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Resistance spot welding (RSW) was applied to non-combustible magnesium alloy, AX41 (Mg-4%Al-1%Ca) to investigate its weldability. The similar material joint of AX41 and dissimilar materials joint between AX41 and aluminum alloy, AA6061 were welded. Tensile shear test was carried out to evaluate joining strength in the similar and dissimilar materials RSW joints. In case of similar material joints, the maximum load obtained with tensile shear test in AX41 similar material joint was higher than that obtained in AA6061 similar material joint. Moreover, higher maximum load was obtained in a similar material joint without surface polishing compared to joint welded with surface polishing in AX41. In case of the dissimilar materials joint, the maximum load obtained was almost comparable with AX41 similar material joint, however scatter in joint strength was large. Weldability of the dissimilar materials joint became poor by applying surface polishing.

In this study, austenitic stainless steel 316L and titanium alloy (ASTM grade 5) were welded together by micro-resistance spot welding at different combinations of welding parameters and by using specifically designed electrode geometry. The welded joints were subjected to tensile shear strength test in order to determine the strength of the welded zones. In addition, micro-hardness and microstructural examinations of the fracture mode (failure analysis) were carried out in order to examine the influence of welding parameters on the welded joints. The results showed that using a combination of 2.0 kN welding current, 100 ms welding time, and 241 N welding force yield the highest load value, 378.25 N by using full factorial design of experiment (DOE). Welding current is the most significant parameter which is obtained through analysis of variance (ANOVA). However, the increase in welding current should be controlled to avoid weld metal expulsion. The microstructure of these resistance-welded metals are detailed and investigated by using SEM and EDS mapping analysis. Based on the SEM observations, columnar dendritic structures can be seen at the fusion zone (FZ) of the welded nugget.

The fatigue strengths at 1000 °C of layered W-Re/TZM, bulk W-Re and bulk TZM for x-ray target materials were successfully evaluated under load-controlled four-point bending by introducing a fatigue failure criterion as two-times increase of initial compliance. The obtained fatigue strengths at 1000 °C for layered W-Re/TZM and bulk W-Re were similar and 280 MPa and 290 MPa at 106 cycles, respectively, while that of bulk TZM was 200 MPa. During fatigue loading at 1000 °C, dominant fatigue damage would be multiple intergranular crack nucleation and propagation, which would induce the increase of compliance. The reasonability of the fatigue failure criterion was confirmed by the fatigue process observations and the results of room temperature fatigue tests of the specimens tested at 1000 °C up to the cycles corresponding to the fatigue failure criterion. Keywords: Fatigue strength, High temperature, Fatigue damage, Microstructure-property relationship, Refractory materials

The texture, microstructure, and mechanical properties of severely deformed aluminum subjected to friction stir processing (FSP) with SiC nanoparticles under ambient and cryogenic temperatures are comprehensively investigated. Even one pass of FSP in the ambient temperature results in the formation of quite large grain structure with random texture in the heat affected zone (HAZ). It suggests the occurrence of static recrystallization and subsequent grain growth because of the appreciable strain stored within the base metal. One FSP pass under condition of cryogenic temperature causes to the development of bimodal grain size distribution in HAZ, including the elongated grains with interior subgrains and very large grains as a result of texture-induced abnormal grain growth. Large grains with random texture are formed throughout HAZ during further FSP passes. In the stir zone where SiC nanoparticles are dispersed during FSP in the ambient temperature, fine grain structure with the shear texture components of C and A 1 ∗ is obtained through the geometrically dynamic recrystallization mechanism. When FSP is performed in the cryogenic temperature, the grain size of stir zone is decreased because of the increased cooling rate. Although the shear texture components developed in the stir zone are the same at both mentioned processing conditions, the texture strength is less in the case of cryogenic temperature, suggesting the contribution of additional grain formation mechanism, namely discontinuous dynamic recrystallization. This mechanism gives rise to the formation of randomly oriented grains. The nano-indentation test results confirm that FSP under cryogenic temperature improves the mechanical properties not only in the stir zone, but also in HAZ.

