Study on the microstructure and wear resistance of AlCrCuFeNiTix high entropy alloys for surfacing welding

In order to investigate the effect of Ti element content on the microstructure and wear resistance of AlCrCuFeNiTix (x = 0, 0.3, 0.6, 0.9) high entropy surfacing alloy, a melting electrode gas shielded welding technology was used to prepare AlCrCuFeNiTix high entropy surfacing alloy on the surface of carbon steel plates. The microstructure, phase composition, and wear resistance of the alloy were analyzed. The results show that the phase composition of the surfacing alloy becomes a BCC+FCC solid solution phase, with a much higher content of BCC phase than FCC phase. The microstructure consists of disordered BCC phase rich in Fe and Cr, ordered BCC phase rich in Al and Ni, and FCC phase rich in Cu. The microstructure exhibits typical dendritic (DR) and interdendritic (ID) structures. With the increase of Ti element, hard Fe2Ti phase precipitates in the interdendritic zone, and the micro hardness of the alloy shows an increasing trend. The maximum hardness can reach 636 HV, which is 2.4 times that of the base material. With the increase of Ti element, the friction coefficient of the alloy shows a trend of first decreasing and then increasing, and the wear amount first decreases and then increases. When Ti is 0.6, the wear resistance of the high entropy surfacing alloy reaches its best.


Introduction
With the development of materials, the performance requirements for alloys are also increasing.Traditional alloy designs often choose a small amount of other elements for alloying to meet application needs.As the types of elements increase, intermetallic compounds containing brittle phases will precipitate in the alloy, resulting in a serious decrease in the alloy's performance [1].In the 1990s, Taiwan, China scholar Ye Junwei et al [2] put forward the concept of high entropy alloy.By designing a reasonable alloy formula, alloy materials with various excellent properties such as high hardness, high strength, high wear resistance, and corrosion resistance can be obtained [3][4][5][6], which has received high attention from researchers in this regard.
In the study of alloys, the types and contents of elements are adjusted to make them have a wide range of microstructures and various properties.Professor Zhang Yong from Beijing University of Science and Technology [7] prepared for the first time an AlCoCrFeNiTi based high entropy alloy with a single BCC structure and conducted relevant research on the basis of composition fine-tuning; Guo et al [8] prepared 5-8 element alloys AlFeCuCoCr, AlFeCuNiCrV, AlFeCuCoNiCrTi, and AlFeCuCoNiCrTiV, and studied the effects of main elements on the microstructure and properties of high entropy alloys.The alloy structure presented a dendritic structure, and the hardness increased with the increase of main elements; Otto [9] prepared CrMnFeCoNi alloy using arc melting and cold rolling techniques, and studied the variation of its mechanical properties with temperature under low temperature conditions.
Ti atoms have a relatively large radius, and the addition of high entropy alloys can increase the lattice distortion effect.Its configuration entropy value is high, which can improve the hardness and strength of the alloy and facilitate the formation of BCC solid solutions.Therefore, high entropy alloys containing Ti have great potential for development.At present, the main method for preparing high entropy alloys is vacuum arc melting [10][11][12][13].However, the high entropy alloy elements prepared by arc melting are prone to segregation, and the alloy ingots are small, which limits their application in certain fields.Therefore, a wear-resistant high entropy surfacing alloy was developed based on the design concept of high entropy alloys and the characteristics of flux cored welding wires; Increasing the content of low-cost Fe elements can reduce the tendency to crack during the welding process and also reduce the cost of high entropy alloys in application; Surfacing manufacturing and remanufacturing technology is a fast and convenient repair process that uses gas shielded arc welding to melt alloy materials with certain properties onto the surface of the base metal, enabling the surfacing layer to achieve metallurgical bonding with the base metal, thus possessing certain special properties.In summary, the focus of this article is on a new high entropy alloy preparation method, which is to prepare AlCrCuFeNiTi x high entropy surfacing alloy through GMAW.The phase structure and microstructure of the alloy were studied, and the relationship between microstructure and wear resistance was discussed.

Preparation of the alloys
Al, Cu, Cr, Ni, Ti metal powders with a purity greater than 99.9% were selected as raw materials for the experiment, and added to steel strips in different ratios for rolling and drawing Φ2.55 mm flux cored welding wire has a powder filling rate of 35%, and the composition of the steel strip is shown in table 1. Prepare according to the ratio of AlCrCuFeNiTi x (atomic ratios of x values are 0, 0.3, 0.6, 0.9, denoted as Ti 0 , Ti 0.3 , Ti 0.6 , Ti 0.9 ).Powder needs to be dried in advance because it is highly absorbent of moisture, and the use of damp powder to produce flux cored welding wires can easily cause defects such as pores and cracks in the weld seam [14].
Then, the surface of the carbon steel substrate is polished to remove the oxide skin.After cleaning with anhydrous ethanol, it is deposited onto the surface of the carbon steel plate using a gas shielded arc welding method to obtain AlCrCuFeNiTi x high entropy alloy.Argon as protective gas, the welding process parameters [15] are shown in table 2. Cut the weld overlay sample into 10 × 10 × 10, ground and polished, and subjected to 4% nitric acid alcohol corrosion for microstructure observation.

