Wire Arc Additive Manufactured Research Articles

Characterization of surface irregularities and fatigue strength evaluation of wire arc additive manufactured high strength steel specimens

This paper aims to study the effects of surface topography on the fatigue strength of wire-arc additive manufacturing (WAAM) components, paying particular attention to the interaction of roughness, waviness, and microstructure. For this purpose, first, a novel surface topography characterization approach is proposed to separate waviness and roughness without distorting the surface features, as smaller-scale irregularities, called secondary crack-like defects, have been confirmed to exist at notch-like valleys of WAAM specimens. This novel approach can practically be employed for any surface data obtained by different measuring methods, and it is not limited to WAAM specimens. Then, Murakami’s area model with a corrected stress intensity factor is introduced to evaluate the fatigue strength of WAAM specimens by considering the interaction between surface features (roughness and waviness) and their simultaneous effect. This approach can also particularly predict the fatigue strength of other engineering components once a small secondary crack or defect exists at the notch tip without entailing fatigue tests or intricate analysis. Moreover, microstructure changes due to the back-and-forth transformations in the microstructure and the formation of a soft phase in the interlayer area at the notches during the manufacturing process with lower hardness values have been considered using this model.

Effect of heat treatment on the microstructure and corrosion properties of plasma wire arc additive manufactured Mg-Gd-Y-Zr alloy

In this study, plasma wire arc additive manufacturing (P-WAAM), a technique known for its high deposition efficiency and material utilization rates, was applied to manufacture Mg-Gd-Y-Zr alloy. An investigation was conducted to examine the impact of heat treatment on the microstructure and corrosion characteristics of P-WAAM Mg-Gd-Y-Zr alloy. The results indicated that heat treatment effectively eliminated twins and residual stresses in P-WAAM Mg-Gd-Y-Zr alloy, while also homogenizing the second phase β-Mg24(Gd, Y)5. Electrochemical test results show that the heat treatment resulted in an increase in the self-corrosion potential (Ecorr) from −1.413 V to −1.311 V and a reduction in the corrosion current density (Icorr) from 2.046×10−5 A/cm2 to 4.766×10−6 A/cm2. Consequently, implementing an appropriate heat treatment regimen significantly improves the microstructure and corrosion resistance of the P-WAAM Mg-Gd-Y-Zr alloy.

In-situ investigation on tensile deformation behavior of hybrid hot-rolled wire arc additive manufactured Al-Mg alloys

Hybrid hot rolling and wire arc additive manufacturing (HR-WAAM) are becoming an effective way for enhancing both tensile strength and plasticity in Al-Mg alloys. This work systematically studies the microstructural evolution and strengthening mechanisms of HRAM Al-Mg alloys during the stretching process. The results show that interlayer hot rolling can effectively refine grains, reduce defects, and promote uniform precipitation of second phases. Within a lower strain range, uniformly distributed fine grains help prevent the formation of high geometrically necessary dislocation (GND) density in localized areas, thereby reducing local stress concentration. With a higher strain range, this microstructure can promote coordinated grain transformation towards softer orientations, reduce local microstructure work hardening, and allow GND to develop within a broader range of plastic deformation, thereby exhibiting excellent strain coordination ability. These factors collectively contribute to the improved strength and ductility of the material. Compared with traditional WAAM samples, the yield strength, tensile strength, and elongation of HRAM samples have increased by 18 MPa, 92 MPa, and 11 %, respectively. Consequently, this study provides new insights and a theoretical foundation for advancing additive manufacturing technology in the production of high-performance Al-Mg alloys.

Research on hot deformation behavior and microstructure characterization of the NiTi alloy fabricated by wire arc additive manufacturing

This study investigates the hot deformation behavior and microstructural evolution of wire arc additive manufactured (WAAMed) NiTi alloy. Hot deformation tests were conducted within a temperature range of 750–900 °C and a strain rate range of 0.01–5 s−1, achieving a height reduction of up to 50%. The Strain-Compensated Arrhenius-Type (SCAT) model and deformation processing maps were developed. The microstructures of compression specimens at strain rates of 0.01 s−1 and temperatures of 750–900 °C were characterized. The results demonstrate that the flow stress of WAAMed NiTi alloys increases with increasing strain rate and decreases with increasing deformation temperature. The calculated values from the SCAT model align closely with the experimental values, indicating a reliable analysis with high prediction accuracy. Flow instability region expands as the degree of deformation increases, and the optimal processing windows for the studied alloy are concentrated within the range of 800–900 °C and 0.01–0.05 s−1. In the deformed WAAMed NiTi alloys, the proportion of low-angle grain boundaries is around 70% and the recrystallized grain fraction remains below 15%. This is attributed to the predominance of dynamic recovery behavior in the softening mechanism. Discontinuous dynamic recrystallization and continuous dynamic recrystallization as auxiliary softening mechanisms are also involved in the softening of NiTi alloys, leading to a steady state of flow behavior.