Fatigue strength characteristic of laser welded joint between austenitic stainless steel and ferritic stainless steel was studied at room temperature and 300 °C.The results showed that fatigue strength at 300°C was higher than that at room temperature in lower applied stress amplitude region. Fatigue crack mainly initiated at stress concentration part at room temperature. However, fatigue crack origin changed toward to the base material and the heat affected zone with increasing testing temperature. Increase in ductility of the base material and the weld metal induced by increasing testing temperature reduces stress concentration effect and resulted in change in fatigue mechanism. Appropriate weld position in a component with temperature gradient can be considered by minimizing the effect of stress concentration and taking into account degradation of strength due to increase in temperature.

The elastic-plastic fracture toughness JIC of two titanium-zirconium-molybdenum (TZM) alloys with different kinds of grain boundary particles was estimated at elevated temperatures using the convenient JIC test method with a suitable depth of side-groove for determining the JIC at the maximum load point. It was found that the convenient JIC test method can be successfully applied to evaluate high temperature fracture toughness at least up to 1200 °C. The JIC values at temperatures ranged from 800 °C to 1200 °C were almost constant regardless of temperature, while the JIC values of the TZM with carbide particles were higher than those of the TZM with oxide particles. The TZM with different forging rates showed similar JIC values, which suggested the effect of forging rate would be not significant at high temperatures.

Severely deformed aluminum sheets were processed by friction stir processing (FSP) with SiC nanoparticles under different conditions to improve the mechanical properties of both the stir zone and the heat affected zone (HAZ). In the case of using a simple probe and the same rotational direction (RD) of the FSP tool between passes, at least three FSP passes were required to obtain the appropriate distribution of nanoparticles. However, after three FSP passes, fracture occurred outward from the stir zone during transverse tensile tests; thus, the strength of the specimen was significantly lower than that of the severely deformed base material because of the softening phenomenon in the HAZ. To improve the mechanical properties of the HAZ, we investigated the possibility of achieving an appropriate distribution of nanoparticles using fewer FSP passes. The results indicated that using the threaded probe and changing the RD of the FSP tool between the passes effectively shattered the clusters of nanoparticles and led to an acceptable distribution of SiC nanoparticles after two FSP passes. In these cases, fracture occurred at the HAZ with higher strength compared to the specimen processed using three FSP passes with the same RD between the passes and with the simple probe. The fracture behaviors of the processed specimens are discussed in detail.

Effect of mean stress on fretting fatigue behavior of 304 austenitic stainless steel has been investigated by conducting fretting fatigue tests at a constant contact pressure of 100 MPa under three different mean stresses i.e., 0, 350 and 450 MPa. For comparisons, plain fatigue tests were also carried out. The influence of tensile overload on fretting fatigue life was also investigated. The results showed that with an increase in mean stress, the reduction in fatigue strength due to fretting increased drastically from 51% at 0 MPa mean stress to 71% at 450 MPa mean stress. The application of tensile overload during fretting fatigue had significant influence on the fretting fatigue lives when the tensile overload was above yield strength. The fretting variables, i.e., tangential stress and relative slip amplitude were measured during fretting fatigue tests. Fracture surfaces were examined using scanning electron microscope. The results have been discussed based on the tangential stress measurement, relative slip amplitude evaluation during fretting fatigue and fracture surface examinations.

The texture development during friction stir processing (FSP) of 1050 aluminum severely deformed at the strain magnitude of 2.32 was comprehensively discussed. It was observed that the component $$ \bar{B} $$ of the ideal shear texture along with the cube texture was developed in the severely deformed base metal. The effects of base metal texture on the texture development of stir zone, thermo-mechanically affected zone, and heat-affected zone during FSP were examined. Also, the developed texture components in the vicinity of the FSP tool and the stir zone were correlated to the deformation induced by the rotating tool which consisted of pin and shoulder. The observed texture components in the longitudinal section of the stir zone were found coincided with the ideal shear ones, but different from those observed in the severely deformed base metal. It could be responsible for the fact that the material beneath the FSP tool is predominantly deformed and stirred by the shoulder rather than the pin. The independency of texture development in the stir zone from pin-induced deformation was also consistent with the observation associated with the stir zone geometry which was independent of the pin geometry. Microstructural evolutions in the regions located ahead of the FSP tool manifested the incident of static recovery and recrystallization as a result of the stored strain in the severely deformed base metal. These led to the development of almost random texture and the deterioration of base metal texture in this region. This suggested the independency of texture development in the stir zone from the texture of severely deformed base metal.

