Láser surface alloying of aluminium-transition metal alloys

Láser surface alloying has been used as a tool to produce hard and corrosión resistant Al-transition metal (TM) alloys. Cr and Mo are particularly interesting alloying elements to produce stable highstrength alloys because they present low diffusion coefficients and solid solubility in Al. To produce Al-TM surface alloys a two-step láser process was developed: firstly, the material is alloyed using low scanning speed and secondly, the microstructure is modified by a refinement step. This process was used in the production of Al-Cr, Al-Mo and Al-Nb surface alloys by alloying Cr, Mo or Nb powder into an Al and 7175 Al alloy substrate using a C02 láser. This paper presents a review of the work that has been developed at Instituto Superior Técnico on láser alloying of Al-TM alloys, over the last years.


INTRODUCTION
High-strength aluminium alloys are widely used for structural applications in high-performance automobiles, railway cars, airplanes and spacecraft, light ships, etc., owing mainly to their excellent mechanical strength, low specific weight, good formability and relatively low cost.However, the widespread use of these materials has been limited by their low resistance to corrosión and wear.Rapid solidification processing has played a major role in recent developments of aluminium alloys, since it promotes a general refinement of the microstructure, extensión of solubility of critical alloying elements in the oc-aluminium solid solution and the formation of useful metastable phases, including metallic glasses and quasicrystals (1).Since these structural modifications are often responsible for an increase in the wear and corrosión resistance of aluminium alloys, rapid solidification is being seriously considered for the preparation of high performance alloys.Amongst rapid solidification techniques, láser surface alloying (LSA) is particularly efficient at producing surface layers with improved wear and corrosión resistance, since it allows to modify the chemical composition and the microstructure to adapt surface properties to service requirements (2)(3)(4).
This paper summarizes results of a study that aims to evalúate láser surface alloying as a tool to produce corrosión resistant and hard aluminiumtransition metal alloys.Chromium and molybdenum were chosen because they exhibit a tendency to form supersaturated solid solutions (5)(6)(7)(8), leading to improved corrosión, and present low diffusion coefficients and solid solubility in Al, therefore, enabling the formation of AlxTMy particles dispersed in an a-Al solid solution matrix that are inherently stable.Niobium is also a very promising alloying element to aluminium, since it leads to the formation of the hard intermetallic compound Al 3 Nb, even for small concentrations (9).The substrates used in this work were commercial puré aluminium and an Al-Zn-Mg alloy of the 7000 series.The latter alloy was chosen because it is widely used in the aircraft industry but is prone to localized corrosión.Furthermore, the 7000 series alloys are particularly suitable for láser treatment, owing to their reduced tendency to overage and the possibility of strengthening by natural aging (10).

EXPERIMENTAL METHOD
Láser processing was performed in two steps: first, láser surface alloying (LSA) of the Al-25 % (Cr, Mo or Nb) powder mixtures into puré Al and ANSÍ 7175 alloy substrates, and second, láser surface remelting (LSR) the alloyed surfaces in a direction normal to the alloying direction.The alloying treatments were carried out with a C0 2 láser, using a power density and an interaction time of 1.5 x 105 W/cm 2 and 0.26 s, respectively.Remelting was performed using the same láser power and interaction times in the range 0.03-0.26s.To reduce moisture the Al substrates were cleaned by sand blasting and the alloying powder was dried in an oven at 70 °C for 3 hours just before the experiments.
The láser treated samples were cross-sectioned and cut longitudinally for observation.Metallographic samples were prepared using standard techniques and etched with Keller's reagent.The microstructure was characterized by optical, scanning and transmission electrón microscopy.The distribution and morphology of the intermetallic compounds were studied by SEM, using backscattered electrón images of unetched samples.The phases present were identified by X-ray diffraction and electrón diffraction and chemical analysis was performed using electrón probé microanalysis (EPMA).Microhardness measurements were made with a Vickers indenter, under a 100 g load.Each hardness valué reported is the average of ten measurements.
The corrosión resistance of the samples láser alloyed with Cr and Mo and the substrates was evaluated by anodic polarization tests, using a 3 % NaCl deaerated solution.The polarization tests were performed in a three-electrode cell.The work electrode was the specimen under study, the refe-rence electrode was a saturated calomel electrode and the auxiliary electrode was platinum foil.