Effects of deposition strategies on microstructure and mechanical properties of wire arc additive manufactured CoCrFeNiMo0.2 high-entropy alloy

High-entropy alloys (HEAs) have great potential for application in various fields due to their excellent performance. However, there are few reports on wire arc additive manufacturing of HEAs. In this study, CoCrFeNiMo0.2 HEA solid wire was prepared and used for WAAM based on cold metal transfer for the first time. Different deposition strategies (namely single-pass (SP), oscillation (OS), and multi-parallel pass (MPP)) were used to prepare metal wall samples of different thicknesses. The HEA samples' microstructure with different deposition strategies were explored by optical and scanning electron microscopy, as well as the electron backscatter diffraction method. Microhardness and tensile tests were used to evaluate the mechanical properties of different samples. The results prove that SP and MPP samples exhibit rougher surfaces due to heat accumulation. The OS sample has the best molding and an effective deposition ratio of up to 91.5 %. The composition of columnar grain grown along the deposition direction with strong {100}<001> cubic texture in both SP and OS samples. Therefore, these two samples have similar mechanical properties, and the mechanical performance in the vertical direction is better than that in the horizontal direction. Due to the complex internal thermal effects and remelting effects between welds, the internal structure of the MPP sample exhibits no obvious preferred orientation. Therefore, the anisotropy of mechanical properties is significantly alleviated.

The role of coherent nano precipitates on stacking fault and deformation twin formation during tensile deformation of wire arc additive manufactured nickel-aluminum-bronze

The strain hardening mechanisms and dislocation interactions of a wire arc additively manufactured (WAAM) nickel-aluminum-bronze (NAB) alloy was investigated using multi-scale characterization approach cross-correlating scanning electron microscopy (SEM), SEM-based electron back-scatter diffraction (EBSD), site- and crystallographic orientation-specific focused ion beam (FIB) nanofabrication, scanning transmission electron microscopy (STEM) and STEM-based energy dispersive X-ray spectroscopy (EDS). Flat, rectangular dog bone specimens were strained under uniaxial tension conditions. To understand the micro- and nanometer scale deformation mechanisms throughout the microstructure, tensile test specimens were interrupted near the yield strength and ultimate tensile strength. STEM microstructural characterization revealed that tensile deformation occurs by planar slip and multiple slip bands interacting on different {111} planes as well as the nucleation of dislocations from the interfaces associated with grains and secondary phase particles. To the authors knowledge for the first time the Center of symmetry (COS) analysis revealed that in both samples the shearing of nanoscale precipitates led to the formation of extended stacking faults including a combination of intrinsic stacking faults and 2-layer extrinsic stacking faults. At high strain levels more dislocations and stacking faults were nucleated, as well as intersecting slip bands further facilitating the activation of the stair-rod cross-slip mechanism and thickening of an ESF to create a 3-layer nano-twin. Activated mechanisms of plastic deformation and associated role of the precipitates are discussed with respect to the microscopic strength-plasticity characteristics as well as macroscopic mechanical properties of WAAM NAB alloy.

Mechanical and Metallurgical Characteristics of Wire-Arc Additive Manufactured HSLA Steel Component Using Cold Metal Transfer Technique

Recently, the application of wire-arc additive manufacturing (WAAM) for the production of metallic products is gaining traction. WAAM is associated with the direct energy deposition technique and therefore has a higher deposition rate (approximately 4 kg/h). For this reason, it is of greater interest than powder-based additive manufacturing techniques. Industrial applications such as marine and offshore structures and pressure vessels for space programs commonly utilize high-strength low-alloy (HSLA) steel. HSLA steel components produced by casting methods exhibit defects due to oxidation. Therefore, cold metal transfer (CMT)-WAAM was adopted in this study to fabricate HSLA steel components. The metallurgical properties were analyzed using microscopic and diffraction techniques. The effects of the evolved microstructures on mechanical properties, such as strength, microhardness, and elongation to fracture, were evaluated. To analyze and test the structure, two regions were selected, namely, top and bottom. Microstructural analyses revealed that both regions were primarily composed of acicular ferrite, polygonal ferrite, and bainitic structures. The bottom region exhibited superior mechanical properties compared with the top region. The improved strength at the bottom region can be ascribed to the formation of a high density of dislocations and finer grains.

