Friction stir welding/processing of metals and alloys: A comprehensive review on microstructural evolution

Increasing demand of lightweight structures with exceptional properties elicits materials processing and manufacturing technologies to tailor blanks in order to achieve or enhance those pre...

Recent Development in Friction Stir Processing as a Solid-State Grain Refinement Technique: Microstructural Evolution and Property Enhancement

While significant progress has been made in laser powder bed fusion (L-PBF) for metal additive manufacturing (AM), there is still limited large scale adoption of this advanced manufacturing techniq

Laser powder bed fusion for metal additive manufacturing: perspectives on recent developments

Magnesium (Mg) alloys have been extensively used in various fields, such as aerospace, automobile, electronics, and biomedical industries, due to their high specific strength and stiffness, excellent vibration absorption, electromagnetic shielding effect, good machinability, and recyclability. Friction stir processing (FSP) is a severe plastic deformation technique, based on the principle of friction stir welding. In addition to introducing the basic principle and advantages of FSP, this paper reviews the studies of FSP in the modification of the cast structure, superplastic deformation behavior, preparation of fine-grained Mg alloys and Mg-based surface composites, and additive manufacturing. FSP not only refines, homogenizes, and densifies the microstructure, but also eliminates the cast microstructure defects, breaks up the brittle and network-like phases, and prepares fine-grained, ultrafine-, and nano-grained Mg alloys. Indeed, FSP significantly improves the comprehensive mechanical properties of the alloys and achieves low-temperature and/or high strain rate superplasticity. Furthermore, FSP can produce particle- and fiber-reinforced Mg-based surface composites. As a promising additive manufacturing technique of light metals, FSP enables the additive manufacturing of Mg alloys. Finally, we prospect the future research direction and application with friction stir processed Mg alloys.

Friction Stir Processing of Magnesium Alloys: A Review

Additive manufacturing (AM) of metallic alloys for structural and functional applications has attracted significant interest in the last two decades as it brings a step change in the philosophy of design and manufacturing. The ability to design and fabricate complex geometries not amenable to conventional manufacturing, and the potential to reduce component weight without compromising performance, is particularly attractive for aerospace and automotive applications. This has culminated in rapid progress in AM with Ti- and Ni-based alloys. In contrast, the development of AM with Al-alloys has been slow, despite their widespread adoption in industry owing to an excellent combination of low density and high strength-to-weight ratio. Research to date has focused on castable and weldable AlSiMg-based alloys (which are less desirable for demanding structural applications), as well as on the development of new AM-specific AlMgSc alloys (based on 5xxx series). However, high strength wrought Al-alloys have typically been unsuitable for AM due to their unfavourable microstructural characteristics under rapid directional solidification conditions. Nevertheless, recent research has shown that there is promise in overcoming the associated challenges. Herein, we present a review of the current status of AM with Al-alloys. We primarily focus on the microstructural characteristics, and on exploring how these influence mechanical properties. The current metallurgical understanding of microstructure and defect formation in Al-alloys during AM is discussed, along with recent promising research exploring various microstructural modification methodologies. Finally, the remaining challenges in the development of AM with high-strength Al-alloys are discussed.

A review of Laser Powder Bed Fusion Additive Manufacturing of aluminium alloys: Microstructure and properties

AlSi10Mg alloy was fabricated by Selective Laser Melting (SLM) using two different atmospheres, argon and nitrogen, both in vertical and horizontal built directions. The influence of work environment and anisotropy are studied in this work by keeping all other process parameters including shielding gas pressure constant. The meso-structural features revealed scan tracks and melt pool layers while the microstructural features have showed cellular structure with α-Al matrix and its boundary made up of the eutectic mixture of Al and Si. These cells were approximately of 500 nm size as revealed by TEM analysis. The grain morphology and texture were studied using EBSD analysis. The XRD analysis revealed similarity in the phases formed in samples built under argon and nitrogen environment. Both the horizontal as well as vertical built samples contain elongated grains with 〈 100 〉 direction oriented along the built direction and 〈 100 〉 fibre texture in the XY plane. Tensile test results showed the maximum strength of 385 ± 5 MPa was exhibited by vertically built sample under nitrogen environment while the lowest strength of 338 ± 2 MPa was exhibited by horizontally built sample under argon environment. Surface roughness and porosity measured in all the conditions exhibited minor variation in the average surface roughness and porosity area fraction values. Among the samples tested under same built orientation, the variation in strength is approximately 5% higher in nitrogen environment while among the samples tested under same working environment, the variation in strength is approximately 7.5% higher in vertically built samples. This states that the influence of working environment on strength is lesser than that of the built orientation. The materials exhibited brittle failure with inclined fracture profile, fracture morphology with pores, river patterns and inter-cellular fracture plus approximately 4.5% elongation in all the tested samples, implied isotropic elongation behaviour. The samples built under both the work environments exhibited the similarity in defect structure, microstructure and nearly isotropic behaviour in mechanical properties. It inferred that nitrogen could be preferred over argon as shielding gas in SLM for its cost effectiveness.