General features of alloys
After LSA many Cr, Mo and Nb particles remain undissolved (partially or totally) (fig.1).This behavior is explained by the difference in melting point between these TM and aluminium and by their low solubility in molten aluminium.The partially dissolved particles are surrounded by a TMrich layer of intermetallic compounds.Cracks often nucleate in this layer and propágate across the material.These cracks result from the difference between the thermal expansión coefficients of oc-aluminium solid solution and of the intermetallic compounds.The láser alloyed layers are chemically and structurally heterogeneous, showing zones enriched in TM and zones depleted in these elements.Moreover, they present considerable porosity (fig. 1) that is due to the reléase of hydrogen during solidification.To eliminate porosity and homogenize the surface layers the samples were láser remelted.
After remelting the structure of the alloyed layers is homogenized and the pores and undissolved particles are eliminated.In fig. 2 a cross-section of an Al-Cr alloy typical of the alloys produced in this work is depicted.The alloyed layer (A) and the remelted layer (R) can be observed.The depth of the remelted layers increases with increasing láser power and decreasing scanning speed (fig.3).When the remelted layer is thinner than the alloyed one, some pores remain in the non-remelted material.The pores can be completely eliminated if an appropriate choice of processing parameters is made.
The examination of the cross-section of Al-Nb alloys (fig.4) reveáis a strong segregation of niobium leading to the formation of two different zones: a top layer, about 700 |am thick, where the niobium concentrated forming a niobium-rich alloy (A), and a bottom layer that is formed essentially by resolidified aluminium solid solution (B).
The average chemical composition of the alloyed and remelted layers determined by electrón probé microanalysis is depicted in table I.In 7175 alloy there are significant losses of Mg and Zn by vaporization (about 50 %), which increase with decreasing scanning speed and with increasing láser power (2).The losses of alloying elements are more significant in the alloyed and remelted layers than in the layers that were only alloyed, due to the fact that the material was melted twice.

Microstructure
The stmcture of the Cr and Mo-alloyed layers (A) consists of intermetallic compounds in a matrix of a-aluminium solid solution.In Al-Cr and 7175-Cr alloys these intermetallic compounds appear with two different morphologies: isolated blocky particles and long faceted needles, a microstructure similar to that reported by Luft et al. (11) for an Al-Cr alloy submitted to láser surface melting.In the remelted layer (R), if the solidification rate during remelting is similar to that used for alloying there is no significant change in the morphology of the intermetallics; they form a dense network of needles, but finer than in the alloyed layer.However, for higher solidification rates during remelting the microstructure becomes very fine and its morphology changes drastically.The stmcture presents a cellular equiaxed morphology, with 6-A17Cr, r|-Al 11 Cr 2 and 8-Al 4 C intermetallics organized radially around  primary cubic particles (fig.5).The solidification path of these alloys was discussed in detail in previous papers (2,12).The solubility of Cr in aluminium is increased to 0.6 at.% after the láser treatment.
In the Al-Mo alloys the intermetallic compounds appear with different morphologies: isolated blocky particles and long faceted needles (or plates).After remelting, the intermetallic compounds are very homogeneously distributed and present a dendritic morphology (fig.6).The average size of their particles decreases with increasing scanning speed.The only intermetallic compound found by X-ray diffraction was Al 5 Mo.However, this intermetallic is observed in different allotropic forms, depending on its temperature of formation (13), and all these forms seem to be present in the samples studied here.The Mo content in the matrix is 0.9 at.%.
In the Nb alloys the structure of the top niobium-rich zone (A) consists of dendrites of an intermetallic compound and a-Al solid solution in the FIG.interdendritic regions.The intermetallic compound was identified as Al 3 Nb by X-ray diffraction.The constitution of this Nb-rich layer is not modified by remelting but its structure is refined (fig.7).A comparison of the relative intensity of the aluminium and Al 3 Nb peaks in X-ray diffractograms obtained for alloyed and alloyed and remelted samples shows that remelting produces a decrease in the volume fraction of a-Al solid solution.This effect is probably due to the complete melting of niobium particles that were only partially dissolved during alloying and subsequent incorporation of niobium in the surface layer resulting from the remelting treatment.The microstructures observed are compatible with the Al-Nb phase diagram but it is not clear that a peritectic reaction has occurred, as predicted by that phase diagram.