β-Grain refinement in WAAM Ti-6Al-4 V processed with inter-pass ultrasonic impact peening

As-deposited Wire-Arc Additive Manufactured (WAAM) Ti-6Al-4V parts typically contain large columnar β-grains on a centimetre scale, with a strong 〈001〉 fibre texture, leading to anisotropic mechanical properties and unacceptable scatter in damage tolerance. Inter-pass deformation, introduced by the application of Ultrasonic Impact Peening (UIP) across each added layer, has been shown to be effective in refining the β-grain structure and achieving a weaker texture. The depth of deformation and the grain refinement mechanism induced by UIP have been investigated by combining advanced electron backscatter diffraction (EBSD) characterization with a ‘stop action’ observation technique. UIP facilitates a similar refinement mechanism and nearly the same depth of deformation as conventional machine hammer peening, with the advantages of a much higher strain rate, lower peak force, and two orders of magnitude lower impact energy, making it a faster and more economical process. β recrystallization is seen within the deformation zone during re-heating through the α → β transition. Although new recrystallized β-grains formed in the UIP surface-deformed layer to a shallower depth than that of remelting, recrystallization initiated ahead of the melt pool and the recrystallized grains grew downwards to a greater depth before remelting. These refined grains were thus able to survive and act as nucleation sites at the fusion boundary for epitaxial regrowth during solidification, greatly refining the grain structure.

Evaluation of mechanical and microstructural characteristics in different regions of wire arc additive manufactured 304L austenitic stainless steel

ABSTRACT The large structural components (304 L austenitic stainless steel) used in nuclear power plants are difficult and expensive to manufacture and machine using standard methods. Wire arc additive manufacturing (WAAM) is a low cost and high deposition method for fabricating large structural parts. Therefore, in this investigation, 304 L austenitic stainless steel (304 L ASS) cylindrical component was fabricated using WAAM technique. The mechanical and microstructural characteristics of the bottom (region ①) and top (region ②) of the WAAM 304 L ASS component are studied. The microstructure of region ① consists of austenite and ferrite with vermicular and lathy morphologies, while region ② consists of skeletal and reticular morphologies. In regions ① and ②, yield strength (YS), ultimate tensile strength (UTS), and elongation (EL) were found to be 350 ± 7 MPa, 562 ± 10 MPa, and 75 ± 1%, respectively. The impact toughness and hardness in regions ① and ② were found to be 112 ± 2 J and 183 ± 6 (Hv0.5), respectively. From the results, it is evident that the tensile properties of the WAAM 304 L ASS component were equal/greater than the values of the forged 304 L ASS material, wrought 304 L ASS alloy, and 304 L ASS filler wire.

Influence of post heat treatment on tribological and microstructural properties of plasma wire arc additive manufactured maraging steels

Abstract Metallic alloys are increasingly being produced using wired arc additive manufacturing (WAAM). In this study, 18Ni300 defect-free maraging steels were produced using the WAAM technique. A traditional solution treatment, direct aging, and cryogenic heat treatment processes were applied to the WAAM produced maraging steels. The influence of conventional and novel cryogenic heat treatments on microstructural, mechanical, and tribological properties were examined. The microstructure of the as-built materials obtained by WAAM thermal cycling has mainly been homogenized through the solution, direct-aging, and cryogenic heat treatments. As a result, homogeneously distributed precipitate phases were obtained and the hardness increased by 30 % with a combination different post heat treatments. The cryogenic heat treatment improved the martensitic transformation and facilitated the formation of various Fe–Ni–Mo–Ti-containing intermetallic precipitates. Similarly, because of the different heat treatments, the wear resistance improved by a factor of 2–5.5 relative to the as-built material. Adding the cryogenic heat treatment to the traditional heat treatment procedure improves wear resistance by a factor of 1.2–2.9.