Notch fatigue behavior of Ti-6Al-4V titanium alloy has been investigated under a load-controlled high cycle fatigue test. The S-N curve was kinked at the critical nominal stress amplitude, where the plastic deformation nucleated at notch root. The plastic zone size at the critical nominal stress amplitude was almost equal to the size of 4 grains of the present Ti-6Al-4V alloy, which were the same findings as in the 2024-T4 alloy used in the previous study. Above the critical nominal stress amplitude, the local stress ratio at notch root decreased with increasing nominal stress amplitude. The critical nominal stress amplitudes normalized by the cyclic yield stress were almost the same for Ti-6Al-4V alloy and the 2024-T4 alloy. The significant decrease of the local stress ratio with increasing nominal stress amplitude was observed in the 2024-T4 alloy with low cyclic yield stress and low cyclic hardening coefficient compared to the Ti-6Al-4V alloy with high cyclic yield stress and high cyclic hardening coefficient.

Fretting fatigue behavior of IMI 834 titanium alloy has been investigated at room temperature, 400 and 590 °C. For comparison plain fatigue tests were also carried out. Finite element analysis was done to evaluate the tangential stress coefficient and relative slip amplitude during fretting fatigue at room temperature and high temperatures. The fracture surface and fretting contact surface were examined using scanning electron microscope. The result showed that, the fatigue strength has reduced due to fretting both at room temperature and high temperature. With an increase in temperature the fretting fatigue strength reduced. It reduced significantly at 400 °C and drastically at 590 °C, due to the increase in severity of damage in fretting contact surface with an increase in test temperature. The results were discussed based on the relative slip amplitude values, tangential stress coefficient values and the surface damage observations.

Formation behavior of bubbles and its effect on strength in PET/SUS304 dissimilar materials laser spot lap joint was studied. Surface of SUS304 was irradiated through transparent PET with fiber laser. Static and cyclic contact force was applied at the welding region during the joining process. Strength of the joint was evaluated by conducting tensile shear test. The formation behavior of bubbles inside PET nearby the interface changed with change in condition of contact force. In case of applying lower static contact force, welded area and failure load of the joint increased with increase in contact force. This result might be due to flow of molten PET by applying compressive force. On the other hand, when higher static contact force was applied, welded area and failure load of the joint decreased, because suppress of the flowing for molten and softened PET occurred. The joint joined with applying cyclic contact force showed lower values of welded area and failure load compared to the joint joined with applying the same maximum static contact force. According to fracture surface observation, crack mainly passed through bubbles to failure in all joints. Apparent strength was calculated by failure load and welded area obtained. The strength of the joint decreased with increase in bubbles density, except the joint joined without contact force. In case of joint joined without contact force, large bubbles were formed and decreased strength. Bubble size formed was small when contact force was applied. Density of bubbles affected strength of the joint and changed by applying contact force, particularly that decreased by applying cyclic contact force. It is considered that applying the contact force is an effective way to control formation behavior of bubbles and results in improvement for reliability of the dissimilar material joint between plastic and metallic materials.

The performance of Pb-free SAC305 solder joint added with porous Cu interlayer was investigated. The porous Cu interlayer was placed in a sandwich-like layer between SAC305 solder alloy and rod Cu as the substrate metal. Two different pores, 15 ppi (pore per inch, P15) and 25 ppi (P25) with the pore size approximately ϕ1.7 and ϕ1.0 mm, respectively were used. The soldering process of solder joint was carried out in three different soldering temperatures of 267, 287 and 307 °C while the holding time was set at 300 s. Tensile test was performed to evaluate the joining strength of the solder alloy with loading rate of 0.5 mm/min. The highest joint strength was recorded at 53 MPa when soldered with P25 porous Cu interlayer at 307 °C. High thermal aging test was performed in an oven, heated to temperature of 150 °C for aging time of 100, 200 and 500 h to investigate the heat and time effect on the joint strength. The results indicate the reduction of strength at the aged sample with the increased aging time. The crack initiates dominantly at the interfaces between the porous Cu/SAC305. In general, solder joint soldered with P25 of porous Cu joints have greater tensile strength (23 MPa) than P15 of porous Cu interlayer after aging test.