Influence of working environment and built orientation on the tensile properties of selective laser melted AlSi10Mg alloy

Multipass friction stir processing (FSP) was performed in as-cast AZ91 Mg alloy (AC) to generate layered microstructure through the thickness and study its effect on superplasticity. The FSP tools with different tool pin length were used to develop three kinds of layered microstructured materials. A full thickness fine grained microstructure (FFG), a half thickness fine grained with remaining half in as-cast condition (HFG). The last variation was one third thickness modified into fine grain from both the surfaces and the middle section having as-cast microstructure (SFG). FSP was performed at a tool rotational speed of 720 rpm and at a transverse speed of 150 mm/min. The coarse α-Mg dendrites with large plate like interconnected β-Mg17Al12 interdendrites were the characteristics microstructure of as-cast AZ91 alloy. Fine grains and uniformly distributed precipitates were the characteristics of FSPed microstructure. High temperature tensile tests were carried out at 350 °C using three different initial strain rates i.e. 5 × 10−3 s−1, 1 × 10−3 s−1 and 5 × 10−4 s−1. The FFG material showed superplasticity at all strain rates and highest ductility of 680% was achieved at the strain rate of 5 × 10−4 s−1. The AC and HFG material displayed very low elongation while SFG material exhibited superplasticity of 388%. The superplastic behaviour in SFG was due to increase in the fraction of fine grained microstructure and modification of as-cast microstructure on both the surfaces. Microstructure and texture studies revealed that grain boundary sliding accommodated by grain boundary migration and grain rotation was responsible for superplasticity in FSPed region.

Effect of layered microstructure on the superplasticity of friction stir processed AZ91 magnesium alloy

Effect of friction stir processing on tensile and fracture behaviour of AZ91 alloy

Selective laser melted (SLM) IN 718 alloy specimens are subjected to heat treatment and shot peening to assess the effect of post processing on the corrosion performance of the alloy in a 3.5 wt % NaCl solution. The four conditions used in this analysis are as-built material (AB), heat-treated as-built material (HT), shot-peened as-built material (SP), and heat-treated and shot-peened as-built material (HTSP). Microstructural studies revealed the presence of a 500 nm sized cellular structure with a γ matrix surrounded by the Laves phase in the AB material. Shot-peening reduced the surface roughness of the AB and HT samples to almost 80%. The potentiodynamic experiments revealed a highest Icorr value of 0.21 µA/cm2 for the AB material and the lowest Icorr value of 0.04 µA/cm2 for the HTSP material. In the Electrochemical Impedance Spectroscopy (EIS) analysis, the Nyquist plot substantiated the increasing corrosion resistance in the same order of decreasing corrosion rate. The Bode plot exhibited two resistance–capacitance (RC) time constants for all four conditions. The solution resistance measured around 30 Ω, with the HTSP specimen exhibiting the highest passive film resistance of 676 kΩ cm2 and the AB specimen exhibiting the lowest passive film resistance of 234 kΩ cm2. This study has shown that elimination of the network of the Laves phase in SLM material through heat treatment and smooth surface morphology achieved through shot peening improves the corrosion resistance of Inconel 718 alloy.

Corrosion Study of Selective Laser Melted IN718 Alloy upon Post Heat Treatment and Shot Peening

The present work investigates the relationship between fatigue crack growth rate (da/dN) and stress intensity factor range (∆K) using machine learning models with the experimental fatigue crack growth rate (FCGR) data of cryo-rolled Al 2014 alloy. Various machine learning techniques developed recently provide a flexible and adaptable approach to explain the complex mathematical relations especially, non-linear functions. In the present work, three machine algorithms such as extreme learning machine (ELM), back propagation neural networks (BPNN) and curve fitting model are implemented to analyse FCGR of Al alloys. After tuning of networks with varying hidden layers and number of neurons, the trained models found to fit well to the tested data. The three tested models are compared with each other over the training as well as testing phase. The mean square error for predicting the FCG of cryo-rolled Al 2014 alloy by BPNN, ELM and curve fitting methods are 1.89, 1.84 and 0.09 respectively. While the ELM models outperform the rest of models in terms of training time, curve fitting model showed best performance in terms of accuracy over testing data with least mean square error (MSE). In terms of local optimisation, back propagation neural networks excel the other two models.

https://www.mdpi.com/2075-4701/10/10/1349/pdf

A. Raja

Papers

Influence of working environment and built orientation on the tensile properties of selective laser melted AlSi10Mg alloy

Effect of layered microstructure on the superplasticity of friction stir processed AZ91 magnesium alloy

Effect of friction stir processing on tensile and fracture behaviour of AZ91 alloy

Corrosion Study of Selective Laser Melted IN718 Alloy upon Post Heat Treatment and Shot Peening

Prediction of Fatigue Crack Growth Behaviour in Ultrafine Grained Al 2014 Alloy Using Machine Learning