Microstructural Analysis and Preliminary Wear Assessment of Wire Arc Additive Manufactured AA 5083 Aluminum Alloy for Lightweight Structures

The proliferation of Wire Arc Additive Manufacturing (WAAM) has significantly enhanced the production capabilities for lightweight and structurally robust components. This study investigates the microstructural characteristics, tensile properties, and preliminary wear performance of AA 5083 aluminum alloy processed via WAAM, focusing on applications for lightweight structures. Using SEM and XRD, microstructural changes during the WAAM process are analyzed, and tensile testing evaluates the mechanical properties, including ultimate tensile strength (UTS) and elongation. The results reveal that the microstructure consists of α-Al and β-(Al5Mg8) phases, with the Al5Mg8 phase distributed along grain boundaries and within grains. Notably, the grain size in the Y-direction (building direction) is larger than in the X-direction (deposition direction) due to temperature variations during processing. Tensile testing shows that horizontal samples (X-direction) have a UTS of 295 ± 5 MPa and elongation of 20.08 ± 0.8%, while vertical samples (Y-direction) have a UTS of 267 ± 10 MPa and elongation of 16.43 ± 2.1%. This results in an anisotropy of 9.4% in tensile strength, reflecting the differences in mechanical properties between the two directions. The WAAM AA 5083 aluminum part exhibits a maximum wear rate of 5.22 × 10⁻³ mm³/m and a coefficient of friction of 0.52 at a load of 3.5 kg and 450 rpm. Under these conditions, deep grooves, layer separation, and load-induced deformation are observed. The primary wear mechanisms include delamination, adhesion, and abrasion. Hardness levels are consistent in the X-direction and show minimal variance in the Y-direction, with an average hardness of 89.4 ± 5.14 HV0.5. The study demonstrates that WAAM-produced AA 5083 aluminum alloy, with an anisotropy below 10%, is suitable for real-time lightweight structures, offering effective performance in engineering applications such as aerospace and automotive industries. Future research should focus on further quantifying wear behavior and optimizing processing conditions to enhance material performance for specific applications.

Effects of residual Laves phase on microstructural evolution and mechanical properties of wire arc additive manufactured GH4169 Ni-based superalloy

The GH4169 Ni-based superalloy is prepared by wire arc additive manufacturing and the sizes and morphologies of Laves phases under different heat treatment strategies are obtained to further investigate the influence of residual Laves phases on the tensile properties of the superalloy. After the solution treated at a higher 1150 °C and followed by aging, the Laves phase is completely dissolved, with limited δ phase with varying sizes distributed at the grain boundaries. While for lower 1020 °C solid solution heat treatment temperature, the Laves phase in the sample changes from chain-like to particle-like shapes by partially dissolving, and the needle-like δ phase exists around the particle-like Laves phase. Such residual "Laves + δ" impede high-temperature tensile crack propagation, significantly enhancing plasticity. This mechanism makes the lower temperature solid solution treated sample exhibit similar tensile stress but ∼2 times of elongation when compared to the sample treated by higher solid solution.

Augmenting microstructure and tribological performance of wire arc additive manufactured PH13-8Mo stainless steel via TiC/TiB2 nano-particles incorporation

This study aimed to investigate the effect of TiC and TiB2 nano-inoculants addition on the microstructure and tribological behavior of PH13-8Mo stainless steel processed through wire arc additive manufacturing (WAAM). The incorporation of TiC/TiB2 inoculants demonstrated notable efficiency in mitigating the anisotropic wear and scratch response and enhancing wear resistance observed in the as-printed state. This improvement was ascribed to the refinement of the grain structure, disruption of the columnar structure, and increased content of the retained austenite. Notably, TiB2 inoculation exhibited superior grain refinement and achieved the highest hardness. However, the TiC-inoculated condition demonstrated the best wear resistance, attributed to its excellent combination of hardness and fracture resistance, and a higher contribution of strain-induced martensite transformation during wear testing. Additionally, the implementation of post-printing solutionizing and aging treatment was found to improve the scratch and wear resistance of the alloy, attributed to the formation of nano-sized β-NiAl precipitates. The main wear mechanism observed involved oxidation wear, adhesive wear, and three-body abrasive wear. The findings of this study highlight the significant potential of incorporating ceramic-based nano-particles for improving the wear resistance of WAAM PH13-8Mo components, particularly in demanding applications like injection molding dies where superior abrasion resistance is paramount.