The status of poor corrosion resistance of die cast AM60 magnesium (Mg) alloys can be improved by applying various types of surface coatings above them. However, coating can sometimes alter the fatigue properties and initiation mechanism, which is not reported so far for this group of alloy. Present study focuses on this issue investigating influence of anodizing layer and chemical-conversion-coating layer on corrosion fatigue properties of Mg alloy, AM60. Coating performance was assessed in two different corrosive environments: high humidity (50 °C, 80% RH) and 5 wt% NaCl medium. The key observation for humid environment was that the chemical-conversion-coating without a painting layer showed better corrosion resistance than that of the anodized layer without a painting layer. Moreover, chemical-conversion-coating with a painting layer can effectively protect the underlying AM60 alloy even under the most severe corrosive environments (5 wt% NaCl). Our results show, for the first time, that chemical conversion coating can be successfully used in machine parts loaded in aggressive medium without severely compromising the fatigue strength.

Fatigue behavior of Mg-Al-Ca-Mn alloy with T6 treatment was studied at room temperature and 150°C by conduction rotating bending fatigue test. Fatigue strength at high temperature was lower than that at room temperature in the alloys with and without heat treatment. However, degradation of fatigue strength at high temperature in the T6 treated alloy was not significant compared to the as-extruded alloy. Fatigue crack initiation and propagation behavior was observed with replication technique by conducting interrupted fatigue test at room temperature and 150°C. Multiple cracking was significantly observed at 150°C in both as-received and T6 treated alloys. Change in grain size and randomization of crystal orientation due to the heat treatment could affect the fatigue property.

Tensile and fatigue behavior of a 6N01 aluminum alloy friction stir welded (FSW) joint was studied. The tensile and fatigue tests were carried out for the large plate and small round bar specimens. The small round-bar specimens extracted from base material (BM), heat affected zone (HAZ) and stir zone (SZ). The HAZ exhibited the lowest hardness. The higher tensile and fatigue strengths of SZ were mainly due to fine and homogeneous grains and significant cyclic hardening. Fatigue failure of the large plate specimen including the whole FSW joint part occurred at the lowest hardness location in the HAZ.

Fatigue crack growth (FCG) behavior at weld nugget zone (WNZ) and heat affected zone (HAZ) in friction stir welded (FSWed) joints joined by using a bobbin type tool in 5052, 6N01 and 7N01 aluminum alloys were investigated comparing to the base materials (BM). Constant stress amplitude fatigue crack growth tests were conducted with stress ratio of 0.1. Crack closure behavior was investigated simultaneously during fatigue test with unloading elastic compliance method. The results in this study showed that in near threshold region, FCG resistance in WNZ of FSWed 5052 and 6N01 joints was lower than that in the BM and the HAZ. In contrast, FCG resistance in WNZ of FSWed 7N01 joint was higher than that in the BM and the HAZ. At high ΔK region, WNZ of FSWed 5052 joint had the similar FCG resistance with the BM and the HAZ. However, WNZ of FSWed 6N01 joint showed lower FCG resistance compared to the BM and the HAZ. FCG resistance in WNZ of FSWed 7N01 joint was the similar with that in the HAZ but was lower than that in the BM. Crack closure behavior was observed in BM and HAZ for all materials tested. In WNZ, crack closure was found in FSWed 7N01 joint but was not found in FSWed 5052 and 6N01 joints. Difference in FCG behavior at different weld region was mainly due to difference in crack closure behavior in FSWed 5052 and 6N01 joints. However, in case of FSWed 7N01 joint, it is considered that effect of microstructure was significant on FCG behavior even at high ΔK region.

