This experiment provided an chance of carry oning bonding, and of larning analysis technique utilizing spectrometry and microscopy. The samples used in analysis are Sn- Bi alloys with different composings. Cooling curves were depicted by chilling down these metals from 300 a„? , and were used to pull stage equilibrium diagrams. SEM and EDS were applied to bring forth surface topographical images and chemical composings of the specimens, doing comparings with the deliberate consequences from the stage equilibrium diagram. Consequences showed that electronic analytical devices may differ from conceptual derivation in quantitative finding of composings.
1.0 Introduction
The stage equilibrium lab introduces basic techniques of soldering and analysing microstructures of metals with scanning negatron microscope ( SEM ) and Energy Dispersive X-ray Spectroscopy ( EDS ) . The soldering portion of the lab helps pupils develop a hand-on experience of fall ining two Cu pipes with Sn-Bi metals. In the interim, a 90Sn- 10Bi metal is cooled down from 300 a„? , and its temperature alterations with clip are recorded with a thermocouple. Afterwards chilling curves are drawn to develop a stage diagram of the metal.
The 2nd portion of the lab involves the usage of SEM and EDS to detect microstructures of Sn- Bi alloys with different composings. The SEM is responsible for bring forthing images of the exaggerated construction of four samples, viz. , eutectic, hypoeutectic, hypereutectic and solder metals. The specimens are placed in the SEM chamber which can be seen from a proctor ; perceivers are able to exchange among the four specimens. A beam of negatrons are bombarded toward ( scan ) the specimens and reflected beams are collected, which are displayed at the same scanning rate on a cathode beam tubing. [ 1 ] EDS is used to find the chemical composing of the solder and each stage by analysing the emitted X raies after hit by charged atoms. [ 2 ]
2.0 Materials
2.1 Soldering and Cooling
Flux
Propane torch
Copper pipes
Solders ( 5:95 Bi: Tin )
Thermocouple:
K type ( Nickel-Chromium Vs. Nickel-Aluminum )
Data Logger Thermometer:
Brand: Omega
Model # : HH306A
Temperature Scope: -200C – 1370C
Hot Home plate:
– Brand: Corning
– Model # : PC-400
2.2 SEM
SEM/EDX:
– Brand: Hitachi
– Model # : S570 & A ; S4500
3.0 Procedures
3.1 Soldering
First, clean Cu pipes until there is no dust and obvious oxides on their surface of both terminals. Flux was so brushed to wet the connection interface, inside and outside, to let formation of lasting bond after hardening of the liquefied solder. [ 3 ] To avoid fire, merely little sum of flux is used. Afterwards, operator used the outer fire of the propane torch to heat the overlapping subdivision of the two pipes, and the solder wire in the opposite way to touch the surface of the joint until the solder was melted followed by runing of flux. Molten solder joined with the two pipes by capillary force ; and chilling in H2O furthered the hardening. Pressure trial was applied to analyze the waterproofing of the pipe connexions. Tap H2O ran into the pipes ; there was no escape connoting that the bonding was done absolutely.
3.1 Cooling of Bi-Sn Alloy
The Bi-Sn metals were cooled down from about 300 grades to room temperature. The solder metal is placed inside a clinker block and the lessening of temperature was detected by a thermocouple and recorded by a information lumberman thermometer. Cooling curves of the Bi-Sn systems are depicted with temperatures against composings ( Fig. 1 ) .
3.2 Scaning Electron Microscopy
Preparation of samples: 1. Use stiff proverbs to cut off the sample
2. Heat sample pieces in Bakelite
3. Polish the sample with emery paper
4. Polish with electro Polish [ 4 ]
The prepared samples are placed in the SEM chamber for analysis. By whizzing in, one can clearly see exaggerated images of the microstructures of the metals.
4.0 Consequences
4.1 Cooling Curves
The curves are developed after chilling the Bi-Sn metal of different composings from ~300a„? .
Fig. 1 Cooling curves matching to different composings of the Bi-Sn system.