Revealing corrosion behavior and mechanism of cold metal transfer-wire arc additive manufactured Mg-10Gd-4Y-2Zn-0.5Zr alloy in 3.5 wt% NaCl

The microstructure and corrosion behavior of wire arc additive manufactured (WAAM) Mg‐10Gd‐4Y‐2Zn‐0.5Zr alloy were comparatively studied to cast alloy. Higher fraction of eutectic (Mg,Zn)3(Gd,Y) phase along the grain boundary, more (Gd,Y)H2, (Gd,Y)2O3 and Zr enriched particles, heterogeneous microstructure composed of melt pool boundary, melt pool zone and heat affected zone were present in WAAM Mg-10Gd-4Y-2Zn-0.5Zr. Localized corrosion was initiated at the interface between the α-Mg and second phase and precipitated particles due to the inner layer differences of oxide films formed on them, resulting in inferior corrosion property and periodic corrosion characteristics of WAAM GWZ1042K alloy in 3.5 wt% NaCl.

Machinability of gas metal arc based 3D printed Al-Mg 5356 alloy using wire-EDM

Wire-Arc Additive Manufactured (WAAM) is relatively new method of metal 3D printing in which the raw material is heated by the gas metal arc and the molten metal pool is deposited layer-by-layer using a computer numerically controlled axis drive system. Since WAAM needs a finishing process for attaining the final dimensions of the components, there is a need to investigate the machinability aspects of WAAM fabricated materials. This work investigates the machinability of Al-Mg 5356 alloy test samples fabricated by WAAM process using wire-electric discharge machining (Wire-EDM). The test samples were subjected to Wire-EDM and the obtained material removal rate (MRR), kerf width (KW) and surface roughness (Ra) were investigated at different Wire-EDM process settings of voltage, current, pulse-on time (Ton), pulse-off time (Toff) and wire speed (Ws). Statistical analysis revealed that current had a significant influence on MRR. Ton had a strong influence on KW and Ra, whereas Toff exhibited a considerable impact on all these responses. Notably, Ws demonstrated a significant impact on Ra. However, voltage was found to have statistically negligible impact on all the machining responses. Microstructural investigations and compositional analysis were conducted providing valuable information on the cut surfaces. The results derived from the present investigation are useful for predicting the optimum process parameter settings for machining of WAAM-based 3D printed Al-Mg alloy in various manufacturing industries.

Pulsed TIG wire arc additive manufactured 17-4 PH stainless steel: Effect of Cu-rich precipitates and δ-ferrite on the mechanical properties

Thin-wall samples of 17-4 PH stainless steel were prepared by pulsed TIG wire arc additive manufacturing technology (P-TIG WAAM), and the microstructure and mechanical properties of the samples in different regions (bottom, middle, top) were studied. The results show that the microstructure of the thin-wall samples are mainly consisted of martensite matrix embedded by worm-like (bottom) and strip-shaped (top) δ-ferrite, with a small amount of austenite. The δ-ferrite content gradually decreases along the building direction, resulting in a gradual increase in microhardness from the bottom to the top. Along the building direction, the precipitation density and size distribution of nano-scale CRPs (Cu-rich precipitates) in δ-ferrite and martensite, as well as at the δ-ferrite/martensite interface are significantly different: the CRPs in δ-ferrite is precipitated with lower density, larger size, and inhomogeneous distribution at the bottom region, while that in martensite is precipitated with higher density, smaller size, and homogeneous distribution. At the bottom region, a chain of large size fcc-CRPs (∼65 nm) is precipitated along the δ-ferrite/martensite interface to coordinate the strain between them and improve the strength and ductility simultaneously, thus excellent mechanical properties (tensile strength = 1356 ± 24 MPa, elongation = 15.16 ± 2.6 %) are obtained. The relationship between the evolution of CRPs and the intrinsic heat treatment (IHT) during P-TIG WAAM is also discussed.