In Order to Improve the Joining Strength of Friction Stir Spot Welded PVC Joint, Sic Particle was Compounded by the Stirring in the Welding Process. the Influence of Sic Particle Sizes on the Joining Strength was Investigated in this Study. in Case of the Specimen Compounded Sic Particle with Size of 14.6μm, Heat Generated during the Process Significantly Increased and Resulted in Increasing Width of Welded Region. however, According to Result of Hardness Test, Excessive Heat Generated during the Process Led to Degradation of Mechanical Property of the Welded Area in the Specimen Compounded Sic Particle with Size of 14.6μm. on the other Hand, in Case of the Specimen Compounded Sic Particle with Size of 6.5μm, Width of Welded Area was Smaller than that of the Specimen Compounded Sic Particle with Size of 14.6μm and was Similar as that of the Specimen Compounded Sic Particle with Size of 0.5μm. it was Observed that Hardness of the Welded Area was Uniformly Higher in the Joint Compounded Sic Particle with Size of 6.5μm. Specimen Compounded Sic Particle with Size of 6.5μm Showed the Highest Strength among the Joints Tested. it is Considered that the Joining Strength was Affected by both Width and Mechanical Property of the Welded Area.

Fatigue Property of an Extruded AZ61 Magnesium Alloy with the Processing Layer Introduced by Machining was Investigated. Rotating Bending Fatigue Tests were Carried out with the Specimen with and without the Processing Layer. According to Results of the Fatigue Tests, Fatigue Life Significantly Increased by Introducing the Processing Layer to the Specimen Surface. Fatigue Crack Initiation and Propagation Behaviors were Observed by Replication Technique during the Fatigue Test. Fatigue Crack Initiation Life of the Specimen with the Processing Layer was Slightly Longer than that of the Specimen without the Processing Layer. Higher Fatigue Crack Growth Resistance was also Observed when the Fatigue Crack was Growing in the Processing Layer in the Specimen with the Processing Layer. the Longer Fatigue Life Observed in the Fatigue Test in the Specimen with the Processing Layer could be Mainly due to the Higher Crack Growth Resistance. it is Speculated that the Fatigue Strength can be Controlled by Change in Condition of Machining Process. it could be Effective way in Industry to Improved Fatigue Strength only by the Cutting Process without Additional Surface Treatment Process.

Fatigue characteristics of bolted joint tightened with a steel bolt have been discussed extensively. However the fatigue characteristics of bolted joint tightened with a nonferrous bolt have not been sufficiently discussed. In this study, two types of fatigue tests, axial fatigue tests and transverse fatigue tests, for nonferrous bolts were conducted. The nonferrous bolts used in this study made of AZ31 and AZX912 magnesium alloy and A5056 aluminum alloy. The results of both fatigue tests showed that the fatigue limits of the A5056 bolt were the highest of all. The fatigue limits of two kinds of the magnesium alloy bolts were almost the same in both fatigue tests. However the ration of the axial fatigue limits to the transverse fatigue limits were different according to the bolt materials.

In the present study, fatigue crack growth tests of Pb-containing [Sn-37Pb (wt.%)] and Pb-free [Sn-3.0Ag-0.5Cu (wt.%)] solders were performed under cycle/time-dependent step loading at a constant J-integral range (ΔJ). The C* parameter was also estimated for discussing time-dependent crack growth behavior. The experimental results indicated that acceleration of the crack growth rate at the beginning of the second loading step was induced when the C* value for the first loading step was high, regardless of time- or cycle-dependent crack growth and for both Sn-37Pb and Sn-3.0Ag-0.5Cu solders. The length of the acceleration region of the crack growth rate for both solders was in good agreement with the creep damage zone size estimated by the creep zone model proposed by Riedel and Rice.

In the present study, scatter behavior of fatigue limit for the die-cast magnesium alloy AM60B was investigated. Twelve fatigue limits were determined based on S-N curves obtained according to the standard fatigue test methods (that is based on the 0-2 or 0-1-2 combinations). The scatter of fatigue limit for the die-cast material was large where the values of Weibull modulus and COV were 16 and 6.8, respectively. This value of COV was in agreement with that obtained by simulated staircase method. The scatter of fatigue limit in the die-cast magnesium alloy was comparable to those in the die-cast aluminum alloy. In the die-cast materials, the fatigue limit determined based on S-N curve should be taken into account to have 6-10% of scatter from the mean value.

