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The Mechanical Performance of an Isolated Copper Braze Joint

Norsell, Johanna LU (2015) KOO920 20151
Centre for Analysis and Synthesis
Abstract
This Master Thesis has been conducted with the aim to investigate the mechanical performance of an isolated copper braze joint. The mechanical performance of the copper brazed heat exchanger is to a great extent determined by the properties of the copper braze joints. It is not yet exactly known how different parameters such as the weight of copper per joint, plate material composition and furnace cycle times during brazing affect the performance of the joint. Hence, this work will be divided into three steps to evaluate the mechanical performance of an isolated copper braze joint. These will consist of (1) an evaluation of the test method with respect to the copper weight per joint, (2) evaluation of how the plate material composition... (More)
This Master Thesis has been conducted with the aim to investigate the mechanical performance of an isolated copper braze joint. The mechanical performance of the copper brazed heat exchanger is to a great extent determined by the properties of the copper braze joints. It is not yet exactly known how different parameters such as the weight of copper per joint, plate material composition and furnace cycle times during brazing affect the performance of the joint. Hence, this work will be divided into three steps to evaluate the mechanical performance of an isolated copper braze joint. These will consist of (1) an evaluation of the test method with respect to the copper weight per joint, (2) evaluation of how the plate material composition affects the mechanical performance of the joint and (3) how the thermal history affects the mechanical performance of the joint.

To produce the samples, coupons of stainless steel were brazed with copper to produce single braze joints. These were tensile tested in order to investigate the mechanical performance. Some samples were further investigated with LOM and SEM to analyze wetting and grain size.

The method produced reliable results with good wetting and low variations in the braze joint width for a fixed quantity of copper. Because of this, the method proved it could be used for further analyses. The mechanical performance was dependent of the braze joint geometry and the wider the joint, the higher the mechanical performance.

The test method was further used to investigate how the variations in titanium content within the 316 specification affected the mechanical performance. Results showed that a higher titanium content resulted in smaller joints compared to joints made of steel with a lower titanium content. The higher titanium content was believed to stabilize the oxide film on the stainless steel surface and by this reduce the wetting ability. The smaller joints led to a slight decrease in mechanical performance. This was in accordance to the previously observed results where a wider joint had a higher mechanical performance.

The brazing of 304 stainless steel was also studied. Compared with stainless steel 316, the 304 samples had smaller joints for the same quantity of copper per joint. However the smaller joint had the same mechanical performance as the joints made of 316 with the same amount of copper per joint and almost the same titanium content. The difference in joint geometry and mechanical performance was believed to be due to the different alloy composition.

Finally, a shorter brazing cycle was designed for brazing of the samples. The samples became covered with a green oxide during the brazing, resulting in insufficient wetting and weak joints. The oxide was probably chromium(ІІІ)oxide formed due to a not fully controlled atmosphere during the brazing. This test showed how small alterations in furnace cycle can affect the brazing and the importance of a controlled atmosphere during brazing. (Less)
Popular Abstract (Swedish)
En kopparlödd värmeväxlare används i fjärrvärmeapplikationer. Ett samhälle under ständig utveckling har lett till en strävan att förbättra dessa produkter, men också till en strävan att minska kostnaden vid både tillverkning och utveckling av nya, effektivare produkter.
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author
Norsell, Johanna LU
supervisor
organization
course
KOO920 20151
year
type
H2 - Master's Degree (Two Years)
subject
keywords
materials chemistry, materialkemi
language
English
id
7513323
date added to LUP
2015-09-07 10:30:17
date last changed
2015-09-07 10:30:17
@misc{7513323,
  abstract     = {This Master Thesis has been conducted with the aim to investigate the mechanical performance of an isolated copper braze joint. The mechanical performance of the copper brazed heat exchanger is to a great extent determined by the properties of the copper braze joints. It is not yet exactly known how different parameters such as the weight of copper per joint, plate material composition and furnace cycle times during brazing affect the performance of the joint. Hence, this work will be divided into three steps to evaluate the mechanical performance of an isolated copper braze joint. These will consist of (1) an evaluation of the test method with respect to the copper weight per joint, (2) evaluation of how the plate material composition affects the mechanical performance of the joint and (3) how the thermal history affects the mechanical performance of the joint. 

To produce the samples, coupons of stainless steel were brazed with copper to produce single braze joints. These were tensile tested in order to investigate the mechanical performance. Some samples were further investigated with LOM and SEM to analyze wetting and grain size.

The method produced reliable results with good wetting and low variations in the braze joint width for a fixed quantity of copper. Because of this, the method proved it could be used for further analyses. The mechanical performance was dependent of the braze joint geometry and the wider the joint, the higher the mechanical performance. 

The test method was further used to investigate how the variations in titanium content within the 316 specification affected the mechanical performance. Results showed that a higher titanium content resulted in smaller joints compared to joints made of steel with a lower titanium content. The higher titanium content was believed to stabilize the oxide film on the stainless steel surface and by this reduce the wetting ability. The smaller joints led to a slight decrease in mechanical performance. This was in accordance to the previously observed results where a wider joint had a higher mechanical performance.

The brazing of 304 stainless steel was also studied. Compared with stainless steel 316, the 304 samples had smaller joints for the same quantity of copper per joint. However the smaller joint had the same mechanical performance as the joints made of 316 with the same amount of copper per joint and almost the same titanium content. The difference in joint geometry and mechanical performance was believed to be due to the different alloy composition. 

Finally, a shorter brazing cycle was designed for brazing of the samples. The samples became covered with a green oxide during the brazing, resulting in insufficient wetting and weak joints. The oxide was probably chromium(ІІІ)oxide formed due to a not fully controlled atmosphere during the brazing. This test showed how small alterations in furnace cycle can affect the brazing and the importance of a controlled atmosphere during brazing.},
  author       = {Norsell, Johanna},
  keyword      = {materials chemistry,materialkemi},
  language     = {eng},
  note         = {Student Paper},
  title        = {The Mechanical Performance of an Isolated Copper Braze Joint},
  year         = {2015},
}