 2
Fig. 2 Influence of Cu content on densification parameter (φ/ %) and relative density (ε/ %) of Ni-Al-CuIn order to assess densification better, two parameters, ϕ and ε, are introduced and defined as follows:

Fig. 3
Fig. 3 Microhardness of Ni-Al-Cu synthesized samples with different Cu contents.

Fig.
Fig.5shows typical morphologies of laser-sintered samples.Two kinds of structure can be found in these figures.One is dentrite-like crystal grains which make up the main part of sintered samples and the other is a net-like structure with a light color.As can be seen, the amount of the net-like structure increases with increasing Cu content.When the Cu content rise to 15wt% or 20wt%, a large amount of net-like structure can be found as shown in Fig.5c and d.The reason for the formation of the net-like structure distributed on the grain boundary may be the result of Cu flowing into pores during sintering.

Fig. 6
Fig. 6 Distribution of Al, Ni and Cu in the sample with a Ni-Al-15wt%Cu composition.

Fig. 7
Fig. 7 XRD results of Ni-Al-Cu compacts with different Cu content after laser induced synthesis and air-cooling.

Fig. 8
Fig.8 shows morphologies of the sintered samples with composition Ni-Al-12wt%Cu and Ni-Al-20wt%Cu, respectively.The grains have equal axis size with a rather regular morphology, especially for the Ni-Al-20wt%Cu sample; the grain size is approximately 15µm less than the original powder size.