Dissimilar materials laser spot joining between polyethylene terephthalate (PET) and aluminum alloy (A5052) was studied by using pulse YAG laser. The results showed that it was possible to make a joint in a wide range of heat input. Molten pool was formed at the interface of A5052 and depth of molten pool increased with increase in heat input. Tensile shear test was carried out to evaluate strength of the joints. The result showed that tensile shear strength increased with increase in the heat input when the molten pool filled with molten or softened PET. On the other hand, instability of joining state inside the molten pool might induce variation in the strength. FEM analysis was carried out to investigate effect of molten pool shape on the strength. The result showed that molten pool radius did not significantly affect tensile shear strength. In contrast, the joint strength increased with increase in the molten pool depth as observed in result of the experiment. It is proposed that taking into account the mechanical effect such as molten pool shape is an important approach to improve the joining strength under tensile shear loading.

Tensile and compressive creep tests of the extruded magnesium alloy AZ91D were carried out in vacuum at 150°C under constant engineering stresses ranged from 60 to 150 MPa. From the test results, the secondary creep rate in tension was found to be significantly higher than that in compression. Moreover, the estimation method of creep curve under a constant true stress was proposed by considering the reduction of cross sectional area during tensile loading where the specimen cross-sectional area and length were measured periodically until the end of creep test. The creep curve under a constant true stress obtained in the present study was still different between tensile and compressive loadings. Therefore, it should be noted that the different creep curves and creep exponents should be used in the creep deformation analysis of structures.

In the present study, scatter behavior of fatigue limit for the die-cast magnesium alloy AM60B was investigated. Twelve fatigue limits were determined based on S-N curves obtained according to the standard fatigue test methods. The scatter of fatigue limit for the die-cast AM60B was large compared to the extruded AZ61: the values of Weibull modulus and COV were 16 and 6.8, respectively, for the die-cast material, whereas those for the extruded one were 159 and 0.7. The COV value of 0.7 for the extruded AZ61 meant that only 0.7% of scatter from the mean value was included in the fatigue limit determined based on the S- N curve. This COV value was comparable to that of 0.6 for the tensile strength of the same material. Therefore, the fatigue limit for the extruded material has small enough scatter to be considered as the material property similar to the tensile strength that can be determined from the average value of 2-3 samples.

Overall fatigue strengths and hardness distributions of the aluminum alloy similar and dissimilar friction stir welding (FSW) joints were determined. The local fatigue strengths as well as local tensile strengths were also obtained by using small round bar specimens extracted from specific locations, such as the stir zone, heat affected zone, and base metal. It was found from the results that fatigue fracture of the FSW joint plate specimen occurred at the location of the lowest local fatigue strength as well as the lowest hardness, regardless of microstructural evolution. To estimate the fatigue strengths of aluminum alloy FSW joints from the hardness measurements, the relationship between fatigue strength and hardness for aluminum alloys was investigated based on the present experimental results and the available wide range of data from the references. It was found as: σa (R = −1) = 1.68 HV (σa is in MPa and HV has no unit). It was also confirmed that the estimated fatigue strengths were in good agreement with the experimental results for aluminum alloy FSW joints.

The joining of lead-free Sn-3.0Ag-0.5Cu (SAC305) solder alloy to metal substrate with the addition of a porous Cu interlayer was investigated. Two types of porous Cu interlayers, namely 15 ppi—pore per inch (P15) and 25 ppi (P25) were sandwiched in between SAC305/Cu substrate. The soldering process was carried out at soldering time of 60, 180, and 300 s at three temperature levels of 267, 287, and 307 °C. The joint strength was evaluated by tensile testing. The highest strength for solder joints with addition of P25 and P15 porous Cu was 51 MPa (at 180 s and 307 °C) and 54 MPa (at 300 s and 307 °C ), respectively. The fractography of the solder joint was analyzed by optical microscope (OM) and scanning electron microscopy (SEM). The results showed that the propagation of fracture during tensile tests for solder with a porous Cu interlayer occurred in three regions: (i) SAC305/Cu interface; (ii) inside SAC305 solder alloy; and (iii) inside porous Cu. Energy dispersive X-ray spectroscopy (EDX) was used to identify intermetallic phases. Cu6Sn5 phase with scallop-liked morphology was observed at the interface of the SAC305/Cu substrate. In contrast, the scallop-liked intermetallic phase together with more uniform but a less defined scallop-liked phase was observed at the interface of porous Cu and solder alloy.

