Rare earth RE elements addition can improve

Rare earth (RE) elements addition can improve the specific strength and modulus of Mg alloys. Among them, Mg–Nd–Zn alloys can generate excellent mechanical performances with only a small amount of Nd and Zn additions [19–23]. In this Bindarit work, a common commercial Al alloy 6061 and a potential Mg–Nd–Zn–Zr alloy were chosen for friction stir lap linear welding (FLSW). Various welding parameters were performed to optimize the rotation and travel speed. Meanwhile, the temperature fields were monitored to study the effect of welding parameters on system heat input. The optimized weld was then subjected to short time solution to achieve better tensile shear properties.

Experimental procedures
6061 Al alloy and NZ30K Mg alloy sheets with same thicknesses of 3 mm were FSLWed. The chemical compositions of the parent materials are listed in Table 1. All welds were made in lap joint configuration of 6061Al alloy on top and NZ30K on the bottom as shown in Fig. 1. The cylindrical threaded welding tool was made from H13 tool steel and the shoulder diameter, pin diameter, and pin length were 15 mm, 4 mm and 3.8 mm, respectively. The tool tilt angle was 2.5° and the shoulder plunge depth was kept 0.8 mm during welding procedure. The right hand threaded tool used in this research was set to rotate clockwise. The rotation speed of 600, 900, 1200 and 1500 rpm and travel speed of 60, 90 and 120 mm/min were performed.
The temperature field during the FSLW was measured at the overlapping part as shown in Fig. 1. During every single run, eight K-type thermocouples (1 mm in diameter) were inserted into eight small holes (1.2 mm in diameter) drilled in two sides of the two layers for different depths respectively. The graphic scheme of the placements and depths distribution of drilled holes is shown in Fig. 2. Two layers named top layer and bottom layer were measured for all the parameters. In the process, only one layer was measured each pass to avoid the deviation from excessive lack of parent materials. As for the temperature of the center, the average value of the two center holes shown in Fig. 2 was adopted. The thermocouples were linked to a system controlled by the PC, which enabled a temperature sampling frequency of 100 Hz.
The FSLW joints were cross-sectioned perpendicular to the welding direction for microstructure observation. The samples were polished with diamond polishing agent and etched by three steps [21,22]. Firstly, the specimens were etched in a solution of 10 ml acetic acid, 10 ml distilled water, 4.2 g picric acid in 100 ml alcohol for 5 s to reveal the microstructure of NZ30K alloy. Secondly, the specimens were etched in a solution of 20 g NaOH in 100 ml distilled water for 40 s to reveal the grain boundary of Al alloy. The last step was to dye the specimens by the solution consisting of 4 g KMnO4 and 2 g NaOH in distilled water for 10 s. The macrostructure and microstructure were obtained with an optical microscope (OM) (Zeiss Axio Observer A1). The details of the joints microstructure, including the element distribution within the weld zone were analyzed using a FEI company produced low vacuum scanning electron microscopy (LV-SEM) equipped with an energy-dispersive X-ray spectroscopy (EDS) analysis system. XRD (RigakuUltima IV) was used to examine the phases of the joints with the analysis angles ranged from 20° to 100°. Tensile shear test was carried out on Zwick Z100 machine according to ASTM D 3164 [24]. The length and width of the specimens were 90 mm and 20 mm, respectively. To ensure tensile loading direction parallel to the weld region, a supporting plate was used. And the displacement rate was 1 mm/min.

Results and discussions

Conclusions
In this work, some parameters were adopted to the friction stir linear welding of magnesium alloy NZ30K and aluminium alloy 6061, and the following conclusions are given:

Acknowledgments

Bindarit PRRSV is an enveloped single stranded

PRRSV is an enveloped, single-stranded, positive-sense RNA virus that is a member of the family Arteriviridae placed with the family Coronaviridae in the order Nidovirales (Cavanagh, 1997; Snijder et al., 2013). The PRRSV Bindarit is approximately 15kb long with a 5′ cap and a 3′ polyadenylated tail and consists of a 5′-untranslated region (UTR), 10 open reading frames (ORFs), designated ORF1a, ORF1b, ORF2a, ORF2b, and ORFs 3 through 7, including ORF5a, and a 3′-UTR (Firth et al., 2011; Johnson et al., 2011; Snijder and Spaan 2007). The two large ORF1a and 1b occupy the 5′-proximal three-fourths of the genome coding for nonstructural proteins (nsps). ORF1a translation yields a replicase polyprotein (pp) la or shorter pp1a products via a −1 or −2 ribosomal frame shift (RFS), whereas ORF1b is expressed by a −1 RFS, which C-terminally extends ppla into pp1ab. These ppla and pplab undergo proteolytic maturation by internal proteases to form at least 16 processing end products (nsp1α, nsp1β, nsp2, nsp2TF, nsp2N, nsp3–6, nsp7α, nsp7β, and nsp8–12) involved in viral transcription and replication. The remaining ORFs in the 3′-proximal genome part encodes 8 structural proteins expressed from a respective 3′-co-terminal nested set of subgenomic (sg) mRNAs (Fang et al., 2012; Kappes and Faaberg, 2015; Li et al., 2012, 2014; Snijder et al., 2013).
The most extraordinary feature of PRRSV is deemed to be high antigenic drift due to a rapid mutation rate. PRRSV strains currently comprise two genetically distinct groups, the European (EU; type 1) and North-American (NA; type 2) genotypes, with approximately 40% sequence differences at the genome level (Nelsen et al., 1999; Plagemann, 2003). Both genotypes circulate worldwide (Kimman et al., 2009; Zimmerman et al., 2012; Nam et al., 2009). Strikingly, strains within each genotype also show extremely high degrees of nucleotide divergence as high as 20% (Faaberg et al., 2006a; Han et al., 2007). The genetic liability of PRRSV leads to considerable biological and pathogenic heterogeneity among field isolates, which is one of the challenges in developing an effective vaccine. Type 2 PRRSV is divided into 9 monophyletic lineages (Shi et al., 2010) that continues to evolve with clinical variations of the disease, such as swine mortality and abortion syndrome or “atypical PRRS” in 1996 (Epperson and Holler, 1997; Halbur and Bush, 1997); the epidemics of highly pathogenic PRRSV (HP-PRRSV) that appeared in China and its neighboring countries, causing “pig high fever disease” with high (∼20%) mortality in pigs of all ages since 2006 (An et al., 2011b; Feng et al., 2008; Ni et al., 2012; Normile, 2007; Tian et al., 2007; Tong et al., 2007); and virulent lineage 1 viruses that have been circulating primarily in the mid-western US since 2000 (Han et al., 2006). The type 2 PRRSV lineage 1 includes virulent MN184 and its relative strains with a notable discontinuous 111-1-19 deletion of 131 amino acids within the middle hypervariable region 2 (HV2) of nsp2 (nsp2 111-1-19 DEL) compared to type 2 prototype VR-2332 (Brockmeier et al., 2012; Han et al., 2006). The MN184-related nsp2 DEL variant first emerged in South Korea likely in the early 2010s and has since spread across the country, causing severe PRRS outbreaks (Choi et al., 2014).
Since live-attenuated PRRSV vaccines have higher efficacies than the killed vaccines, they have been used worldwide to prevent and control the disease. After sequential passages (at least 70 times) of PRRSV on non-host cell lines, the field wild-type strains can be efficiently attenuated but still retain immunogenicity (Nielsen et al., 2001). The attenuated virus can be used to prepare a modified live virus (MLV) vaccine. Indeed, the first type 2 live vaccine, Ingelvac PRRS MLV, was obtained by continuous passages in MARC-145 cells (Opriessnig et al., 2002). More recently, live attenuated HP-PRRSV vaccines were developed by in vitro serial passages in MARC-145 cells and are now available in Chinese markets, showing effectiveness and protection by reducing the economic loss against PRRS (An et al., 2011a; Han et al., 2009; Leng et al., 2012a,b; Tian et al., 2009). In the present study, the virulent Korean PRRSV nsp2 DEL strain CA2 was culturally passaged 100 times in MARC-145 cells for attenuation so that it could be further used for the development of an MLV vaccine. We aimed to characterize phenotypes in low- to high-passage derivatives of CA-2 in vitro and to evaluate the virulence and immunogenicity of the attenuated CA-2-P100 (100th passage of CA-2) strain in vivo. In addition, the complete genome sequence of CA-2 was determined at selected passages to understand the potential relations of genetic mutations with PRRSV attenuation.