Hardness
The hardness of the alloys is higher in the remelted layers than in the alloyed ones and it varíes in the ranges: 110-220 HV for Al-Cr, 170-320 HV for 7175-Cr, 65-350 HV Al-Mo and 500 to 650 HV for Al-Nb alloys.The increase of hardness depends on the content of alloying element in the alloy and also on the remelting speed (fig.8).Alloys with higher content of alloying element present a higher fraction of intermetallic compounds and therefore a higher hardness.For alloys of similar composition the hardness increases with the remelting speed.Higher scanning speeds lead to finer particles/dendrites and, consequently, to harder structures.The hardnesses obtained for the Cr and Mo-alloys are higher than those reported by previous authors (14), a discrepancy that is explained by the fine dispersión of intermetallics formed during remelting, since the hardness valúes obtained in the alloyed layers are similar to the ones reported by these authors.
In the Nb alloys, the high hardness valúes result from the large volume fraction of Al 3 Nb present in the structure and also from the refinement of the microstructure resulting from the remelting treatment and are similar to those found in alloys produced by láser cladding (15).Despite the brittleness of Al 3 Nb intermetallic compound (16), no cracks were observed in Al-Nb alloys and this is probably due to the existence of the thin interdendritic film of oc-Al solid solution.The aluminium solid solution is soft and ductile and contributes to a better accommodation of stresses.

Corrosión resistance
Anodic polarization curves for aluminium and 7175 alloy before and after láser alloying are presented in fig.9.The pitting potential of untreated 7175 alloy is -730 mV while in the Cr-alloyed layers it increases up to -270 mV for 12 wt.% Cr in the alloyed layer.These results show that láser surface alloying improves the pitting resistance of the Cr-alloyed layers.This improvement depends on the chromium content in the láser alloyed layer and is more significant for 7175 alloy than for Al.The higher the amount of chromium in the alloyed layer, the higher the pitting potential is, since the formation of CrOOH and/or Cr 2 0 3 enhances the passive film formed.This film is less soluble in acid Solutions than the film containing only aluminium oxide and thus a higher potential is needed to propágate the pits.The pitting potential of the molybdenum-alloyed layers is only slightly enhanced (about 35-40 mV).This improvement is due to the formation of a passive film containing Mo0 2 that is less soluble in acid solutions than the film containing only aluminium oxide.However, this enhancement is less significant than the one observed for samples alloyed with chromium.
The results obtained suggest that alloying with chromium is interesting for corrosión applications, while all three alloying elements tested may find application to increase the wear resistance of aluminium alloys.On the basis of the higher hardness valúes obtained and the absence of cracks, niobium is probably the most promising alloying element for applications where wear resistance is critical.Furthermore, since the increase in hardness observed in these alloys is due to the formation of stable intermetallic compounds that contain an element with a low diffusion coefficient in aluminium, they should not be seriously affected by heating to moderately high temperatures.To investígate its potential, abrasive wear tests are being carried out.

CONCLUSIONS
1. Láser alloying with Cr, Mo or Nb powder produces very hard surface layers on aluminium and 7175 high strength alloy substrates.However, the distribution of alloying element is not homogeneous and the alloyed layers present defects like undissolved particles and pores.2. The structural integrity of the surface layers is largely improved by láser remelting: most defects are eliminated, the material is homogenized and a fine and dense dispersión of intermetallic compound particles forms.3. The structure consists of idiomorphic particles of intermetallics in a matrix of oc-aluminium solid solution for Al-Cr and Al-Mo alloys and dendrites of Al 3 Nb intermetallic compound with interdendritic oc-aluminium solid solution in Al-Nb alloys.4. The láser alloyed layers present very high hardness as compared to bulk materials produced by other techniques due to the formation of fine and dense distribution of intermetallic compounds.5.The corrosión resistance of 7175 alloy and aluminium is significantly enhanced by LSA with Cr while there is only a slight improvement in the Al-Mo layers.