In recent years, magnesium alloys are widely used for automotive applications as structural components due to its lightweight property and high specific strength. In this regards, magnesium alloys are subjected to high velocity and impact loads during accident. Hence, understanding the impact and dynamic behaviours of magnesium alloys are essential. In this study, the effect of loading rates on the fracture behaviour of Mg-Al-Zn alloys was investigated using pre-cracked single-edge notched bending (SENB) specimens. Three-point bending tests were conducted at different loading rates of 5, 50 and 500 mm/min. The Mg-Al-Zn alloys that used in the present study were extruded AZ31 and AZ61 magnesium alloys. From the load-load line displacement results, both alloys exhibited nonlinear fracture behaviour. The maximum load (Pmax) of these two alloys increased with increasing loading rate. Comparing both alloys, AZ61 exhibited higher Pmax than that of AZ31 due to the higher volume of b-phase and smaller grain size in AZ61. Fracture surface observation revealed that both alloys fractured in ductile manner with large scale yielding and high shear lips ratio at all loading rates.

In this research, polyethylene terephthalate (PET), as a high-density thermoplastic sheet, and Aluminum A5052, as a metal with seven distinct surface roughnesses, were joined by friction spot welding (FSW). The effect of A5052’s various surface states on the welding joining mechanism and mechanical properties were investigated. Friction spot welding was successfully applied for the dissimilar joining of PET thermoplastics and aluminum alloy A5052. During FSW, the PET near the joining interface softened, partially melted and adhered to the A5052 joining surface. The melted PET evaporated to form bubbles near the joining interface and cooled, forming hollows. The bubbles have two opposite effects: its presence at the joining interface prevent PET from contacting with A5052, while bubbles or hollows are crack origins that induce crack paths which degrade the joining strength. On the other hand, the bubbles’ flow pushed the softened PET into irregularities on the roughened surface to form mechanical interlocking, which significantly improved the strength. The tensile-shear failure load for an as-received surface (0.31 μ m Ra) specimen was about 0.4–0.8 kN while that for the treated surface (>0.31 μ m Ra) specimen was about 4.8–5.2 kN.

The weld strength of thermoplastics with aluminium alloy, such as high density polyethylene and polypropylene sheets, is influenced by friction stir welding parameters. This paper focuses on the preliminary investigation of joining parameter at various levels as well as the mechanical properties of friction spot joining (FSJ) of aluminium alloy (A5052) to polyethylene terephthalate (PET). A number of FSJ experiments were carried out to obtain optimum mechanical properties by adjusting the plunge speed and plunge depth in the ranges of 5–40 mm min−1 and 0·4–0·7 mm respectively, while spindle speed remains constant at 3000 rev min−1. The results indicated that A5052 and PET successfully joined with the aid of frictional heat energy originated from the friction spot welding process. The effect of plunge speed on the joined area and the effect of formation of bubbles at the interface of joints on the shear strength of joint are discussed.

In this study, the effects of CeO2 nanoparticle addition on tensile strength and formation of interfacial intermetallic compound layer for lead-free Sn-3.5Ag-0.7Cu solder joint were investigated. The results showed that the thickness of intermetallic layer was decreased with increasing weight percentages of CeO2 reinforcements, and the growth of the intermetallic layer was remarkably suppressed during isothermal aging at 150 °C. It was also found that the tensile strength of the solder joint gradually increased with the addition of CeO2 particles, reached to the maximum value at 0.5 wt% CeO2 and then decreased drastically with further increase in CeO2 content. Microstructural investigation of 1 wt% CeO2 reinforced solder joint revealed that due to particle pushing under the influence of interfacial tension between the particles and the matrix, the brittle cluster-like regions with high concentration of CeO2 particles were formed inside the joint. These brittle clusters act as preferential site for crack initiation during tensile test which would degrade the tensile strength. Hence, based on the results obtained in this study, addition of CeO2 particles up to 0.5 wt% provide an optimized solder joint with improved strength together with suppressed intermetallic growth and long-term reliability.