3.2 Bi-Sn Phase Diagram
Fig. 2 Phase diagram of Bi-Sn system with their hypereutectic and hypoeutectic compositons
3.3 Compositions
Solder
Hypo-
eutectic
Eutectic
Hyper-
eutectic
Overall composing from
EDS [ wt % Bi ]
4.88
67.65
47.28
19.55
Composition of primary stage from EDS [ wt % Bi ]
N/A
92.31
N/A
N/A
Area fraction of primary
stage, calculated from SEM micrograph [ % ]
N/A
56.25
N/A
N/A
Weight fraction of primary
stage calculated from country
fraction [ wt % ]
N/A
62.50
N/A
N/A
Weight fraction of primary
stage calculated from tie line drawn on stage diagram [ wt % ]
N/A
34.21
N/A
N/A
Composition of Sn-rich stage from EDS [ wt % Bi ]
3.50
15.82
15.71
5.12
Composition of Sn-rich stage as determined from stage diagram [ wt % Bi ]
2
2
2
2
Weight fraction of entire Sn-
rich stage calculated from tie line drawn on stage diagram [ wt % ]
90.72
28.87
42.27
80.41
Composition of Bi-rich stage from EDS [ wt % Bi ]
98.28
92.31
95.16
96.67
Composition of Bi-rich stage as determined from stage diagram [ wt % Bi ]
99.00
99.00
99.00
99.00
Weight fraction of entire Bi-rich stage calculated from tie line drawn on stage diagram [ wt % ]
9.28
71.13
57.73
19.59
Table 1: Composition from EDS analysis and country and stage fractions for Bi-Sn Alloy
Sample computations:
( The denseness of Sn used is 6.99g/cm3 )
4.0 Discussion
4.1 Analysis of the chilling curves
The chilling curves in Fig. 1 shows that the stop deading point of the Bi-Sn metal decreases with increasing proportion of Bi. For all the instances, the rate of chilling slows down before making the freeze point, and under chilling, where liquids exist below the freeze point, occurs merely earlier transforming into solids. There are five curves deserving observing. In the instance of 95Sn5Bi, the first pointer indicating to the stop deading point of the metal, while the 2nd one indicates the place of maximal solubility of Sn in Bi in the stage diagram, which is somewhat below 150 a„? . However, the clip for 40Sn60Bi to turn from liquid to solid is comparative longer than the others, and the graph is about horizontal in the freeze procedure where energy is released ; the temperature matching to the line is eutectic temperature. This particular instance is resulted from the composing of 40Sn60Bi metal approximates to that of eutectic mixture ( 43Sn57Bi ) . Curves four and five as labeled in Fig. 1 have similar tendencies of chilling down, with upper pointers bespeaking some solid Bi, but Sn stayed molten and lower pointers demoing hardening of mixtures. [ 5 ]
4.2 How cooling curves are used to build stage diagrams
Pull a graph with composing being x-axis and temperature y-axis, and the temperature scales lie on both terminals of the graph
Decide the co-ordinate of the eutectic point by alloy composing and eutectic temperature
Determine the freezing temperatures against the proportion of the mixture.
Note: For non-eutectic metals, the freezing temperature corresponds to the “ little hill ” after the under chilling point.
To point the maximal solubility point of Sn in Bi, find the chilling curve of 95Sn5Bi in Fig. 1, and the lower pointer indicate the point where bezant line and solvus line intersect. The maximal solubility point of Bi in Sn can be found by the chilling curve of 20Sn80Bi with the same method.
Solidus and solvus lines are determined by the freeze point at appendages and the maximal solubility points while liquidus by eutectic point.
Pull a horizontal line go throughing through the eutectic point and ends at maximal solubility points.
4.3 Accuracy of EDS
No significant disagreements are found in the consequences of EDS and phase diagram computations for the composing of Bi-rich stage, most important per centum mistake is within 7 % . Whereas, there are noteworthy differences for hypoeutectic and eutectic mixtures when comparing composings of Sn-rich stage utilizing EDS and stage diagram, with a disparity of about 14 wt % Bi for both instances. The disagreements could be resulted from two parametric quantities. For sample A, thin beds of Bi in the eutectic construction besides reflect the negatron beams of X-ray and therefore do the crisp addition in the sum of Bi nowadays in primary stage. Besides, one can see the black musca volitanss, which are drosss, in the SEM image of sample B. Most of the drosss are near the boundaries of primary and eutectic construction, and thereby accounted as primary stage by reflecting X raies to the sensor.
4.4 Reliability of SEM Image
A 28.29 wt % spread between the weight fraction of primary stage derived from SEM image and tie line indicates that the informations manipulated from SEM disagrees with conceptual 1s. The major cause is that the inaccuracy of the grid numeration technique of finding the weight fraction. The figure of intersections of lines that fall in the atoms are well affected by the figure of grids drawn and magnification of the image. The unsmooth estimate of this method consequences in undependable informations produced.
4.5 Comparison of the stated composing and EDS informations
The consequences from EDS are by and large accurate except for sample B, which is the eutectic mixture. EDS showed 9.72 wt % Sn higher than stated. This divergence could be explained by drosss engagement. As noticed before the lab, the metals may represent of more than two constituents and hint of drosss will besides be present in the alpha + beta stage, ensuing in a misrecognition of Sn, whose peak displayed may overlap with those of drosss. Therefore, this colored analysis gives a high proportion of Sn in the mixture.
4.6 Practical usage of pipe bonding
The Bi-Sn solder is mostly used in pipe bonding and circuit board printing as the runing point of this metal is comparatively low, for case, SnBi58 thaws at 138 A°C. Building codifications today prohibit utilizing lead solder for H2O pipes, though traditional tin-lead solder is still available. “ Surveies have shown that lead-soldered plumbing pipes can ensue in elevated degrees of lead in imbibing H2O. “ [ 6 ]
5.0 Decision
Based on the observation and analysis stated above, a decision can be drawn:
Microstructures of Bi -Sn alloys determines the belongingss ( strength ) of the solder, which contribute to its common application in pipe bonding. Meanwhile, EDS and SEM are effectual in look intoing the microstructures of metals, diagrammatically and quantitatively, if the technique of construing the consequences is sensible and dependable.