Experimental methods
The phase structure of the surfacing layer was analyzed using x-ray diffraction (XRD).The specific parameters were: pure copper target, voltage 40 KV, current 30 mA, step size 4 degree/min, scanning range 20°to 90°.Use a scanning electron microscope (SEM) equipped with energy dispersive spectroscop (EDS) to characterize the microstructure of the alloy.A microhardness tester was used to study the microhardness of the surfacing alloy under a 500 g load for 15 s.The MFT-4000 reciprocating friction and wear testing machine was used for wear testing.The test parameters were: load 10 N, speed 120 mm min −1 , time 30 min, and reciprocating distance 7 mm.Compare the mass of the sample before and after wear using a BL410F electronic balance (1 mg).

Results and discussion
3.1.Phase structure Figure 1 shows the XRD spectrum of AlCrCuFeNiTi x (x = 0, 0.3, 0.6, 0.9, denoted as Ti 0 , Ti 0.3 , Ti 0.6 , Ti 0.9 ) based high entropy surfacing alloy.The phase group of high entropy surfacing alloy becomes the BCC+FCC solid solution phase, where the content of BCC phase is much higher than that of FCC phase.The microstructure is composed of disordered BCC phase rich in Fe and Cr, ordered BCC phase rich in Al and Ni, and FCC phase rich in Cu.With the increase of Ti element, a new phase Laves phase was precipitated in the alloy.By comparison with PDF cards, the newly precipitated phase was Fe 2 Ti.In addition, there is a small amount of TiC precipitation.The diffraction peaks of the Laves phase gradually increase, resulting in a decrease in the intensity of the diffraction peaks of the matrix BCC.

Microstructures
Figure 2 shows the microstructure of AlCrCuFeNiTi x (x = 0, 0.3, 0.6, 0.9, denoted as Ti 0 , Ti 0.3 , Ti 0.6 , Ti 0.9 ) based high entropy surfacing alloy.It can be seen that the overlay layer has a dendritic structure, with typical gray  dendritic (DR) and white interdendritic (ID) structures clearly visible in the alloy.DR is composed of Fe-Cr phase in the matrix and basket like AlNi phase, with Cu rich phase segregation and ID zone.In addition, with the increase of Ti element content, two phases precipitated in the microstructure of high entropy surfacing alloy, the first being phase A, which is dispersed and distributed in the microstructure.The second type is phase B, which is distributed along grain boundaries and transforms from striped to layered with increasing Ti content.The chemical composition of different regions was obtained by EDS, as shown in table 3.In the AlCrCuFeNiTi x high entropy surfacing alloy, dendrites (DR) are rich in Fe, Cr, Ni, and depleted Cu, while interdendritic (ID) is rich in Cu and Ti.According to EDS analysis, it can be seen that the small precipitate phase A is TiC phase, because Ti is caused by the reaction between strong carbon compound forming elements and the diffusion of carbon from the base material.The layered phase B is Fe 2 Ti.As the Ti content increases, the number of precipitated phase TiC gradually increases.Figure 3 shows the EDS element distribution of AlCrCuFeNiTi 0.9 high entropy surfacing alloy.From the figure, it can be seen that the green bright colored block like particles enriched in Ti are TiC, and the layered phase distributed in the interdendritic zone is Fe 2 Ti.Consistent with the above results.