Scatter behavior of fatigue life in a die-cast AM60B magnesium alloy was investigated. For comparison, those in a rolled AM60B alloy and a die-cast A365-T5 aluminum alloy were also studied. Scatter behavior of pore size was also investigated to discuss about dominant factors for fatigue life scatter in the die-cast materials. Three-parameter Weibull function was suitable to explain the scatter behavior of both fatigue life and pore size. The scatter of fatigue life in the die-cast AM60B alloy was almost comparable to that in the die-cast A365-T5 alloy, while it was significantly large compared to that in the rolled AM60B alloy. Scatter behavior of pore size observed on the specimen cross-section was comparable to that of fatigue life. Therefore, the scatter behavior of fatigue life can be estimated, if the scatter behavior of pore size on the cross section of specimen would be evaluated, as proposed in the present study.

As one of the key technologies for high performance electronic devices, composite solders have been recently developed to improve thermal and mechanical properties of solder joints. In this study, accumulative roll bonding process was used as an effective alternative method for manufacturing high-strength, finely dispersed, void-free and highly uniform Sn–Ag–Cu/CeO2 nanocomposite solders. Microstructural investigation of nanocomposite solders revealed that homogenous distribution of CeO2 nanoparticle has been achieved and the eutectic as-cast morphology of the solder changed to recrystallized fine grained structure. As a result of severe plastic deformation during rolling, brittle and elongated intermetallics crushed into fine particles with an average diameter of a few hundred nanometers and dispersed uniformly in the solder matrix. Mechanical test results showed that the microhardness, 0.2% yield stress, and ultimate tensile strength of the composite solder increased with addition of CeO2 nanoparticles, while the ductility of the composite was decreased.

Spark plasma sintering (SPS) technology was used to determine the appropriate conditions for SPS sintering of commercially pure magnesium as well as the magnesium alloy AZ31. It was found that the sintering temperatures of 585 °C and 552 °C were the most suitable sintering temperatures for the magnesium and the AZ31 alloy, respectively. Magnesium matrix and AZ31 alloy matrix composites reinforced with SiC particles were then successfully fabricated by the SPS method at sintering temperatures of 585 °C and 552 °C, respectively. A uniform distribution of SiC particles was observed along the boundary between matrix particles. The mechanical properties, i.e. hardness and tensile strength increased with increasing SiC content up to 10 wt%. However, when the SiC content was larger than 10 wt%, the tensile strength decreased due to the agglomeration of SiC particles. The agglomeration of SiC particles was found to lead to the degradation of the interfacial bonding strength between matrix and reinforcement.

Magnesium (Mg) and its alloys have a long tradition of use as a lightweight material in the field of commercial and especially automotive construction. Unfortunately its poor corrosion resistance restricts its use under corrosive atmosphere. So far, most of the research works on corrosion fatigue properties of Mg alloys have dealt with effect of pre-existing pit or effect of pitting corrosion. In our previous work, it has been found that fatigue cracks initiate and propagate from corrosion-induced surface defects rather than pits. In the present study, fatigue tests of as-cast and shot-blasted AM60 Mg alloy specimens were carried out in three environments: low humidity (relative humidity 55%RH), high humidity (relative humidity 80%RH) and 5% NaCl. Fatigue strengths were degraded in the high humidity and NaCl environments for both the as-cast and the shot-blasted specimens. Cracks were nucleated from corrosion-induced surface defects which then propagated until final failure. Considering the corrosion-induced surface defect as a stress raiser, the stress intensity factor value was estimated. The estimated value was almost coincided with the threshold stress intensity factor range. The shot-blasted specimens showed improved fatigue strength under all the three environments compared to the as-cast specimens. It was speculated that improvement of fatigue strength due to shot-blasting resulted from the hardening and compressive residual stress near surface region induced by shot-blasting.

A new direct joining method for dissimilar materials between a metal and a plastic by using friction stir heating technique is introduced. A5052 and polyethylene terephthalate (PET) were used as materials used in this study. The joining strength was evaluated under tensile shear loading condition. The fracture surface and the cross section of the joined specimens were observed in order to understand characteristics of the joints. The results show that A5052 and PET were successfully welded by the present joining method. Similar strength of the joints was obtained regardless of the joining conditions. Melting and softening behaviors of PET and formation of bubbles observed in welded area might affect the joining strength.