Fusion is the alloy add-on for SimpleCore/Simple Ores2. It adds steel (and steel items) and the fusion furnace (an alloy furnace); from Simple Ores 2, it adds 3 more alloys and items made from them: bronze, thyrium and sinisite. Fusion recycling, from akkamaddi's Addenda (formerly Simple Fusion Recycling), was merged into Fusion around Minecraft 1.14.4/1.15 or so.
For Minecraft Forge 1.16.3 and later, SimpleCoreLib is required.
For Minecraft Forge 1.14 through 1.16.1, Simple Ores2 is required.
SimpleCore API is no longer required for Minecraft 1.12.2 or earlier.

Please see the original Minecraft Forum post for information. Or the wiki.
Automation notes by side:
- Top: inserts into catalyst slot
- Bottom: extracts from output slot
- Back, Front: inserts into fuel slot.
- Left: inserts into left input slot
- Right: inserts into right input slot

Fusion originally only had the bronze door; recently, I added steel, sinisite and thyrium doors
Classic appearance. We updated the armor textures in later versions. left to right: bronze, sinisite, thyrium, and steel armor.
From before I added JEI support for fusion furnaces.
In very recent versions, if you have Netherrocks installed, you can do fusion recycling of netherrocks gear.
clockwise from upper left: Sinisite, bronze, steel, thyrium. Fusion Furnace to the right.
Image of various chunk combination recipes.
Fusion Furnace showing hopper inputs and output. Also shows off new 1.18.2 texture for furnaces.
Steel was a late addition to Fusion, relatively speaking; thus, we had a cheat sheet for it.
Fusion Furnace requires 4 brick blocks, 2 iron, 2 coal, and a furnace to build.