Microhardness and wear resistance
Figure 4 shows the lateral microhardness curve of AlCrCuFeNiTi x (x = 0, 0.3, 0.6, 0.9, denoted as Ti 0 , Ti 0.3 , Ti 0.6 , Ti 0.9 ) based high entropy surfacing alloy.As the Ti content increases, the microhardness of the high entropy surfacing alloy gradually increases, with the highest microhardness reaching 642 HV, which is about 2.4 times the hardness of the matrix.The thickness of the overlay layer is approximately 3.0 mm.In addition, the hardness of the welding fusion line is smaller than the surface hardness of the weld seam.Due to the variety of alloy elements in the deposited metal of the weld seam, the effects of solid solution strengthening and precipitation strengthening significantly increase the hardness.The fusion zone is located in the transition zone between the weld seam and the base metal.Due to dilution, the hardness is lower than that of the deposited metal, but still higher than that of the base metal.Therefore, the hardness of the fusion line is smaller than that of the weld seam.
Figure 5 shows the comparison curve between the average hardness and wear weight loss of AlCrCuFeNiTi x (x = 0, 0.3, 0.6, 0.9, denoted as Ti 0 , Ti 0.3 , Ti 0.6 , Ti 0.9 ) based high entropy surfacing alloy.The visual display shows an increasing trend in the microhardness value curve of the alloy, with the highest average hardness of 636 HV in Ti 0.9 ; The wear curve shows a trend of first decreasing and then increasing, with the smallest wear amount at 0.037 g in Ti 0.6 .The higher the hardness, the better the wear resistance of this alloy.Ti is a strong carbideforming element that easily forms TiC, increasing the resistance to dislocation movement and making the alloy much harder than the substrate [16,17].As the Ti content increases, Laves phase appears in the alloy, and with  the change of Ti content, the shape of Laves phase changes.The wear weight loss of Ti 0.9 high entropy surfacing alloy is abnormal because the number and morphology of Laves phases change with the increase of Ti element, and layered segregation occurs in the ID region.During the wear process, the hard and brittle Laves are more prone to fracture due to stress concentration, leading to an increase in wear weight loss [18,19].
Figure 6 shows the friction coefficient curve of AlCrCuFeNiTi x (x = 0, 0.3, 0.6, 0.9, denoted as Ti 0 , Ti 0.3 , Ti 0.6 , Ti 0.9 ) based high entropy surfacing alloy.From the figure, it can be seen that the friction coefficient of high entropy surfacing alloy shows a trend of first decreasing and then increasing, which is consistent with the law of  wear amount.The higher the friction coefficient of an alloy, the more wear-resistant it is.At Ti 0.6 , the friction coefficient of high entropy surfacing alloy is the smallest, with a minimum friction coefficient of 0.244.When Ti 0.9 , the friction coefficient is particularly small at the beginning of wear, because there are more hard Laves phases in the alloy structure, which hinder the movement of dislocations; Over time, the layered Laves phase peels off as a whole, and the friction coefficient of the alloy suddenly increases, resulting in a decrease in the wear resistance of the alloy.
Figure 7 shows the wear morphology of AlCrCuFeNiTi x (x = 0, 0.3, 0.6, 0.9, denoted as Ti 0 , Ti 0.3 , Ti 0.6 , Ti 0.9 ) based high entropy surfacing alloy.From the wear morphology, it can be seen that the main wear mechanism of high entropy surfacing alloys is abrasive wear, which forms furrows of varying depths during the wear process.When Ti 0 is present, the alloy mainly consists of a harder BCC matrix phase.As wear progresses, deeper furrows appear, accompanied by a large amount of debris falling off.With the increase of Ti content, Laves phase in the dispersed distribution of hard TiC and ID regions was produced, with higher hardness.The furrows in the wear morphology also gradually decreased, and the shedding of wear debris also decreased, increasing the wear resistance of the alloy.When the Ti content increases to Ti 0.9 , too many Laves phases are formed, exhibiting a layered distribution.During the wear process, flakes fall off, reducing the ability to hinder dislocations and reducing the wear resistance of the alloy.The change in wear morphology also proves that the higher the hardness of the alloy, the better its wear resistance.

Conclusions
(1) The AlCrCuFeNiTi x high entropy surfacing alloys was prepared on the surface of low-carbon steel using the melting electrode gas shielded welding technology.The phase group of the surfacing alloy becomes the BCC +FCC solid solution phase, where the content of BCC phase is much higher than that of FCC phase.The microstructure is composed of disordered BCC phase rich in Fe and Cr, ordered BCC phase rich in Al and Ni, and FCC phase rich in Cu.
(2) The microstructure of AlCrCuFeNiTi x high entropy surfacing alloys exhibits typical dendritic (DR) and interdendritic (ID) structures.As the Ti element increases, the precipitated TiC disperses and the hard Fe 2 Ti phase precipitates in the interdendritic zone in a layered distribution.
(3) The microhardness of AlCrCuFeNiTi x based high entropy surfacing alloy shows an increasing trend, with a maximum hardness of 636 HV, which is 2.4 times higher than the base metal.With the increase of Ti element, the friction coefficient of the alloy shows a trend of first decreasing and then increasing, and the wear amount first decreases and then increases.When Ti is 0.6, the wear resistance of the high entropy surfacing alloy reaches the best.

Figure 5 .
Figure 5.Comparison of microhardness and wear mass loss of the AlCrCuFeNiTi x HEAs.

Table 3 .
Chemical compositions of the AlCrCuFeNiTi x HEAs at the various regions in figure 2 in atomic percentage.