How can i only lerp the CFrames rotation?

the following code lerps PartA's orientation to match PartB's orientation:

local PartA = workspace.PartA
local PartB = workspace.PartB

local start = PartA.CFrame
local _end = CFrame.new(PartA.Position) * (PartB.CFrame-PartB.CFrame.p)

for i = 0,1,.1 do
    PartA.CFrame = start:Lerp(_end,i)
    wait(.1)
end

This is the line of code that figures out what PartA with PartB's rotation would be:

local _end = CFrame.new(PartA.Position) * (PartB.CFrame-PartB.CFrame.p)

1) CFrame.new(PartA.Position) creates a CFrame consisting of only PartA's position and a rotation of (0,0,0).

2) (PartB.CFrame-PartB.CFrame.p) subtracts the position from PartB's cframe so we are left with a cframe containing only PartB's rotation.

When you multiply CFrame.new(PartA.Position) and (PartB.CFrame-PartB.CFrame.p) you're left with a CFrame value containing PartA's position and PartB's rotation.

we can use that as the CFrame value as our 'end' position for the Lerp()