Fully equiaxed grain structure and isotropic mechanical properties in wire arc additive manufactured Mg–Al-RE alloy

Wire arc additive manufacturing (WAAM) of Mg–Al-RE alloy is an essential technology for producing high-quality large-scale components with high manufacturing flexibility. In this study, WAAM was utilized to fabricate a thin-wall component made of Mg-8.2Al-0.6Y-0.4Ce (AE81) alloy. The microstructure and tensile properties of the fabricated component were investigated. The results showed that the as-built AE81 alloy exhibits superior isotropic tensile strengths and a homogeneous equiaxed α-Mg grains structure. Additionally, the alloy demonstrated a minimal growth in grain size after high-temperature solid solution treatment due to the presence of thermally stable Al11/rare earth (RE)3 phases at grain boundaries. Further ageing treatment induces the formation of high-density of β-Mg17Al12 precipitates within the α-Mg grains, resulting in a considerable precipitation strengthening effect. Consequently, the heat-treated AE81 alloy demonstrated significantly enhanced tensile strength of 280 MPa and acceptable ductility of 4.6%. The results also revealed that the formation of equiaxed grains during WAAM contributed to the isotropic mechanical properties. These findings can facilitate advancements in AM processes and the development of high-performance Mg alloys.

Effect of build orientation on the microstructure and mechanical properties of wire arc additive manufactured grade 91 steel/monel 400 bimetallic components

This study explores the influence of build orientation on the microstructural and mechanical properties of Wire Arc Additive Manufactured (WAAM) Grade 91 Steel and Monel 400 bimetallic components, which are used in combined thermal and corrosion resistance applications. Microstructural analysis was conducted using techniques such as Optical Microscopy (OM), Field Emission Scanning Electron Microscopy (FE-SEM), and Transmission Electron Microscopy (TEM). The research specifically examines how different orientations (0°, 45°, and 90°) affect the as-built properties of these bimetallic components. Mechanical properties were evaluated through tensile testing, revealing significant variations: Grade 91 steel displayed a tensile strength of 1235.2 ± 12 MPa at a 0° orientation and 1314.7 ± 5 MPa at 90°, indicating anisotropic characteristics. Similarly, Monel 400 exhibited an increase in tensile strength from 491.5 ± 12 MPa at 0° to 517.7 ± 14 MPa at 45°. Fractographic analysis indicates a predominantly ductile fracture mechanism. The results of the study are important for using WAAM to create bimetallic components for environments that combine strength, thermal resistance, and resistance to corrosion.

Fatigue response of wire-arc additive manufactured nickel-aluminum bronze (NAB) in the post-annealed condition

Nickel-aluminum bronze (NAB) is chosen for critical applications like marine propellers, pump components, and offshore structures due to its exceptional mechanical properties and corrosion resistance. Considering the utmost importance of the fatigue performance of NAB in such applications, this study investigates the fatigue performance of wire arc additive manufactured (WAAM) NAB in the annealed (675 °C for 6 h then furnace cooling) condition. To this end, static mechanical properties were evaluated using uniaxial tensile testing, while fatigue properties were assessed through fully reversed tests conducted at ambient temperature. Electron backscatter diffraction (EBSD) was utilized for the examination of the texture, and grain size distribution in annealed WAAM NAB. Additionally, scanning electron microscopy was employed to inspect fatigue-fractured specimens, while the analysis of EBSD was conducted to examine the deformation behavior beneath the crack initiation zone. The results provide insights into the fatigue life, crack initiation, and propagation characteristics of WAAM NAB relative to the cast counter material. The findings of this research provide valuable insights into the mechanical properties, microstructural characteristics, and fatigue response of WAAM NAB, informing the design of fatigue-resistant components and supporting the reliable utilization of WAAM NAB in demanding applications such as the marine and naval industries.

On the Enhancement of Material Formability in Hybrid Wire Arc Additive Manufacturing

This paper is focused on improving material formability in hybrid wire-arc additive manufacturing comprising metal forming stages to produce small-to-medium batches of customized parts. The methodology involves fabricating wire arc additive manufactured AISI 316L stainless steel parts subjected to mechanical and thermal processing (MTP), followed by microhardness measurements, tensile testing with digital image correlation, as well as microstructure and microscopic observations. Results show that mechanical processing by pre-straining followed by thermal processing by annealing can reduce material hardness and strength, increase ductility, and eliminate anisotropy by recrystallizing the as-built dendritic-based columnar grain microstructure into an equiaxed grain microstructure.